When cameras still used film, the photographic sensor was the same size for (almost) everybody, with rolls of 35 mm film in use everywhere from the simplest compacts to the most advanced SLRs. That isn't true any more with digital cameras, and it's often said that 'SLRs have better sensors'. But why?
Published: August 25, 2009 11:00 PM
By Franck Mée
The Basics

A bigger sensor means:
  • you can take better photos in darker conditions;
  • you have more control over the depth of field to try out different effects.
It means you have to:
  • be careful not to reduce the depth of field too much.
Why different sizes?

Even on the earliest compact digital cameras, manufacturers tried to reduce costs.  It's logical enough that a smaller sensor costs less to produce.  You can get more out of a single wafer of silicon, and if an individual component is defective, there's less wasted.

On SLRs, though, it was difficult to adjust the size of the sensor, because it uses the same lens as the viewfinder, and therefore can't be reduced without making the viewfinder harder to use.   Also, it was important that the expensive lens kits that professional photographers had invested in remained compatible with their new equipment.

24 x 36
APS
4/3''
1/1.6''
1/2.3''
1/2.5''

As a result, there's a clear distinction between cameras with 'big sensors', measuring at least 13 x 17 mm and those with 'small sensors', which are at most 6 x 8 mm.  The former (in green and blue on the diagram above) are used on SLRs, hybrid micro four-thirds cameras and a handful of other cameras.  The latter, meanwhile (in yellow and red) are reserved for all other digital cameras: compacts and bridges.

The Main Factor: Sensitivity

A sensor is made up of thousands and thousands of tiny pixels which can transform light into an electrical signal.  Each individual pixel can be thought of as a tiny solar panel that captures the light of one primary colour: red, green or blue.

Thinking about it this way makes sense: a large solar panel catches more sunlight than a smaller one and can thus provide more electricity.  Apart from a few technical details, the same is true for each individual pixel.  It's obvious that if you put 12 million pixels on a sensor that measures just 4.5 x 6 mm (like on the Canon Ixus 110 IS), each one will be much smaller than if you put the same number on a larger 15 x 23 mm sensor (like on the Nikon D5000).

 
Pitch 5 μm 2 μm 1.54 μm
 Sensor 14 Megapixels - CMOS 12 Megapixels - Super CCD 12 Megapixels - CCD
 Size  APS 1/1.6'' 1/2.3''
 Example Pentax K20D Fujifilm F200EXR Panasonic FX40

These diagrams show how the pixels are arranged on three different cameras that are currently widely available.  The arrow represents the pitch, the distance between individual pixels.  From left to right, we have a digital SLR followed by two compacts.  Note the differing sizes of the sensitive pixels (the coloured areas).

A little bit of maths show that the D5000's pixels are around twelve times bigger than those on the Ixus 110 IS.  That's an extreme comparison, because SLRs generally have a higher resolution (more pixels) than compacts to start with.  In general, though, each pixel on an SLR will be eight to twelve times bigger than on a compact, and so if they both receive the same amount of light, the SLR will manage to capture 8 to 12 times more, and therefore produce a stronger current.


The same resolution, the same sensitivity (1600 ISO), but a different sensor size.
There's an SLR with an APS above, and a compact below.
 
Now, if the current is too weak, it's hard to measure accurately.  The signal needs to be amplified, which increases the chance of small random errors creeping in as it travels.  The end result is 'noisy' photos.  In the example above, you can see how grainy the bottom picture is.  A stronger current, by contrast, won't need amplifying and can be effectively measured without any extra noise.

Don't forget depth of field

We like our landscape shots to be sharp at any distance.  But sometimes, we want to make the background or the foreground a little blurry, or try to hide a detail in between the camera and the subject.  The ability to adjust the size of the area that's in focus--we call it 'depth of field'--is linked to the physical size of the image that ends up on the sensor.  To cover a sensor that's 15 mm high, the image itself will need to be three times bigger than it would be if there were a 5 mm sensor.


Different sensor sizes: compact on the left, SLR on the right

In this case, the focus is on the man's nose and both photos are taken from the same distance, with the same settings, a focal length equivalent to 105 mm and an aperture of f/5.  On the left is the result of a compact with a 1/2.3'' sensor, while on the right is an SLR with an APS sensor.

Note that on the left, the straps hanging down in the cupboard in the background are still clearly visible, while they're entirely blurred on the right.  On the left, the background distracts the eye's attention and can be annoying, but on the right, the eye is attracted to the sharp details of the face and can ignore the background.

Be careful, though, as reducing the depth of field too much can be problematic: in our examples here, it's so reduced on the SLR that the face isn't entirely sharp.  To avoid this effect, you need to use the camera's manual settings to close the diaphragm.

Measuring Sensor Size

All that remains, then, is to measure how big the sensors actually are.  And it's a very difficult job.  Because manufacturers still use techniques inherited from making cathode ray tube televisions, what's quoted is the diameter of a circle into which the frame fits, measured in inches.

It's not worth trying to understand why or how it works, but we can review the most common sizes and give you an example of the cameras that use them:

Standard Diagonal Dimensions Examples
1/2.5" 7.18 mm 4.29 x 5.76 mm Panasonic TZ6
1/2.3" 7.7 mm 4.62 x 6.16 mm Nikon P90, Canon 110 IS
1/2" 8 mm 4.8 x 6.4 mm Fuji F70EXR
1/1.7" 9.5 mm 5.7 x 7.6 mm Canon G10
1/1.6" 10 mm 6 x 8 mm Fuji S200EXR
4/3" 21.6 mm 13 x 17.3 mm 4/3 SLRs and Micro 4/3s
APS 24.8 mm
to
28.4 mm
13.8 x 20.7 mm (Sigma)
to
15.8 x 23.6 mm (Nikon, Sony)
General SLRs
24 x 36 43.3 mm 24 x 36 mm Nikon D700, Sony Alpha 900
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