Reaching the limit of depth of field

 I think I have reached the limit of the depth of field I can get from single shots. I might get more in due course if the software I am using improves or I get better at using the software. For the moment though the limit appears to be at around f/45 when using a pair of 2X teleconverters with the Laowa 100mm 2X macro. (That f/45 is for the camera/lens - the lens is set to f/11 and the camera knows that there is 4X teleconversion in use and adjusts that f/11 to f/(11*4), and shows an f-number of f/45).

I have two reasons for thinking that this is the limit.

First, I did a session with f/57, which is two thirds of a stop up from f/45, and I didn't want to keep any of the images from the session. Despite the heavyweight processing they were still too soft for my liking and lacked some detail that I had been getting in similar shots at f/45, for example in ants' eyes.

Second, a very helpful and insightful analysis of one of my tiny aperture images by Rik Littlefield. Rik is immensely knowledgeable in this area. He is I believe the author of Zerene Stacker, which is one of the two highly specialised focus stacking software applications. He also runs the photomacography.net web site. Rik is very actively involved with discussions on the web site and has a talent for explaining complicated issues in accessible language for people (like me) who don't have the background to understand the underlying optics and mathematics.

Rik's analysis was in this thread, in which I had posted five of my recent tiny aperture images. (I don't post much on that site, but I'm very glad I did this time round.) Another forum member asked me to do a comparison between a focus-stacked image and one of my tiny aperture images of the same subject, which I did. (I'm not very skilled at focus stacking at the macro scale, but fortunately what I managed was good enough for the purpose in hand.)

In this post Rik did a detailed comparison of a couple of areas in the images and showed that even at the reduced image size of 1300 pixels high it was not just that the tiny aperture image was losing fine detail, but that for the detail that remained it was rearranging some elements of it so that it was different from the actual structure of the subject. However, you had to look very carefully at a like for like comparison with a more accurate rendition to tell this was going on. As Rik put it, 

I don't see that any of this impacts Nick's use of this technology for his purposes. Small aperture one shot is the only technique I know for capturing high DOF images of small active critters. But it does seem clear that this image has reached beyond the limit of accurately capturing fine detail, even at the 1300 pixels size.

I'm content to live with this fine detail inaccuracy as it doesn't in my view significantly alter the overall impression of the scene and the subject. And given my approach of "pretty pictures" and "credibility" rather than "accurate records" and "authenticity" I think that is good enough. But, coupled with my f/57 experience, I do think I'll draw the line at f/45 for now. 

The curious issue remains that using a constant camera/lens f-number of f/45 seems to work even though this means that the effective f-number is changing as the magnification changes, and so the depth of field and diffraction softening are changing too. Why this constant f-number should work is a mystery to me. It may be that I haven't looked at this carefully enough. I'll try some more variations away from f/45 to see if I can get more depth of field at some magnifications. If I can then I can adopt a more nuanced approach to setting the camera/lens f-number, although to be honest the simplicity of using a single f-number is very appealing because it lets me react faster to what is going on.

Update 28 May 2021. 

User ThrillaMozilla has commented very helpfully on this mystery in this post at dpreview.com:

ThrillaMozilla wrote:

gardenersassistant wrote:

It is very convenient to not adjust the aperture as the magnification changes. Given that this means the effective aperture changes with magnification, I don't understand why this works as well as it does in terms of apparently (difficult to test) getting a similar depth of field in relation to subject size whatever the magnification.

Image-side geometry to the rescue.

It's late, but if I did the math. correctly on the small scrap of paper, for a given focal length the diffraction blur diameter and the depth of field are both proportional to 1+1/M. How convenient. You want them both to stay proportional to the size of the sample, hence constant with changes magnification. For M between 1.5 and 5.6 that varies between 1.18 and 1.67 -- thus constant enough. It's just your good luck that it works out that well. 

New kit - Collecting evidence Part 2 - flowers etc

 The third of the three new pieces of kit I was thinking about was a micro four thirds Panasonic Leica 50-200. In this post I said that "For flowers etc I need to do some practical comparisons between the 60mm macro and the 45-175." 

Three weeks ago I photographed 9 flowers hand-held in our garden using a Panasonic 45-175 on a G9, with and without a Canon 500D close-up lens depending on the size of the subject, and with an Olympus 60mm macro on a Panasonic G9. 

I used aperture bracketing. With the Olympus 60mm macro this gave me a set of 7 shots for each shutter press. With the 45-175 each aperture bracket set contained 5 shots.

