Sunday, August 15, 2021

M3 Landscape Bracketing Script: Where the hell am I?

If you read my blog then you are likely one of those photographers that understands focus distance is accessible in the Canon EXIF data and if you have Magic Lantern or CHDK then you can use this in realtime in a Lua script.

Great: but here's the catch, Canon gives you two focus distances, conveniently (:-)) called Upper and Lower.

Graphically (many thanks to @c-joerg on the CHDK forum) the data looks like this (on the 11-22mm EF-M lens):

Two things are obvious. The first is the quantised nature of the focus feedback. Secondly, that the upper and lower distances lose their 'integrity' as you approach infinity, ie they are more consistent towards the macro end.

Up until now my M3 Landscape Bracketing Script has used the lower focus distance, as I'm primarily interested in contiguous focus bracketing from near to infinity. By using the lower focus distance I'm estimating depths of field on the conservative side: thus minimising focus gaps, albeit at the cost of an extra few brackets.

However, there may be those that wish to use the upper focus distance value. In addition, there may be a few who wish to use some value between the lower and upper values.

To account for such needs I've added a menu item that allows the user to select from three focus distance options:

  • Lower
  • Upper
  • Harmonic Mean

In addition, you can now register any lens, ie not just EF-M lenses. For example, I've pre-registered the Canon 24mm Mk II TSE lens into the script. As usual, when registering a lens, enter the magnification, minimum focus distance and longest focal length (if a zoom), plus, of course, the lens name, which the script can get for you.

One caveat is that some third party AF lenses may not work with the script.

Finally, if you really wish to fine tune the lens information, you can also, as before, add in a 'guess’ at the pupil magnification, ie just eyeball the lens, or measure it. If you don't wish to guess it, then pop over to photonstophotos optical bench for some useful lens info, including pupil magnification (many thanks Bill ;-))

The general rule is to use a pupil mag of unity for a telephoto lens and guess the pupil magnification for a retro focus lens.

The latest version of the M3 LBS can be downloaded from the right hand link.

As usual I welcome any feedback on this post or any of my posts.

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