With 35mm film cameras, the “sensor” size was the size of the film negative itself—36 x 24mm (incidentally, the 35mm is the width of the film strip which gives a negative frame height of 24mm, the perforations sitting either side down the film). In today’s digital market, only semi-professional and professional cameras would have a sensor this size—the sensors on the vast majority of DSLRs available today are much smaller, whilst those on a smartphone are typically minuscule in comparison.

Here are a selection of sensor sizes stacked up on top of each other to give an relative comparison (all in ratio but not 1:1 scale).

These varying sensor sizes relative to a full frame can also be expressed as a crop factor. This is simply the ratio of the diagonal of the sensor to the diagonal of a full frame sensor (~43.3mm). For instance, the diagonal of the DX sensor is approximately 28.3mm. So we have 43.3 / 28.3 = 1.53 ~ 1.5x. In practice, this means that a 50mm lens for a full frame camera will have a focal length of roughly 80mm.

This does have it’s advantages. Firstly, long focal length lenses cost a lot. A Canon 500mm f/4 lens can go for over £8,000! It’s 300mm cousin retails for around the £1,000, but on a 1.6x APS-C sensor camera this £1,000 lens now has a focal length of 1.6 x 300mm = 480mm! This is why many nature photographers (e.g. bird photographers) will opt for a smaller sensor DSLR over a full-frame.

Secondly, all lenses are sharper in the middle than they are at the edges (especially at larger apertures). On a smaller format camera the sensor is not big enough to reach the edge of the circle of light that passes through the lens and so only the “middle part” is used to capture the image. This mean that you can open the lens to its maximum aperture with less worry about image quality at the edge of the photograph.

There are two sides to the coin, however. If you’re buying a 16-35mm wide angle lens for nature photography you won’t be pleased when you realise that this is in fact a 26-56mm on your DX camera. Depth of focus is also increased with a smaller format sensor, so it’s not quite so easy to get those blurry backgrounds.

In case you’re wondering, whilst you can use a standard lens on DX (or APS-C) cameras you cannot use a DX lens on full-frame cameras otherwise you’ll get vignetting.

Vignetting typically refers to the light fall-off seen at the edge of the lens (particularly pronounced for wide-angle lenses), but in the above photo the light has simply been blocked by the DX lens

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