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The effective aperture

Written by Administrator. Posted in Contents

Have you ever seen one of your high magnification shots and thought , "there is no detail at all"? I have

The day I realized it was when I took this picture

It was taken with a EOS 40D, a EF 100/2.8 macro USM and a raynox MSN-202; apart from that a full set of extension tubes (65mm) and a 1.4X Kenko teleconverter; 10X magnification on sensor with no detail at all

This is the day I discovered what "effective aperture" meant and how it affects final image quality. The shot was taken at f32 but effective aperture was around f100 (correct me if I am wrong); if you have read the article about diffraction you then know that this is quite insane.

A few days later I sold the 1.4X TC and started to take my first focus stacks

Some people still think that the aperture shown in the camera is the real one (nikon bodies correct effective aperture, at least on automatic macro lenses)

There is a very simple formula: fe=fn(Mag+1) which means effective aperture=nominal aperture * (Magnification +1)

If we take as an example a Canon MP-E 65 with aperture closed to f8 the real effective aperture values would be as follows

  • at 1X would become f16 8(1+1)
  • at 2X would become f24 8(2+1)
  • at 3X would become f32 8(3+1)
  • at 4X would become f40 8(4+1)
  • at 5X would become f48 8(5+1)

By using the focus stacking technique we can avoid diffraction effects (to some extent); for example: If we take a single shot with the MPE at 5X and at f3.5 we would get a very thin depth of field (most of the image would be a colourful soup)

By stacking we can get a huge depth of field (in macro terms) only limited by the number of shots and still be working with and effective aperture of f21, quite reasonable for macro work (at least on a full frame cameras)

Most extreme macro photographers try to work with the widest aperture possible (some lenses benefit from closing 1-2 stops); with microscope lenses that is wide open (no Iris)

As an example, I try to keep effective aperture around a maximum of f21 up to 20X work (in the studio), f32 for up to 40X getting a maximum of effective aperture of f60 at 100X (with a f0.6 lens)