Digital cameras – why sensor size matters

A wide range of sensor sizes are used in cameras today. The size of the sensors is not related to their resolution (measured in megapixels). The smallest, 1/3.6″, has a tiny light-sensitive surface of only 4mm x 3mm. That is just 1/72 of the area of a 35mm frame. Needless to say there are some compromises in performance. Other common sizes are 1/3.2″, 1/3″, 1/2.7″, 1/2″, 1/1.8″, 2/3″, 1″, 4/3″ and 35mm (the same size as a 35mm film frame). The inch sizes are the diagonal size of the sensor, 1/3.6″ being equivalent to about 7mm. In comparison a 35mm sized sensor is about 43mm diagonally.

Most sensors have an aspect ratio of 4:3 – the same shape as a standard TV or the original “academy” feature film ratio. The 35m stills film format is wider, at 3:2. You might think that in this multimedia age somebody might have produced a widescreen 16:9 aspect ratio camera but that is clearly too much to hope for!

The Four Thirds (4/3″) camera format was developed jointly by Kodak and Olympus and is also used by Fuji. The intention was to create smaller, lighter cameras and lenses that were functionally equivalent to 35mm SLRs. Standardising the lens mount meant lenses could be made that would fit all 4/3 cameras. The sensors are about half the height and width of a 35mm film frame.

One consequence of the differing sensor sizes is that lenses offer a reduced angle of view compared to a 35mm film camera. Even with the relatively large sensors fitted to most digital SLRs the effective lens focal length is around 1.6 times that for 35mm film. A 50mm lens designed for use on a 35mm film SLR would act as an 80mm lens on a digital SLR. This is an advantage at the telephoto end of the lens range as the lenses are much smaller. Unfortunately it is pretty unhelpful in making a true wide-angle lens. The smaller the sensor the worse these effects get.

The depth of field available also depends on the sensor size. Small sensors give large depths of field. Great for snapshot cameras with fixed-focus lenses or cheap auto-focus with few steps. Not so good for taking pictures through chicken wire or dirty glass – all those obstructions that would have been invisible or vague blurs with a 35mm SLR are now in sharp focus. Another result is that artistic use of differential focus by controlling the lens aperture (to adjust depth of field) is virtually impossible even if the camera allows the relevant adjustment. The serious photographer is left attempting to emulate the effect in Photoshop.

The smaller the sensor the better the lens performance has to be to focus fine detail onto the sensor with good contrast. In fact there is a finite limit to the resolution of a lens at a given aperture and the nearer the limit it gets the lower the contrast. The measure of lens performance in reproducing fine detail is called the Modulation Transfer Function and it reduces as we get closer to the limit of the lens’s capability. Even the highest quality lenses are incapable of delivering adequate detail to the smaller sensor sizes at f16.

Unfortunately it also works out that the smaller the sensor the smaller the amount of light collected too. That’s the reason that pictures from cameras with small sensors produce noisy (aka grainy) pictures. Comparisons of the image noise in pictures from large and small sensor cameras can sometimes be seen in reviews on the website.

In summary small sensors have advantages in terms of price and convenience but these are paid for in image quality and camera sensitivity.