For each subject I selected the image that appealed to me most from each camera and processed the selected images from raw using my then current flower workflow of Lightroom followed by Topaz DeNoise AI.

For each subject I selected the image that I liked best. This turned out to be four captured with the 45-175 and five captured with the 60mm macro. (They are in this album at Flickr.)

It is possible that the 50-200 would render colours, textures and out of focus areas better than either the 60mm macro or the 45-175, but I obviously could not test this. What I did expect to see was a difference in the way backgrounds were rendered, with the longer focal lengths I typically use with the 45-175 throwing the backgrounds more out of focus, but looking at the test shots nothing of that sort jumped out at me.

Basically, in this test I did not see any evidence of a particular advantage of using a telezoom with/without a close-up lens compared to the macro lens. It looked like any significant benefit of a 50-200 would have to come from better rendering of colours, textures and out of focus areas. But about that I had no evidence to go on.

That was for stills. For focus stacking the 50-200 might be pretty much as good as the 60mm macro, although a lot of the time it would be using a slightly smaller aperture than the f/2.8 that I use with the 60mm macro for focus stacking, and so backgrounds would be slightly less out of focus than with the 60mm macro (possibly counteracted by the effect of using a long focal length) and in some cases shutter speed would be slightly slower or ISO slightly higher.

What was definitely the case was that at 655 grams the 50-200 is (to me) significantly heavier than the 60mm macro at 186 grams.  It is also expensive, at almost £1,500.

To my mind the potential benefits are too much in doubt to outweigh the definite disadvantages, especially as I have no particular problems I need/want to solve with the Olympus 60mm macro. In fact, I rather like using it and am content with the image quality I get from it. I will therefore not be pursuing the option of a Panasonic Leica 50-200.

So for now, I have no additional hardware under consideration.

A more substantial test of the A7sii

 Yesterday I spent two hours in the Church grounds opposite our house testing the A7sii. For the first time this year there were plenty of subjects around. Mind you, I wouldn't necessarily have thought that if I had been using one of my close-up lens setups, because a lot of the subjects were rather small compared to what I found comfortable to deal with using my close-up lens setups.

The session convinced me that I had not been my imagination - I really did seem to be getting stronger focus peaking signals, and easier focusing because of it, including a good degree of success with subjects that were moving around, two of which were rather small and moving a lot.

The post processing was unproblematic. I have a feeling that the raw files may be a bit more malleable than with the A7ii. Looking at the outputs I also get the impression that the A7sii may be picking up more from dark areas that are far from the flash. I did not raise shadows for any of the images beyond what is done by my presets. Nonetheless, I'm not seeing the deep black areas that I dislike.

I captured 529 shots in the session. The 93 that kept (over 17%) are in this album at Flickr. Here are several images from the session.

I don't have much experience yet with the A7sii, so perhaps there are problems I haven't registered yet, but as it stands I expect the A7sii to take the place of the A7ii as my camera of choice for invertebrates.

 

A surprising new camera

 In this post a month ago I mentioned that I was considering two new cameras, a Sony A7r ii and an Olympus E-M1 III. In this post 10 days ago I wrote "So far it is going well enough with the A7ii that I'm not inclined towards the Olympus E-M1 III or the Sony A7rii at this stage. I'm going to put those possibilities on hold for now."  

I've gone further than that now; I really don't think either would work for me. 

As far as the A7rii goes, yes I could use larger apertures to get less diffraction softening than with the tiny apertures I'm using at the moment, but I would want to capture from further away and crop in order to get back the depth of field I had lost by using the larger aperture. With the image quality loss from a big crop (and it would have to be a big crop to make sense), I strongly suspect this approach would simply take me back to roughly where I am with the A7ii. And yes, I could use larger apertures to get better resolution, and put up with the reduced depth of field. But I know I wouldn't be happy with reduced depth of field, and in any case if I want to do this I can do it with the A7ii. All in all, I really don't think the A7rii would be useful for me.

And the Olympus E-M1 III? The potential benefit is with focus stacking for invertebrates using flash, for which none of my current kit is suitable, at least not for my preferred way of working - hand held. The thing is though, I am continuing to get results I like with tiny apertures, and finding that it works with subjects that are moving around, or on foliage that is blowing around in the breezes that are so common here, or grooming, or dealing with prey, or struggling to release themselves from a spider's web, or blowing bubbles ...... in fact anything that involves movement or action of some sort; those are the opportunities that attract me the most, and focus stacking isn't suitable for any of them. The more I think about it, the more I have to conclude that an E-M1 III would not be useful for me.

