Back Focussing and Optical Low Pass Filters

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Mark Forman
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Back Focussing and Optical Low Pass Filters
6 months ago

Back Focussing and Optical Low Pass Filters

I just got off the phone with one of the engineers with Sony who is responsible for digital cinema cameras.

The topic was why adaptors may or may not work with the Alpha 7R.

It comes down to whether the adaptor was designed for the use with or without a low pass filter. When a low pass filter is introduced the optical path is interrupted and the back focus point changes usually in microns. The result is when setting a lens to infinity it will not achieve perfect focus. To correct this condition all lenses must be adjusted to compensate if a correctly made adaptor cannot be used.

It is possible that Leica lenses may suffer this condition when used on the Alpha 7R and it is also possible in my tests with Canon lenses and the Metabones III this has also occurred.

It also means that any adaptors designed for all E mount cameras other than the Alpha 7R will be slightly off when used on the Alpha 7R.

Food for thought.

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Mark Forman
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