Article | September 2005
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Few topics in the world of digital photography are shrouded with more misconception and myth than the dynamics of different sensor sizes. In this article, I will try to cut through the vast amount of information and disinformation floating around on various Internet discussion forums by first providing some basic facts, then analyzing the effects of these facts.

The sensor in a digital camera is the device that captures light and creates an image. In short, the sensor has taken the place the film used to have in analogue cameras. But unlike film, which had a pretty standardized size of 36.0(h)x24.0(v)mm (I will not be discussing medium and large formats here), digital sensors come in many different sizes. So the question for buyers of digital cameras is: How does this matter to me?

The first thing we need to do, is make clear how we measure sensor sizes. Digital camera makers have not been able to agree on a common measuring system, therefore, sensors are sometimes measured in inches, sometimes in millimeters, sometimes horizontally and vertically, and sometimes diagonally. To optimize clarity, every sensor discussed in this article will be measured in millimeters, horizontally and vertically. Some of the names of the sensors, however, still reflect the diagonal inch measurements, like the 2/3" type CCD or the 4/3" type CCD. Don't worry too much about this though, because the horizontal and vertical measurements in millimeters will always be specified.

A digital camera sensor has a certain number of light-gathering photodiodes. Each photodiode creates one pixel in the final image, so a digital camera with five megapixels has five million photodiodes on the sensor. As digital camera manufacturers cram more and more photodiodes onto each sensor in order to meet the continuous demand for more megapixels, they are faced with two alternatives: One is to make each photodiode smaller, and the other is to make the sensors larger.

The most common sensor sizes in digital cameras are 7.18(h)x5.32(v)mm (the so-called 1/1.8" type CCD) and 5.76(h)x4.29(v)mm (the so-called 1/2.5" type CCD). In other words, those are really tiny sensors compared to the old films of 36.0(h)x24.0(v)mm, and that's why a digital camera can be built much smaller than a film camera ever could. If you buy a reasonably advanced digital camera today, it will probably have a sensor of one of these sizes. Examples of cameras with these sensor sizes are shown below:

Sony CybersShot DSC-V3
Sensor size: 7.18x5.32mm

Canon PowerShot S2 IS
Sensor Size: 5.76x4.29mm

Very cheap camera models, as well as cellular phones with built-in camera functionality, usually have even smaller sensors. These will not be discussed here.

Some of the more advanced and expensive digital cameras have somewhat larger sensors, usually 8.8(h)x6.6(v)mm (the so-called 2/3" type CCD). This is needed because these cameras usually have a higher pixel count and need room for more photodiodes. A couple of well-known cameras with this sensor size are shown below:

Nikon Coolpix 8800
Sensor size: 8.8x6.6mm

Sony CyberShot DSC-F828
Sensor size: 8.8x6.6mm

The diagram below compares different types of sensors. The two smallest ones at the bottom are the ones we have discussed so far, the 2/3" type and the 1/1.8" type (the 1/2.5" type is not shown, but it is somewhat smaller than the 1/1.8" type). Notice the huge size difference between these and the 35mm film negative of 36.0x24.0mm. The other sensors shown, the 4/3" type and APS-C type, are usually found in digital single lens reflex (DSLR) cameras with interchangeable lenses. These will be discussed further later in this article.

While the 2/3" type, 1/1.8" type and 1/2.5" type sensors are the most common in digital cameras without flipping mirrors or interchangeable lenses (so-called digicams), larger sensors are usually used in DSLR cameras. The two most common DSLR sensor formats, the 4/3" type and the APS-C type, are shown in the diagram above. The 4/3" type measures 18.0(h)x13.5(v)mm, and Olympus is so far the only manufacturer to use this type of sensor in their DSLR cameras. The APS-C type, usually measuring 22.7(h)x15.1(v)mm (with some negligible differences between manufacturers), is much more widely used. Camera producers like Nikon, Canon, Konica Minolta and Pentax have all adopted this sensor size for most of their DSLR cameras. In addition, Canon has a few cameras with sensors the same size as 35mm film, but these are rare and very expensive. A couple of cameras with the 4/3" type sensor and the APS-C type sensor are shown below:

Olympus E-1
Sensor size: 18.0x13.5mm (4/3")

Canon EOS 20D
Sensor size: 22.7x15.1mm (APS-C)

Looking at the diagram above, we soon realize that these DSLR sensors are much larger than the sensors normally used in digicams. Why is this? It is certainly not the pixel count of the DSLR cameras that makes this a necessity. The DSLR cameras pictured here have five (Olympus E-1) and eight (Canon EOS 20D) megapixel sensors, while the digicams pictured earlier in this article all have five, seven or eight megapixel sensors. In other words, pixel count (and thereby sensor photodiode count) is virtually the same for the DSLR cameras and the digicams pictured here, yet, the DSLR cameras have much larger sensors. As we shall see, DSLR manufacturers have a good reason for doing it this way.

Cramming a lot of photodiodes onto a sensor is not the only reason for making a sensor physically larger. The size of each photodiode is important here, because the larger a photodiode is, the more light it is able to capture. And capturing more light is a good thing, because it reduces image noise, and improves sensitivity and dynamic range. It is very difficult, if not impossible, to obtain the same level of image quality with smaller photodiodes. This is why DSLR manufacturers choose to make sensors larger. Not because they need room for more photodiodes, but because they need room for larger photodiodes.