With my mind released from thinking about those two bits of kit it wandered off to a claim I had heard 8 months ago. It was in this thread that I started in the Photographic Science and Technology forum at dpreview.com. (Scary place, there are some fearsomely knowledgeable people there. It was the first time I had ever dared to post there.) I wanted to know whether, compared to my Sony A7ii, a camera with the same sensor size but fewer, larger pixels might be better for my purposes. This generated a lot of discussion, most of which I didn't understand, with much of it, as far as I could tell, not being relevant to my particular issue. 

However, in this post a fascinating possibility was raised. It was argued about, as to whether it was true or not, but I got the impression that the general idea was agreed by at least one person who is deeply knowledgeable about such things. The claim was (in my translation) that when dealing with an image captured at a low light level, what you can get out of post processing depends on how large the pixels are; the larger the better.

Now, my A7ii has a 24 megapixel sensor. In contrast, the Sony A7sii has half the number of pixels. Despite this, for a given scene and a given aperture and shutter speed (or in my case for a given aperture and flash level), both cameras will capture the same amount of light, and the image quality, the noise particularly, depends on the amount of light captured. 

However, post processing is done on individual pixels. Post processing involves arithmetic using floating point numbers. Each time a calculation is done for a pixel there will be one or more rounding errors. With repeated calculations these rounding errors will accumulate and the result may go further and further away from what it should be. Critically, the extent of this degradation depends on how good the data was that was captured by each pixel. 

Each pixel in the A7sii will capture twice as many photons as a pixel in the A7ii. There will therefore be a better starting point for the calculations, and the degradation will be less.

That at least is what I thought I could infer from those parts of the argument that I thought I understood. As you will gather from the way I phrased the previous sentence, I was by no means certain about this, but it did seem to make sense to me.

The thing is, because I use such small apertures I can't throw enough light on my scenes to get anywhere near using base ISO. I use quite high ISOs. This means that the camera is collecting less light than it would at base ISO, so even though I'm using flash I'm essentially working in a low light situation. And in addition to the low light, I use very strong post processing on my images to try to mitigate the effect of severe diffraction softening, and also to control highlights and bring up dark areas. Strong post processing very likely has a lot of potential for accumulating calculation errors and degrading the images. I couldn't help thinking that with an A7sii I might be able to get better results with the peculiar approach that I'm using. 

There is of course the fact that the A7sii is "only"12 megapixels. However, I know from experience with my bridge cameras that 12 megapixels is plenty for what I do, with my outputs being only 1300 pixels high. I sometimes do substantial cropping, but it isn't extreme and 12 megapixels is enough for the cropping I want to do. And in any case I seem to be cropping rather less with my new, tiny aperture approach with its greater than usual depth of field.

I idly wondered how much an A7sii would cost, so I went over to eBay to have a look. They looked affordable, and there was one with a low-ish shutter count (under 5,000) and less than 3 hours video capture, which is not much for a camera which is generally considered as a video camera rather than a stills camera. For £1,000 it also had an almost unused Sigma MC-11 Canon EF to Sony E mount converter. These are good. I have one. Having another would let me use non-Sony lenses on both the A7ii and the A7sii if I wanted. They cost around £200 new. And there were three batteries, two of which were Sony branded (around £50 each new). The seller had owned it all from new.

I bought it. 

It arrived very quickly, two days ago, and from the way it was packed I would guess that the owner was the type who looks after his kit very carefully (unlike me). 

The sensor was a bit spotty, but not as bad as I expected it to be. One wet clean got rid of most of the spots and I decided to leave it at that  and go and test it in the garden rather than trying to clean it more and risk getting it worse again and getting into a cleaning loop rather than getting out and testing it.

The A7sii has exactly the same body shape and button arrangement as the A7ii. I set it up in the same way as the A7ii. I used the Yongnuo flash setup that I had sorted out for the A7ii, described in the previous post.

There was not much around in the garden so I had a relatively short session of 40 minutes or so, but there was enough to show that the camera appeared to be working properly. I did notice one difference from the A7ii; I seemed to be getting stronger focus peaking signals, which made focusing easier. But perhaps it was just my imagination, seeing what I wanted to see from my new purchase. Anyway, the post processing went fine and the results certainly seemed no worse than with the A7ii. Perhaps they were a bit better, but that could have been my imagination too.

