Infrared Filters = Light Loss or IQ Loss?

[Adam Reuter]

Hey guys (and gals, although I don't think I've seen a female name on this board ever),

With all the talk about using an infrared filter on the EX1/EX3 to take care of the burgundy color issues on dark suits/clothing I never read anything about light sensitivity loss. Do these filters cause any?

Also, what's the best one to get so that the EX1's awesome optics aren't affected too much by it?

[Les Nagy]

A filter that stops light is going to stop light. Sorry, had to say it :P

The interference type filters such as the 486 from B+W or the 750 True-cut filter stop a certain set of frequencies or colours of light from passing through and instead reflect them back. The fact that the EX series is so sensitive to some of the infrared spectrum and that gets through as IR contamination makes the cameras see more light. Blocking that light does lessen the sensitivity of the camera, but only in that colour range where the filter stops the light. There is a very small loss of light though whenever light passes through another layer of glass. With the exception of the desired effect of removing unwanted IR and the slight loss of light that happens when light passes through the glass, these filters can be considered lossless for all practical purposes.

The other types of filters are dye based. They stop light by absorbing it. Whereas an interference filter can be very selective on what colours it rejects, a dye based filter affects much more of a colour range than just the ones not wanted. So not only does a dye based filter kill the unwanted IR and has the small light loss from just the glass part, it also attenuates light far away from the undesired IR and has the effect of losing a stop or two of light. So a dye based filter loses a lot of light. They can also cause colour balance issues if they are not designed properly and will need their own white balance.

As far as what one is the least degrading optically, it depends on conditions. If you need all the light the camera can get, then the interference ones will be the best choice if you don't face the problem of wide angle shots and the filter not being effective or even wrong at the edges of those shots. If you are out in bright daylight and/or under bright lighting and can afford to lose light or even desire it, then perhaps a dye based filter is best. It will kill light and allow the use of wider apertures for less depth of field. The actual quality of the manufacturer is usually a point of debate but you can't go wrong with the quality supplied by B+W for the 486 IR filter, nor can you go wrong with Schneider Optics for their filters especially once they get the optimum design in place for the EX series. Dye based filters should be purchased from the big names and have multi-coatings to prevent flare.