IR Conversions and White Balance.

Getting your camera converted can be a intimidating prospect.  Unlike many things you can experiment with in photography it alters your camera in a way that makes it no longer capable of functioning as the factory intended.  This can be especially scary when you have only one 'good' camera or when you are looking at purchasing a new one and have to decide if you will get it converted or not.

Luckily there is a way to get your 'normal' functionality back.  There is nothing special about the hot mirror inside your camera, it is just a UV/IR cut filter, thus it is possible to get an external cut filter and put it on front of the lens instead.  There is, however a catch.  Well, two if you want to count the difficulty in using filters in fisheye lenses.

The catch is that external UV/IR cut filters are not going to exactly match what the camera manufacturer put in the camera.    The hot mirror inside most cameras is both  UV/IR cut filter AND a colour correction filter.   Even worse, many hot mirrors are intended to be used to further reduce UV/IR contamination on a normal camera, and just like, say, a blue filter doesn't truly remove all red and green from a scene, these filters do not remove as much UV/IR as the camera is expecting.

So I decided to sit down and compare 3 different hot mirrors.  There are many more out there, but these are the ones I happened to physically have on hand.

The first is the Tiffen Hot Mirror.  The Tiffen Hot Mirror is not really designed as a full scale UV/IR cut filter, it is more intended to reduce the UV/IR pollution entering a traditional picture, thus reduce haze and such. The transmission chart shows it gets kinda lazy in the IR range.

The second is the B+W 486 UV/IR Cut Filter.  The B+W filter is designed as a full fledged cut filter to be placed on cameras that do not otherwise have a hot mirror.  B+W does a lot of work with things like machine vision and has a wide range of filters designed for use on cameras that are not exactly consumer stuff.  However, it is intended to simply cut out UV/IR, not duplicate the hot mirror inside a camera.

Finally we have the Max Max CC1 filter.  Unlike the other two this one is designed to be both a UV/IR cut filter and a colour correction one, thus acting as a replacement for the original hot mirror.   While the other two are clear to the human eye, the CC1 has a distinct blue tint to it, just like the built in hot mirror.

Now, this brings up two questions... which one actually performs the best, and how much does it actually matter?

First let us start with the laziest test, full auto mode.. well, apature priority, but close enough.  In this case I let the camera pick the shutter speed (to check metering) and, more importantly, using the auto white balance.




In the first panel we can see how the camera reacts to no filter being present.  The poor thing has no idea what to do with the data it is getting and IR dominates the image.  The Tiffen and B+W filters do better (with the B+W edging the Tiffen out) but still produce images heavy in reds and, while not quite 'unnatural' looking, do not really look like the particular scene.  The CC1 on the other hand behaves pretty much as what you would expect the camera to do.

This, however, is really only a test of what the camera can do automatically.  For the second test I used a BRNO Balens white balance cap to provide the camera with a custom white balance setting, then retook each shot getting this:



While a discussion about abuse of white balance is beyond the scope of this piece, I should briefly explain that the Canon (as well as many other) firmware has the ability to take a sample image and use that to calibrate the white balance of subsequent images.  In Canon's case it uses the center of the image, which is why the  Balens works so well.  

Anyway.... it should be noted the pictures were taken at two different times of day, so comparing the sets directly to each other doesn't quite work.  Here while it can be argued that the CC1 is still producing the most accurate colour representation, the Tiffen and B+W are not that far off, which suggests that the last of the difference could easily be corrected in post processing.

So my final take?  The camera's built in image processing depends on a particular colour correction filter being in place, so if you want the body to react just as it would out of the factory and utilize all the automatic features it has, the CC1 wins hands down.  However, the only real difference between the CC1 and B+W is the colour correction, which if one is comfortable doing in post processing renders the filters functionally equivalent.  The Tiffen, on the other hand, doesn't really stack up.. and any of the 3 gets better results then the rather IR contaminated 'no filter' situation.

I hope this was useful.  As always one can go to Infrared Club's External Links journal entry for a variety of followup information.

Next time.. comparing a variety of filters for use in broad spectrum or multispectral imaging.