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Understanding Focal Lengths


If you are new to photography and interchangeable lenses, then this is usually where most people start to stumble. However the first thing to grasp is how the size of your camera's sensor will affect its stated focal length.

In the days before digital this wasn't an issue as 35mm film SLRs all used the same 24 x 36mm film stock. A standard 50mm prime therefore produced the same field of view regardless of the camera it was mounted to.

In the digital age, however, sensors come in a number of different sizes, which in turn affects the effective focal length of the lens. This is because sensors that are smaller than 35mm only capture a middle portion of the image generated by a lens.

Put simply, they effectively crop out the sides and magnify the middle. This has the effect of increasing the stated focal length of any given lens. The extent to which each type of sensor does this is usually referred to as its ‘crop factor'.

Because full-frame sensors are the same size as 35mm film they will capture images at the stated focal length of a 35mm lens. APS-C sensors are slightly smaller, however, which gives them a crop factor of 1.5x (Nikon, Pentax and Sony) and 1.6x (Canon). Cameras that use Micro Four Thirds sensors (Panasonic and Olympus) are smaller still, which gives them a 2x crop factor.

Thereby a 50mm prime will shoot at 50mm on a full-frame DSLR, but 75mm on an APS-C equipped camera, and 100mm on a Micro Four Thirds model.

For the sake of understanding, we will refer to the effective focal length; as in what you see.

The chart below should help in understanding the kind of focal range you want in the more popular ranges of lenses.

This is a generalisation.  There is much more to this than meets the eye, and the breakdown lines are (more often than not) a little more blurry.  There are specialist lenses, Macro Lenses, and All-In-One / Super-Zooms. 

For more information, please see our Buyer's Guide on Buying A Lens