My 50D works great in good light, ideal for sunny days and the studio. Like many high resolution dSLR cameras it can suffer from a little bit of noise in low light conditions. In researching image noise and resolution I came across a number of documents that try to explain the issue in great detail. A simple “in a nut shell” summary being:

Higher Resolution with the same sensor size = Higher Noise

Imagine your kitchen draw open and a hand full of marbles. If you throw the marbles into the air above the draw you’ll end up with quite a few marbles in each section. If each of these sections were pixels it would be the equivalent of exposing them to lots of photons of light. Lots of charge would be created to tell the camera what the light was like. Now if you double or quadruple the resolution, making the sections of the draw much smaller, you’ll catch fewer of the marbles in each. With marbles bouncing off the walls of the cells many may not get caught. Make the cells smaller and the ratio of wall to cell surface area grows towards the walls.

This is what’s happening with the pixel size in a lot of current dSLRs. Consumers that think just in terms of pixel count not image quality are pushing the market in the resolution direction.

The table below shows the sensor size on a number of Canon dSLRs and effective pixels. Some simple maths has been used to give an approximate pixel size. There are other factors to take into account, such as inter-pixel gaps (like our draw cell walls), but that’s been left out to keep things simple.

Model 60D 7D 50D 1DMkIV 40D 1000D 1DsMkIII 5DMkII 5D
Effective pixels 18.0M 18.0M 15.1M 16.1M 10.1M 10.1M 21.1M 21.0M 12.7M
Sensor Width 22.3 22.3 22.3 27.9 22.2 22.2 36.0 36.0 36.0
Height (mm) 14.9 14.9 14.9 18.6 14.8 14.8 24.0 24.0 24.0
Sq cm 3.32 3.32 3.32 5.19 3.29 3.29 8.64 8.64 8.64
Pixel Size 0.18 0.18 0.22 0.32 0.33 0.33 0.41 0.41 0.68

What you’ll notice is that the recent 60D and 7D models have smaller pixels. If the other sites warnings re pixel size and noise were all true you’d have to move further to the right of the table to get images with less noise. So why are the images from the cameras on the left half of the table not taking a major step backwards as suggested by the other web sites? The answer is simple; the other sites all assume the same simple sensor technology is in use. The basic principle is sound though, the bigger the pixel the more light it will catch and the less noise you’ll get.

Using our analogy again, what if you were to place a set of square funnels above the kitchen draw that touched each other on all four sides? Almost every marble would be guaranteed to count, none bouncing off the cell walls of the draw.

Canons gapless micro-lenses are doing the same job for the pixels in the Canon 50D and new 1D Mk IV sensors, enhancing their light capturing capabilities to counteract any drop in pixel size. Add an extra Canon technique and you can drop the noise even further. The camera takes two images. One with the shutter open, the other closed. The noise from the closed image is subtracted from the first.

From Canon’s official 1D Mk IV press release:

Image Quality and Performance
The heart of the EOS-1D Mark IV camera’s outstanding image quality is a newly developed 16.1-Megapixel CMOS sensor featuring Canon’s latest and most advanced proprietary technologies. These technologies include improved photodiode construction to enhance dynamic range and gapless microlenses that are positioned closer to the photodiodes for improved light gathering efficiency. The transmissive quality of the color filter array has been enhanced to improve sensitivity. Canon has also upgraded the sensor circuitry to improve noise reduction before the image data is exported from the CMOS sensor to the rest of the image processing chain.

So now when you go to pick your next dSLR think not just in terms of resolution, but also image quality. What is the image sensor size, how it’s designed in terms of photo diodes and micro-lenses. Check review sites for comments on image noise.

From all the previews I’ve seen the EOS-1D Mark IV will raise the bar way past many peoples expectations. Imagine a camera that can see colour and detail in darkness the human eye can’t even see. I’m going to need a larger piggy bank.