Infrared with the Canon PowerShot A650 IS & Photographic Noise Reduction

A few weeks ago I tested the A650 IS for response in the infrared, and was really pleasantly surprised to see it had usable response. The exposures are long, roughly 1 second or longer under sunny-16 conditions. But with a tripod or decent support against a fencepost, it's usable.

For my first tests I used an old tried 'n true, a stack of a Wratten #25 red gel and a Wratten #47A blue gel. This combination has almost zero UV leak, and gives you an 85% throughput IR long-pass filter that crosses 50% right around 770nm. It worked:





With the success of that first test, I bit the bullet and ordered a 58mm Hoya R72 infrared filter, which arrived a few days ago. It has a 720nm cut-on, so you get a bit more light than with the Wratten stack. Plus, it's an AR-coated glass filter that threads onto the end of the lens adapter, so it's a lot more robust. It also works well:





The only gotcha is that low-light exposures like that, even at low ISO, are noisy. Open patches of uniform sky look like they were shot on Kodak HIE (which is cool, in my mind), meaning very very noisy (ok, maybe not so cool.)

So I tried out a software trick that's used with astrophotography: You take a series of exposures and add them together. You average out a lot of the pixel-to-pixel variation this way, though things that move frame-to-frame come out looking blurred.

Unfortunately this can't be done with typical photo software because if you add a bunch of properly exposed images, you exceed the 8-bit math most image software uses. If you've got a program that does 16-bit math, you're probably set. I wound up using some astrophotography software, Iris. Iris plays nicely with JPGs, though it'll also work with RAW files, which the A650 IS can produce if you're running CHDK on it.

A stack of five JPG images knocked down the noise a lot. This is a stacked image I did using this technique:





And a 1:1 pixel-for-pixel side-by-side of one of the original images next to the resulting stack:





(You have to view it original-size. It's two 1000x800 images side-by-side.)

So all in all I'm more than happy with the IR performance of an unmodified A650 IS. Granted, the exposure times are long, but for a lot of landscape work that's entirely acceptable.

If you used to do B&W film photography and have a bunch of Wratten fgel filters lying around, you can experiment with IR on the A650 without having to buy anything by using a #25 / #47A stack. If not, the Hoya R72 filters are less than $50 these days, making infrared photography with the A650 pretty darned affordable.

Tom