Choosing a camera Part 2: is a bigger sensor better?

When looking at pixel size, we saw that there's little difference between having a few large pixels and having lots of small ones, once you consider the whole image. This is because sensors have the opportunity to capture the same amount of light per-whole-image, regardless of how many pixels they have.

However, when looking for a new camera, there often is a way of getting more light and therefore better image quality: a larger sensor. This is because, at the same exposure settings, a large sensor is given the same amount of light per unit area, but has a greater sensor area capturing this light.

Key takeaways:

  • Two cameras with the same exposure receive the same light per square mm, and larger sensors have more square mm.
  • Every object in your scene will be projected onto more square mm of sensor if those two hypothetical cameras have the same field of view.
  • This means every object is described with more photons of light, which gives the potential for a cleaner image.
  • Differences in sensor performance mean one camera may over- or under-perform expectations but these differences are usually smaller than the differences made by changing formats.

The effect of sensor size:

In this instance we're comparing the Nikon D810 and the Nikon D7000, which have the same sized pixels but different sized sensors. The D810 has a full-frame sensor that's around 2.3x larger than the APS-C chip in the D7000.

ISO 1600
D810 whole frame
[Raw File]
D7000
[Raw File]
D810 (resized: 16MP)
[Raw File]
ISO 3200
D810 whole frame
[Raw File]
D7000
[Raw File]
D810 (resized: 16MP)
[Raw File]
ISO 6400
D810 whole frame
[Raw File]
D7000
[Raw File]
D810 (resized: 16MP)
[Raw File]
ISO 12800
D810 whole frame
[Raw File]
D7000
[Raw File]
D810 (resized: 16MP)
[Raw File]

As you might expect, the two cameras look similarly noisy at the pixel level because they received the same amount of light per square mm and each pixel is the same number of square mm.

But when you downscale the D810's images (as you would if you wanted to view or print at the same size), the benefit of its bigger sensor starts to appear.

Compare the D810's output to the D7000 image from one ISO setting lower and you'll see they look very similar, but with the D810 still a fraction ahead. This is consistent with the 1.2EV difference that the sensor size difference would lead you to expect.

Size differences outweigh performance differences

If shot from the same position, using a lens with the same angle-of-view, every object in the scene will be captured by a greater area on a bigger sensor, so with the same exposure a larger sensor will have more photons shone on it to describe the scene. As such it will tend to look cleaner if you view them at the same size.

There will be some differences in how well each sensor design can turn these photons into a digital signal (even though most modern sensors are excellent), but there are fairly large gaps between most popular sensor sizes, and these size differences tend to be greater than the differences made by sensor performance.

Now this might sound like bigger is always better. But it's not that simple...