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What is the crop factor?

Before digital cameras we used film and film came in standard sizes, the most frequently used being 35mm film. Therefore, when digital came along, the size of a piece of 35mm film (specifically 36 x 24mm) was used as the standard for a ‘full frame’ sensor – the sensor being the silicon chip that captures your image. But not all sensors are this size – in fact, many of them are a lot smaller.

I’m still confused. Why does it matter?

It matters because it has an effect on focal length. If you have a full-frame SLR and you put a 35mm lens on it, then that is exactly what you get in terms of focal length. But that’s not the case if you have a crop factor to account for.

How do I know what crop factor my camera is?

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Each manufacturer will quote the sensor size in the specifications for the camera model. As a general rule, full-frame sensors are used in pricier, pro-level cameras, while enthusiast and entry-level SLRs have sensors with a crop factor. Taking Canon and Nikon, the two biggest camera manufacturers as an example, their non-full frame SLRs have crop factors of 1.6x and 1.5x respectively.

How then do I work out the real focal length of my lenses?

The effective focal length of your lens is simply the quoted (35mm equivalent) focal length x the crop factor of your camera. So if you own a Nikon D5500 with its DX sensor and you buy a 16-35mm FX lens to go with it, the effective focal length of that is actually 24-52mm.

What are the pros or cons?

The pro is the fact that your lenses are giving you a longer reach because the focal length is magnified by the crop factor. This can be handy if you want to shoot wildlife or sports, where your subject is often a good distance away. But conversely, the con is that if you want to shoot wider, the crop factor is going to narrow your field of view.

Surely manufacturers have thought of this?

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Yes, and these days you can get lenses with much wider focal lengths to allow for the crop factor. For example, if you wanted a wide-angle zoom for a crop-factor Canon camera, you could opt for something like the EF-S 10-22mm which is the 35mm film equivalent of 16-35mm.

Do all lenses work on all cameras?

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I’m afraid not. Lenses designed specifically for crop-sensor cameras (DX for Nikon and EF-S for Canon) will not work correctly with a full-frame camera and have the potential to damage the camera’s mirror if mounted onto the camera. However, lenses suitable for full-frame cameras can happily be used on crop-factor cameras.