The 34 images ( a rather high 20% keeper rate) I kept from the 174  shots I captured during the session are in this album at Flickr. Here are several of them, including one natural light botanical shot from an interlude when I couldn't find any animals to photograph.

My first impressions, of handling, processing and outputs were promising.

A random walk to a new flash setup, possibly

 My experiments tend to be a bit haphazard. One consequence of this is that it can take me much longer to realise something than it would have had I been more methodical. I know that, but it's just not my style to be buttoned down methodical, carefully pre-planned, thorough records etc. It's the same with my style of shooting. Persistent, yes. Very persistent. But methodical, no. I try to be methodical sometimes, but I get bored with it, and wander off, lose track of the plan, can't be bothered to keep records, forget what happened, don't see the patterns. Bumbling along. Rather inefficient.

There can be an upside to this though Sometimes I wander along from one thing to another, and suddenly realise that I've ended up somewhere rather unexpected, but rather promising. Somewhere that had I been methodical, logical, rational, I probably wouldn't have reached. 

So it was yesterday.

I intended to go over the road to the church grounds to do some aperture variation tests as mentioned at the end of the previous post.  However, it kept raining on and off, which put me off. So I decided to do some indoor testing. That way I could be more, yes, methodical, and should be able to home in quickly on good apertures for different magnifications. I did need a suitable subject though. None of the man-made things I tried had the appropriate type and level of detail. But I searched the window sills and found a dead wasp, about 10mm long. That would do nicely, it's eyes in particular.

Until it's head fell off.

But even headless it was useful. Until it fell apart even more. But I did do some comparisons. I couldn't do enough to be sure about it, but to my surprise the comparisons I did manage to do seemed to support the continued use of the constant f/45 that I had happened upon by .... by accident? Out of laziness? Through some sort of unconscious evaluation? Who knows. Anyway, I should look at this more thoroughly when I can find another suitable subject. Should. Will? Maybe. We'll see.

Anyway, With the very small apertures I'm using I'm finding it difficult to get enough light on to the subject, especially when testing even smaller apertures than the f/45 I've been using recently. One thing that can lead to over-optimistic results is putting the subject on a somewhat reflective surface that can bounce light on to the subject, especially its underside. To try to avoid this I'll sometimes put the subject right on the edge of the table, bookshelf or whatever. A related problem can arise when the subject's surrounding are much lighter than they might be out in the field. To try to avoid dropping into that trap I put subjects on to dark (and not very reflective) doormats. I have several of these spread around in my study to try to protect the light coloured carpet from the muck that I bring in from the garden on my shoes. 

The doormat I used for the dark surroundings tests was in a position where it wasn't getting much light. That made it impossible to see enough to focus properly. The obvious solution would have been to move the mat into the light. But (fortunately) that never entered my mind. Instead the obvious thing seemed to be to turn on the KX800's focusing light. I realised that the KX800 I was using (I have three) was the one with a faulty focusing light which only comes on for a moment every second or so. It is unusable. (The flash itself is fine. It is the one I have been using for a while now. Just not the focusing light.)

One of the other two KX800s has a faulty arm. The KX800s have loc-line (or similar) arms. One of the arms on this one has broken several times, in different places, and each time I have superglued the broken ball and socket joint together. It is now rather inflexible and I wasn't keen on trying to bend the arms into the shape needed for the very long lens setup I'm using on the A7ii. So, I used the third KX800.

The focusing light started acting up. Not as bad as the first one, probably still usable, but I didn't like the way it was going. I decided to try a different direction.

I have a Yongnuo YN24EX twin flash. Like the Canon twin flashes, it can be mounted on the front of the lens, which gets the flash heads near to the subject. That had been sitting on the shelf for many months because although the power of the flash was fine given its front mounting, despite numerous attempts I couldn't arrange diffusion for it which got me illumination I liked. This was despite help from John Kimbler (@dalantech), who uses this type of flash setup to very good effect. However, I could live with not so good illumination for this exercise; it was how detail came out at different apertures that I was concerned with, and the YN24EX has focusing LEDs built in to each flash head. 

The simple, quick and straightforward thing would have been to mount the YN24EX flash heads on the front of the lens as I had done previously. But I looked at the pathetic little two-polystyrene-layer diffusers that were still on the flash heads as the last thing I had tried before giving up on the YN24EX, and I decided on a whim to try something else.

