Several rules of thumb

There are a couple of terms that are going to be important in some upcoming posts about photographing insects and other invertebrates: equivalent f-number and effective f-number. This can get very complicated, especially equivalence, but I'm just going to use a couple of rules of thumb relevant to the particular options I'm considering.

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I am considering setups involving cameras with three sensor sizes: 

• 1/2.3", around the size of sensor used by many smartphone cameras
• micro four thirds
• full frame

For insects etc I use f/8 (minimum aperture) almost all the time with my 1/2.3" sensor cameras. In order to get around the same depth of field with a micro four thirds setup I have to use f/22, and with a full frame camera I have to use f/45. These are the equivalent f-numbers that matter most to me. 

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Effective f-number needs a bit more explanation. (This explanation does not apply to Nikon kit, but I don't use any Nikon kit.)

When you photograph a small scene using a macro lens, extension tubes and/or a reversed lens or bellows, the f-number you are actually using (known as the "effective f-number") is smaller than the one you set on the camera (known as the "nominal f-number"). 

The effective f-number can be calculated using the following formula. (This formula is an approximation, but it works pretty well in my experience).

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

So for example if I set the camera/lens to f/11 and I shoot at 1:2 (so the magnification is 1/2), the effective f-number is 

    f/ (11 * ( 1 + 0.5) ), which is around f/16.

If I set the camera/lens to f/32 and shoot at 3:1, the effective f-number is

    f/ ( 32 * ( 1 + 3 ) ), which is around f/128  

When it comes to depth of field, it is the effective f-number that matters.

I often use setups which have a close-up lens mounted on a telezoom lens. These include the zoom lens on a bridge camera and a 45-175mm lens on micro four thirds. 

The above calculations do not apply when using close-up lenses on telezoom lenses. In that case the effective f-number remains unchanged, irrespective of the magnification.

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In the upcoming posts about invertebrates I will be comparing close-up lens kit on bridge and micro four thirds cameras with macro lens kit on full frame. There is a nice simplification for these comparisons:

• For the close-up lens setups I only need to consider equivalent f-numbers, because the effective f-numbers don't change with magnification.
• For the full frame macro setups I only need to consider effective f-numbers. This is because I will be using full frame equivalent in the comparisons, so the full frame options don't need any equivalence adjustment.

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Here are a couple of other rules of thumb that I'll be using.

• Depth of field roughly doubles with each two stops reduction in aperture.
  
  
  
• ISO 100 with my bridge cameras has roughly the same amount of noise as ISO 800 with my micro four thirds cameras and ISO 3200 with my full frame camera.