Sunday 9 May 2021

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. 







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