Addendum: The crop factor:
There is also another reason why DSLR manufacturers have chosen to put large sensors in their cameras, and that is their lenses. DSLR manufacturers sold lenses for many years when analogue film of 36.0(h)x24.0(v)mm was the only medium available, and when digital sensors made their entrance, millions of photographers were deeply invested in such high quality optics designed for the 35mm film negative. If manufacturers had then started producing DSLRs with 2/3" or 1/1.8" size sensors, all that glass, especially the wide angles, would have been rendered useless because of the crop factor that is introduced at smaller sensor sizes. I will explain further what this is.

The crop factor can be defined as "the number that you need to multiply the actual focal length of a lens with in order to obtain 35mm equivalent focal length", and the crop factor doubles as the diagonal of the sensor is halved. A 35mm film negative thus has a crop factor of 1.0 because this is the size the lenses were originally designed for. A 4/3" sensor has a crop factor of 2.0 because its diagonal is exactly half the diagonal of 35mm film, while a 2/3" sensor has a crop factor of approximately 4.0 because its diagonal is about one fourth the diagonal of a 35mm film negative (see the sensor size diagram above). Of course, lenses on small-sensor digicams are not designed for sensors as large as 35mm film at all, but their actual focal length is still defined in accordance with the 35mm film negative because this is the format that has been standardized upon for decades, and when comparing different focal lengths it is useful to have such a universal standard.

35mm equivalent focal length is the number that you get when you multiply actual focal length with the crop factor. If the sensor is the same size as a 35mm film negative, actual and 35mm equivalent focal lengths are the same (because the crop factor is 1.0). The smaller the sensor gets, the smaller is the actual image-capturing area, so if you were to put a 2/3" sensor (which has a crop factor of 4.0) in a DSLR with 35mm lenses you would have to multiply the actual focal length of the lens (could be 28-80mm) by 4 in order to obtain 35mm equivalent focal length (thus getting a 35mm equivalent focal range of 112-320mm). This might be ok if you need a lot of tele, but wide angle lenses would be effectively crippled. Of course, DSLR producers realized this, so they never actually put 2/3" sensors in their DSLRs.

I have now gone through some basic facts about digital camera sensors, but we still need to look at how these facts translate into actual use. All sensor types have their advantages and limitations, and these will be discussed below.

Small sensors like the ones normally used in digicams have one great advantage, and that is portability. Cameras with small sensors can be made smaller, and lenses constructed for small sensors can have an extensive zoom range without becoming too bulky (11x or 12x zooms are not uncommon). The huge size and weight advantage this actually translates to is shown in the example below. The Panasonic FZ-30 digicam has a bright 12x, optically stabilized Leica lens with internal zoom and focus, and the entire camera weighs in at 740 g. Here compared to a Canon DSLR lens with comparable features, constructed for sensor sizes common in DSLR cameras.

Panasonic FZ-30 digicam
Sensor size: 7.18x5.32mm
Zoom range: 35-420mm (35mm equiv.)
Brightness: f2.8-3.7
Optically stabilized: Yes
Weight (entire camera): 740 g.

Canon 28-300mm f3.5-5.6L IS USM
Sensor sizes: 36.0x24.0mm or 22.7x15.1mm
Zoom range: 28-300mm (x1.6 for APS-C)
Brightness: f3.5-5.6
Optically stabilized: Yes
Weight (lens only): 1670 g.

As we can see, the Panasonic FZ-30 with its small sensor provides an extensive zoom range in a relatively small and light package without compromising on optical quality. The Canon 28-300mm L-lens provides a comparable zoom range and optical quality for large sensors, but its size, weight and price is very high. Cheaper and smaller DSLR lenses with similar zoom ranges exist, but are largely unable to provide good optical quality for such a large sensor, and optical stabilization is a rarity among them. The alternative with a DSLR camera, of course, is to use multiple lenses with smaller zoom ranges, because the smaller the zoom range, the easier it is to get good optical quality. However, that adds weight as well, so there is no way for a large-sensor camera to actually match the portability and conveniency of a small-sensor one.

The size, weight and price advantages of smaller sensors are undoubtedly great, but of course, put this top-of-the-line Canon lens on a DSLR camera with a large sensor, and you get a whole other level of image quality than what is possible to achieve with a small-sensor digicam. This, as I mentioned earlier, is the great advantage of large sensors with large photodiodes. Pictures look sharper and cleaner (providing you have a good lens, of course), dynamic range is much improved, and it is possible to bump the ISO to achieve hand-held shutter timings even in dark environments (something that's virtually impossible with a small-sensor digicam due to excessive noise).

Still, as research moves forward and sensor technology improves, more and more photodiodes are crammed onto even large sensors with seemingly no quality loss even though each photodiode inevitably becomes smaller. The world of photographic science is not a static one, and what seems impossible today might become possible with tomorrows technology. The laws of physics, however, do not change. A large sensor will always capture more photons per photodiode than a small one (given the same pixel count) and optics made for small sensors will always be smaller, lighter and cheaper than what's possible with large-sensor cameras.

See the article Depth of Field Explained for more info about sensor sizes.