I remembered that I had bought a flash bracket with bendy arms like this one. 

The arms on the one I bought are very stiff indeed. Also, the configuration of the base and the arms made it uncomfortable to use hand-held. It was not an appealing piece of kit. I had not found a use for it. For some reason I decided to try using it with the YN24EX. 

I reused the diffusers from the KX800, and after an hour or so of testing and adjustments, the setup ended up looking like this.

The piglets illustrate the working distances. From 8:1 down to 2:1 or so the working distances are similar enough that it is easy to arrange the flash heads so they can provide satisfactory illumination without needing to adjust the position of the heads. However, as you approach 1:1 the working distance gets bigger faster, getting to around 350mm by 1:1, at which point the working distance is increasing very fast indeed. This makes flash illumination difficult for low magnifications, partly because the flash heads are pointing in the wrong direction (or if they are pointing in the right direction they are pointing in the wrong direction for higher magnifications). And even if they are pointing in the right direction the illumination level decreases with distance according to the inverse square law, and 1:1 is a lot further away than 2:1. Given that I am using very small apertures, this makes it difficult to provide enough light at lower magnifications. This is a general problem with this double teleconverter setup, not specific to the YN24EX.

What surprised me was that it seemed, at least from indoor testing, that this setup could actually provide (just) enough illumination out at 1:1, and enough light to aid focusing too. In dark scene tests I had to let the ISO go up beyond the ISO 3200 that is equivalent to what I have been using with close-up lenses for years, but not by much, and only right out at 1:1. It only needed a quite small increase in magnification to get it back to ISO 3200.

There were also a couple of hints that the quality of illumination might be slightly better than with the KX800. This seems unlikely, given that I was re-using the KX800 diffusion arrangement. However, more by accident than design, the outer diffuser was mounted further away from the flash head boxes than with the KX800, and previous tests indicated that greater inter-layer distances increase the diffusion effect. 

Here is what it looks like from the subject's perspective.

Desktop tests are one thing. How things work in the field can be quite another, so earlier today I went across to the church grounds to see if I could find anything to give the setup a reality test. It did ok.

• The flash bracket was not as uncomfortable to use as I thought it might be. I think that is because of the length of the lens setup and the fact that I keep my left hand at the front of the barrel, on the focus/magnification ring. With the left hand bendy arm running up close to the lens barrel, and my hand out at the front, the bendy arm didn't get in the way.
• The flash provided enough illumination for me to run the flash at 1/4 power as usual to keep recycle times fast enough for my shooting rate without having to raise the ISO so far as to be troublesome.
• The quality of the illumination was no worse than with the KX800, and might possibly have been just a little better with reflective surfaces. 
• The way the outer diffuser was mounted, a bit higher than with the KX800, left just enough of a gap underneath which I could look through to help with lining up subjects.
• I didn't need to use them, but it was nice to know I had focusing lights available should I need them.
• If I really wanted to, I could alter the direction the flash heads were pointing because there are flexible connectors at the ends of the bendy arms. This is not possible with the KX800 because the flash heads are in a fixed position at the end of its bendy arms. In order to rotate the flash heads you have to manipulate the arms, which moves the flash head to a different location. It can be tricky to get the flash head back to the right place but pointing in a different direction.  

I captured 285 shots during the test session and ended up keeping 60 of them, a keeper rate of 21%. Given that it was distinctly breezy, and that the majority of the subjects were fairly small, that seemed very satisfactory. 

Here are some of the images from the session. The rest are in this album at Flickr.

Using tiny apertures - Part 8, So what f-number to use?

 

In this post I wrote "Where next with invertebrates?  More of the same I think with the A7ii, with some f-number variation. I'll try to go to one of the local nature reserves to see if I can find some more varied subjects and shooting situations."

Well, I went to one of the local nature reserves a couple of days ago. It was the first time in almost two years that I had been to any of the reserves and lovely it was to be there again. However, I didn't do much by way of varying the f-number. It turned out that I really didn't want to waste the opportunity that I had been denied for so long, and I was afraid that varying the f-number would risk losing shots. I knew that using f/45 the whole time would probably work well enough, even if it wasn't optimal, and so that is what I ended up doing. I did use f/64 a handful of times late on in the session, but that was not enough to draw any conclusions from.

