Sensor size and noise
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It's been a while since I started a thread here, so I thought I may as well throw in my two cents' worth about sensor sizes and noise.
Some common sensor sizes are:
- Full frame (35mm equivalent): e.g. Canon 1D and 5D series, Nikon D3, D700, Sony A900
- Crop sensor: lower-end models, e.g. Canon 50D or 500D, Nikon D300, Sony A350, Sigma SD14
- Four-Thirds: various Olympus and Panasonic models
- 2/3”, ½”, 1/3”: various compacts and point-and-shoots
While a few compacts like Sigma’s DP1 have crop sensors, by and large DSLR sensor sizes are massive compared to those found on compacts, as illustrated below:
Image source: www.cambridgeincolour.com
So guys keep saying size doesn’t matter, right? Rubbish.
Regardless of whether you’re using film or digital sensors, photography boils down to capturing light in order to form an image, and the more light you capture, the better.
In a digital camera, the data for each pixel is captured by a photosite, millions of which make up a sensor. As a rule of thumb, the larger a photosite, the more light it collects, improving the quality of the image.
Now of course there have been improvements to sensor technology, with more sensitive photosites, gapless microlenses and better design all working to increase the amount of electronic signal that sensors convert from a given amount of light, but all things being equal, a larger photosite is always better by virtue of the larger amount of light collected.
So that’s all well and good if you’re designing a camera sensor, but what does that mean for the guy behind the viewfinder? Simple – the less light your camera is able to collect, the darker or noisier your photos are going to be.
On precisely the same settings, a small photosite will be able to collect less light than a large one, resulting in a darker picture. There are a few ways to make the picture brighter, of course, but each has its own disadvantages:
1) Use a larger aperture: this is probably the best way to get more light in. However, this will present problems if you’re looking to get your whole picture in focus, since the depth of field decreases with larger apertures.
2) Use a slower shutter speed: well, we’ve all seen blurry and shaky pictures taken in low light. Let’s move on.
3) Turn up your sensitivity. Sure, crank that ISO up. You might get a brighter picture, but some photosites are going to get oversensitised and then you get overexposed spots – this is noise. Sure, use noise reduction. All that does is blur the image till the spots aren’t obvious. Never mind that it blurs the picture itself – dumping a freezing man in boiling water’s a sure way of dealing with hypothermia.
Regardless of the tricks you can use, there’s no substitute for a well-exposed shot, and that itsy-bitsy little sensor in that compact just isn’t going to do. That’s why you’ll see a lot of reviewers shrug off the fact that the compact they’re testing is no good above ISO 200 – there’s little more that can be expected from small sensors.
By now, some of you may have twigged on to the issue of megapixels. For any given sensor size, the higher the pixel count, the more photosites occupy the same space and the smaller each photosite will be. Higher pixel count is therefore often a bad thing; if the design of each photosite is not improved, using smaller photosites is just a step backwards.
Anyway, there's more to sensor size than just low light and noise, but I'll chuck that bit in another day.