Seeing in Infrared. Sort of.

March 17, 2010 · Leave a Comment 

The Glasgow Necropolis in infra-vision. Camera: Canon 40D; aperture: f/8; exposure: 30 s; ISO: 250, focal length:24mm

A few months ago a purchased a new addition for my collection of photography accessories – an infrared lens filter. I’d seen some photos taken with such a filter and thought I’d give it a try myself. The idea is that the IR filter blocks out all light accept that in the infrared spectrum which is normally invisible to the human eye. The results can be quite striking, especially when photographing green vegetation because it tends to come out a strange whitish colour. Since picking up the filter I’ve had a chance to play with it a bit so I thought I’d post some of my results.

But first, a word of warning. If you are purchasing an IR filter online, be careful about what you are actually buying. I bought mine on EBay after doing a search for “Hoya r72”, which is the name of a well-known IR filter for digital SLRs. If you do this, numerous listings will come up with “Hoya” in the title, but they will say things like “for a Hoya lens” or “Hoya-like” or “Hoya-esque”. I stupidly didn’t read the item description properly and ended up getting some generic IR lens instead of the Hoya r72 I thought I was getting. Surprisingly, the filter itself appears to be solidly built, but it came in this sad, non-descript plastic case with no brand name whatsoever that looked like it had been salvaged from somebody’s garage.

After initially feeling like a total sucker for being duped, I have since decided it wasn’t such a bad purchase after all. It only cost me about £12, and it actually seems to work okay (though I don’t have anything to compare it to at the moment), so I can at least practice with it and decide whether or not I want to invest a little more in an actual Hoya filter. Plus, it came with a free lens cloth (or, depending on your perspective, maybe I bought the lens cloth that came with a free generic IR filter).

The first thing I noticed when using the IR filter was the extremely long exposure times required to get a usable shot. This is due to the fact that most of the available light is actually blocked out by the filter and only a small fraction ever makes it to the camera sensor. A tripod or a stable place to rest the camera is pretty much required if you are going to use an IR filter. A tripod will also be useful for composing and focusing your shot: you won’t actually be able to see anything when the filter is over the lens so it would help to frame your shot without the filter, and then screw on the filter once you have everything set up.

Another thing you’ll notice is that the results straight out of the camera will look nothing like the picture at the top of this post or other digital pictures you may have seen using an IR filter. They will probably have a strong red or magenta tint that you will have to get rid of using some sort of post-processing. Here is what the photo above looked like straight from the camera:

The initial red tint on my pictures is probably even stronger than normal due to my crappy generic filter. Even so, I was able to remove this colour cast in Photoshop using a few easy steps. After opening your file in Photoshop, add a channel mixer adjustment layer. For the “Red” output channel, change the red value to 0 and the blue value to 100. Then for the “Blue” output channel, change the blue value to zero and the red value to 100. This should mostly get rid of the redness, but if you have a crappy generic filter like mine you may also need to desaturate the reds a bit prior to this procedure using a Saturation adjustment layer or play with the redness hue.

After removing the red colour cast you can then do any regular contrast and sharpening adjustments to finish things off. I’m actually pretty happy with my results so far and am probably going to splurge for a real Hoya r72 sometime soonish. Here are a few other infrared images, and there are some others in my infrared gallery located here.

Glasgow Botanic Garden in infrared.  Camera: Canon 40D; aperture: f/4; exposure 30 s; ISO: 320; focal length: 24mm

More of the Glasgow Botanic Garden in infrared.  That's not snow, it's grass.  Camera: Canon 40D; aperture: f/4; exposure 30 s; ISO: 320; focal length: 24mm.

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