The session reassured me that the A7ii setup was usable out in field, for example crawling around in the midst of long grass which was moving around in the breeze and which was laced with brambles, dead branches etc that transmitted my movements elsewhere, including to where the subject was. This was normal working conditions, and the setup worked with a similar balance of success and frustration as my previous setups.

When I did the selection and processing for the session I was reassured that the A7ii setup gave me at least as good a success rate as my previous setups. It also made it easy for me to get some sequences of a type that I only rarely got with my previous setups, such as the one below. 

With my previous setups I would have probably needed to stop and change the close-up lens somewhere between the first and the last image, and every time I wanted to cross that boundary again. With the A7ii setup I could move freely back and forth right across the range of framings/magnifications I wanted to use. The same would apply if I needed to switch between a relatively large subject such as this one to a very small one that turned up, or from very small to much larger. You need to be quick with such switches sometimes in order not to lose the opportunity, which may only last a few seconds. 

This is definitely advantageous.

What I did notice though was that with fairly large subjects such as this the depth of field I was getting with the lower magnification shots such as the first of these didn't look much, or possibly any, greater than with my previous setups. For a side-on shot like the first one above that might not matter too much, but where the subject is at an angle like the next one I would definitely like more in focus.

At the other end of the magnification scale, looking at an image such as the one below I wondered whether the f/45 I was using was forcing the effective aperture so small that I was losing too much detail, even for my fairly lax standards in that regard. Mind you, if that was the case, then this might be a lose-lose situation, because opening up the aperture would reduce the depth of field, and that was already so thin at f/45 that I had severe doubts as to whether I really wanted to keep this image. This might be the sort of case where the only solution is a low image count stack with a larger aperture, of the sort that I believe Thomas Shahan uses some of the time. I could try that, although I don't know if I'm good enough to make it work. I suspect not.

This got me thinking about f-numbers again. 

As described in this post, I am now confident that I can work out what effective f-numbers I am using with the A7ii double 2X teleconverter setup. The table below shows how the constant f/45 I used for recent sessions pans out in terms of effective f-number.

As shown in the table, I previously thought that as the magnification with the A7ii setup at f/45 ranged from 1X to 8X, the effective aperture ranged from f/90 to f/405. As shown by the bottom row of the table, my close-up lens setups all max out at f/45 full frame equivalent, whatever the magnification. 

Since depth of field roughly doubles for each two stops reduction in aperture, this would mean that at 1:1 the A7ii setup would have twice the depth of field of the close-up lens setups, and this disparity would increase all the way to 8:1, by which point the A7ii setup would have over 8 times the depth of field of the close-up lens setups.

It didn't feel like I was getting this much of an increase in depth of field, and this feeling was reinforced when I looked at the images from the latest session.

The comparison with the correct method values in the table shows that this feeling was entirely justified. Using the correct method of calculating the numbers, at 1:1 with f/45 the A7ii setup has only slightly larger depth of field than the close-up lens setups. It gets to around twice the depth of field at 4:1. By 8:1 it is still less than 3 times the depth of field according to my back of the envelope calculations.

I decided to try to work out what f-number I would need to set at various magnifications on the A7ii setup to get depth of field that was a bit more than twice what I could get with my close-up lens setups. 

I decided to work this out to match the markings on the Laowa 100mm lens barrel so as to make it practical to make the appropriate adjustments out in the field. The Laowa 100mm is marked with the following magnifications: 1:4, 1:2, 0.75:1, 1:1, 1.5:1 and 2:1.

I decided to add intermediate values between 1:1 and 1.5:1, and between 1.5:1 and 2:1. These are the numbers I came up with.

I intend to try using these f-numbers on my next outing. 

It is likely to be a bit hit and miss because sometimes things happen fast enough that I need to keep concentrating on focusing and framing rather than making other adjustments. However, hit and miss is ok, or can be. With plenty of repetition, if there is a strong enough signal then it will come through the "noise" caused by imperfect execution. 

Using tiny apertures - Part 7, the apertures are definitely not so tiny

 In this post I mentioned the uncertainty over what f-numbers I am actually using ("effective apertures") when I am setting tiny apertures (large f-numbers) on the camera/lens.

It seems that my modified formula is giving the correct answers. I'll explain here in terms that I can understand. If you want to know more about the technical/optical factors in play please refer to this thread at dpreview.com, particularly the posts by alanr0.

