Seeing in low light

I love my compact IXUS 220HS digicam that does most of the pictures used for this blog. It’s got this supposedly cool feature called low-light mode. According to Canon, it allows you to take great pictures in low light, for example a candle lit room. It’s only fitting that the menu icon is a little  candle, which reminds me of an old Kodak ad introducing their Kodacolor VR 1000 film once upon a time. The problem with this low light mode is that it doesn’t quite work as advertised (or should I say expected?), so Wall-E suggested doing some real measurements to untangled the ball of marketing yarn 🙂

In short, the camera trades pixel resolution for sensitivity in low light mode. By limiting the picture resolution to 3MP from the standard 12MP, the sensor is supposed to capture more light per pixel. Such a reduction suggest pixel binning, a method used to increase the light capturing capacity of the camera by combining the light received by four sensor ‘pixel’ to give one image ‘pixel’. It’s like using a fishing net four times the size to capture more fishes in a single swoop. Only that I don’t understand why I still get the same number of fishes with my bigger net.

In photographic terms, the camera should exhibit a two stop increase in sensitivity. This translates into either a shutter speed four times as fast at the same ISO setting. Or conversely, at the same shutter speed, it can shoot at an ISO setting four times higher. I got neither. There was an increase in something but nowhere near the expected value. In fact, most of the time, the low-light mode produces mushier or noisier images than the standard 12MP output. See for yourself in the cropped images below:

12MP output at ISO 1600 3MP low light mode at ISO 1600

The EXIF data in the images clearly shows that the shutter speed 1/25 is not four times faster than 1/15. Nor is the perceived noise better than the 12MP output. It appears that the trade off is not giving the expected returns. As I repeated further tests, I am convinced that pixel binning is not properly exploited in this case. I find that it tends to choose an ISO setting too high to be optimum in most lighting conditions except extreme low light. Luckily, the back-illuminated CMOS sensor of this camera is so good that there is practically little noise artifacts up to ISO 1600.So I could simply stay in Program mode set to Auto ISO for 99% of my shooting.

When light is really low, ISO 1600 starts to use slow shutter speed. I personally can hand hold down to 1/4″ in most conditions at short focal lengths up to about 50mm. When I start getting blur, I know I have to switch to low light mode or use the ISO 3200 setting.  The picture of teapots that I took the other day was an example of low light mode giving satisfactory result.

PS: There is another mode, hand-held night mode, that attempts to take multiple shots of a picture in low light at high shutter speeds to avoid hand shaking – for example, instead of taking a 1/4″ shot, it will take three 1/12″ shots in succession and merge the exposures. It’s an alternative to low light mode and your mileage may vary because it is still susceptible to hand shake in most cases.

 PPS: If time permits, I might do a more in-depth test with CHDK installed to get to the bottom of this.

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2 thoughts on “Seeing in low light

  1. I really love those Canon IXUS compact cameras. I’m using a 1000 HS and an old 40 for the pictures on my blog. So handy to use.

    1. Totally agree, especially for the relative low budget. It’s so small I carry it everywhere! I occasionally miss the ability to tweak manual settings, like manual focusing and exposure compensation, but I can live with that. Thanks for reading the post 🙂

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