IR block filters on AutoFocus Cells and TTL Flash Metering Sensors

["Rolland Elliott" <rolland_elliott@xxxxxxxxxxx>]

Just to let you know why Cameras can't autofocus IR light very well and why 
TTL flash pictures are not always spot on with IR photography.

I disassembled my Nikon N90s camera (again) and removed the bottom mirror 
box portion which houses the AF and TTL sensors. The AF sensor has a filter 
(about 6x9mm) which is coated to reflect IR light. It is a clear filter and 
the coating looks somewhat irridescent. When viewed with an IR camcorder the 
filter looks like a silver color mirror to IR light.  The TTL sensor has a 
blueish green IR block filter (about 2x3mm) over it to block IR light. It is 
not a very strong blocker of IR light, but I'm guessing it would cause a 1 
to two stop reduction in IR light reaching the sensors.

I removed both of them and replaced them with a 25 Red gelatin.  If Nikon 
uses filters to block IR light from reaching the AF sensors and TTL flash 
sensors it is a safe bet that many other camera manufacturers do also.

I bet many cameras could be made infrared compatible by just removing a few 
IR block filters like I have done.

Peace, Rolland



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