Question about losing autofocus without an IR filter

KingRoach · Joined: 21 Dec 2013 Posts: 87 Location: United Kingdom

Hello.

This is a question about the theory of losing autofocus without an IR filter. I would like to distinguish between SLR cameras on the one hand, where the autofocusing chip is separate from the sensor, and the compact and mirrorless cameras where the focusing pixels are on the sensor itself.

I often hear about losing autofocus when removing the IR filter (hot mirror) altogether, and how it is something that can only be remedied by filling the space with a replacement clear glass (or whatever mod you want) filter of the same thickness. I read somewhere that the reason for this is because of the diffractive quality of the medium, so that light bends or breaks just as it was supposed to by design.

This kind of theory makes perfect sense when you think of autofocus as something which is designed to work in sync between three different points in an SLR: the sensor, the focusing screen on top of the mirror, and the actual focusing chip which is normally nearer the bottom of the camera under the mirror. The camera design makes it so that the chip focuses the image at the right distance for both the focusing screen (view finder) and the sensor. If ONLY the sensor design is changed by removing the filter, the chip would still focus the right image for the unchanged focusing screen, so you see the right image in the view finder, but the image on the sensor is unfocused.

This is how I personally think of it, however, I could be wrong so please tell me if I am. Does the camera LOSE autofocus completel (and why) or is it just as I described it, where it autofocuses but captures a defocused image?

Because this takes me to the second part of my theory: IF the above is true, shouldn't a pocket camera or a mirrorless camera which has its focusing pixesl (phase detection?) built in the sensor find the right focus no matter what filter is present or absent in front of it?

I'm confused because some pages desribe retaining autofocus on an IR modded camera by removing only the coating (not the filter) from a POCKET camera!

If anybody has any knowledge on this please share. I am considering to mod my EOS-M as well as any pocket camera I can lay a hand on for IR photography, and would like to do it without having to buy a very expensive replacement filter. If the above is correct, I would presume the EOS-M and other mirrorless interchangeable lenses systems (NEX, M4/3, etc.) and pocket cameras should not lose autofocus, but apparently this is not what actually happens?

Thanks in advance.