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Infrared backfocus

I’m still sorting out which lenses work well and which ones don’t on my converted Canon EOS 5D. As noted in an earlier entry, I had it converted for pure infrared use by maxmax.com.

Although MaxMax adjusts their converted cameras for infrared focusing, there is enough lens-to-lens variation (between different lens models eg 50mm vs 85mm) that this adjustment might not be very helpful in many cases. I will be compiling a table of lenses and how they perform with infrared on both the 5D and D200 (no converted D200 just as yet however).

Below is an example of the focusing error with the Canon EF 50mm/f1.4 at f1.4, taken using a custom target I built this evening with stepped bars 5mm apart (front to back):

Target* Four (4) centimeters behind target

There is about a 40mm focusing error at a distance of around 3-4 feet. Stopping down at least 3 stops would be needed to render crisp results at the chosen focus area. This is why I shoot much of my infrared work at f11—to minimize the number of ruined shots.

I will be exploring this issue in great detail going forward, as well as double-checking for accurate visible light focus with non-modified camera bodies.


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