From: "Rolland Elliott" To: infrared@a1.nl Subject: IR block filters on AutoFocus Cells and TTL Flash Metering Sensors Date sent: Mon, 30 Oct 2000 09:24:22 EST Send reply to: infrared@a1.nl 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