I have been testing the formula 

effective f-number = nominal f-number * 

                 ( 1 + magnification/ (pupil magnification * teleconverter power )  )

I have been using it with the assumption that pupil magnification is 1 (because I don't know what it actually is, and 1 is a sensible value to assume and seems to work fine). So with this assumption the formula becomes 

effective f-number = nominal f-number * 

                 ( 1 + magnification/ teleconverter power  )

However, I have been using magnification so as to include both the magnification provided by the lens and also the magnification provided by the teleconverters. So 

magnification = Lens magnification * Teleconverter power

This means that magnification/teleconverter power can be written as 

Lens magnification * Teleconverter power / Teleconverter power, 

which is simply Lens magnification.

This means that my formula can be written as

effective f-number = nominal f-number * ( 1 + Lens magnification )

This is true only if nominal f-number is the f-number set on the lens adjusted to take account of any teleconversion being used. So for example minimum aperture of f/2.8 when using a lens without a teleconverter becomes minimum aperture of f/5.6 when a 2X teleconverter is used. This is how my setups report f-numbers when I'm using teleconverters.

This formula gives exactly the same answers as the modified formula I used in the previous post, and these answers match the practical results of the test exercises I have been doing.

This means that, for example, when I use f/45 with 8X magnification, using a pair of 2X teleconverters, the effective aperture is around f/135 rather than around f/400 as I previously thought.  

New kit - Collecting evidence Part 1 - Invertebrates

 In this post I mentioned that I was considering three new pieces of kit: a Sony A7rii, an Olympus OMD E M1 III and a Panasonic Leica 50-200. I came to two conclusions:

• For invertebrates, I need to do some practical comparisons in contexts that are more typical than I've tested before, switching between the A7ii and one of my close-up lens setups. If the success rate with the A7ii turns out to be extremely low for my typical subject matter compared to the close-up lens setups then it doesn't matter how much better the images are; potentially nice images that you can't actually capture are not much use!
• For flowers etc I need to do some practical comparisons between the 60mm macro and the 45-175. 

I'll talk about invertebrates in this post. I have done a flower exercise that I'll talk about separately.

I have now had my first session this year with invertebrates, spending two hours in the church grounds opposite our house. Rather than trying to do like for like comparisons between the A7ii setup and one of my close-up lens setups I just used the A7ii. 

I captured around 550 images and ended up keeping 92 of them, which are in this album at Flickr. That was a keeper rate of around 17%, which is quite high for my stuff. That is a bit misleading though because there were two very long sequences, one sequence of 14 images of a woodlouse wandering around and the other, much longer (52 images) of a fly grooming (or possibly flies - not sure it was all the same fly). The woodlouse shots were difficult, so I would expect a low keeper rate for them, but the fly grooming shots were easy, as the fly stayed in the same place for extended periods as I captured shot after shot, so I would expect a higher keeper rate for them. 

Taking the two long sequences out of the equation gives 26 keepers from 228 shots, a keeper rate of around 11%. Keeper rates can vary a lot from session to session, but 11% is comparable with what I would often get with my close-up lens setups. It provides no suggestion of the low keeper rates that I was fearing for my normal subject matter. 

I had also been concerned about whether the flash would be powerful enough for larger subjects. That is because of the need to throw extra light on the scene because of the very small apertures I'm using with the A7ii, coupled with the longer working distances for larger subjects captured at lower magnifications. There were two potential issues with longer working distances: because of the differences in working distance between the typically 1X or so and 8X magnifications that I would be using, it seemed highly likely that the flash heads would need adjustment to keep them pointing in a suitable direction; and there would be severe  (inverse square) drop-off in illumination hitting the subject as the working distance increased. 

As it turned out, that was not a problem. One reason for this was that I was more relaxed about letting the ISO go higher than I had previously been comfortable with. It struck me that ISO 3200 on the A7ii is very similar to base ISO of 100 with my bridge camera close-up lens setups or the ISO 800 that I typically used with my micro four thirds setups. So up to ISO 3200 with the A7ii I would be no worse off than previously. As it turned out, for the 92 keepers only 3 of them were higher than ISO 3200 (ISO 5000), and 70 of the them were lower than ISO 3200.

Another concern had been whether the flash diffusers would get in the way more than with the close-up lens setups because they were mounted a bit further forward so as to try to reduce the illumination drop-off. This too turned out to be unproblematic. 

