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.