A recurring issue turned up, again - very ugly reflections of the flashes on reflective surfaces. This has been troubling me for years; I have never found an arrangement that would rid me of this. So, I have learnt to live with it. However, I suspected it was a bit worse with the A7ii setup. I noticed that the outer diffusion layer was very battered and had developed folds which made it impossible to keep it at a nice distance from the flash heads across the whole width of the setup. I wondered whether that was causing a problem so I made a new front layer, which might or might not make a difference. 

With the new front layer in place I recalibrated the setup for colour cast. I captured an image of the grey panel on a ColorChecker Passport, loaded that into PhotoLab and used the eye-dropper to set the white balance. I then put those values for Temp and Tint into the PhotoLab preset I use for flash-based A7ii invertebrate captures.

Given that I seem incapable of providing illumination which removes the flash reflection issue I've resigned myself to making greater use of cloning to mitigate the problem. It often isn't practical to completely remove the reflections, but they can be much improved by using a mixture of full opacity cloning where practical (problem disappears if done well, needed particularly where an area is completely blown out, which unfortunately does happen sometimes) and low opacity cloning where full opacity cloning isn't practical (so as to tone down the highlights with a light wash of a suitable colour). The wasp at images 10 to 13 of the session's Flickr album are examples of the sort of subject for which this is a problem, and on which I used cloning, low opacity cloning in this case.

Given the "Great shot" culture on photography forums one needs to be careful about interpreting the response to one's photos, but I posted some of the images on three sites and the responses are consistent with my view of the images, that they seem very acceptable to my eye. Particularly telling for me are the references to good detail and large depth of field. There is of course no fine detail in the images because of the diffraction from the tiny apertures, but the level of detail looks no worse to me than with my close-up lens setups, and it might be a bit better in some cases, particularly for the higher magnification shots. As to depth of field, this looks significantly better to me at the higher magnifications. 

As an experiment I had decided to use a fixed f-number for the session, namely f/45. The effective f-number therefore varied as I changed magnification, getting larger as the magnification increased. This seemed to work well enough at the higher magnifications. At the lowest magnifications it was difficult to convince myself there was much if any significant increase in depth of field compared to my close-up lens setups. On my next outing I will try varying the f-number for some lower magnification shots so as to see if I can increase the depth of field while retaining adequate detail. 

One issue that was surprisingly problematic was dust spots on the sensor. I had put a lot of effort into cleaning the sensor and had got it down to just two spots. However, even though I had not since then opened the camera up there were half a dozen or more additional spots to deal with. I think there must be dust in the camera that gets moved around as I used it. I have even noticed extra dust spots appear on the sensor between successive test shots during the course of sensor cleaning. (I take three shots after each cleaning attempt, with the camera kept in the same position for each shot. It is between two of three such shots that I have seen an additional spot appear.)

I have done what I can to clean the sensor, again, but this can be frustrating, with spots increasing as well as reducing in number between cleaning attempts. I have resigned myself to just getting the number of spots down to only a handful and living with that. (I don't have much choice.) One thing that has made this much less aggravating and time-consuming is my discovery of "spot visualisation" in Lightroom. It makes dealing with spots far easier and faster, with a much lower probability of missing spots, which can be really annoying as, depending on when I notice them, dealing with them can mean backtracking not just through elements of the processing workflow but, more time-consuming, also updating the primary image set on my PC, two backup sets (one on the PC and one in the cloud) and a Flickr album. 

Where next with invertebrates?

More of the same I think with the A7ii, with some f-number variation. I'll try to go to one of the local nature reserves to see if I can find some more varied subjects and shooting situations.

So far it is going well enough with the A7ii that I'm not inclined towards the Olympus E-M1 III or the Sony A7rii at this stage. I'm going to put those possibilities on hold for now.

Here are a few of the images from the latest session.

I didn't achieve close-in shots like these three with my close-up lens setups. Partly that may be because I've only recently got more into this sort of shot, but mainly I think because I probably couldn't achieve this sort of result with my close-up lens setups because of not being able to get sufficient depth of field for my taste.

I'm also having more success with small subjects than with my close-up lens setups. The next one was the only small subject I found during the session, and I didn't actually realise it was a subject. I did a quick shot and then moved on because at a cursory glance I thought it was just a piece of detritus on the leaf. I suspect this was  actually based on my impression/assumption before taking the shot rather than really looking at the image I had captured. Especially given the single and rather inattentive attempt, I'm happy enough with the outcome.

On the other hand, these whole body shots of medium-sized subjects look rather similar to me to what I got with my close-up lens setups. These are the sort of shots for which I intend to try out smaller apertures.