Understanding Focal Length

When parallel rays of light strike a lens focused at infinity, they converge to a point called the focal point. The focal length of the lens is then defined as the distance from the middle of the lens to its focal point.

The focal length of a lens is usually displayed on the lens barrel. Below is a picture of a Canon lens with a focal length of 50mm. The maximum aperture is f/1.8.

Lenses are usually categorized as having a wide-angle, normal or telephoto focal length. A normal SLR lens covers a 24x36mm film frame with a field of view that corresponds approximately to our normal vision; a lens with a focal length of 50mm (55mm is also popular) is considered as normal.

Any lens with a focal length less than 50mm (or 55mm) can be considered as wide-angle; any lens with a focal length greater than 50mm (or 55mm) can be considered a telephoto. A zoom lens offers a range of focal lengths. This table lists some of the more popular focal lengths:

Take a look at the two Nikon lenses below; the lens on the left is an SLR camera zoom lens with a focal length of 24-85mm. This is a popular zoom lens because it provides a good wide-angle (28mm) as well as enough telephoto reach for a good portrait (with 105mm, you can fill the screen with a face without getting in your subject’s face, so to speak).

The lens on the right comes with the Nikon Coolpix 5400 digital camera and has zoom focal lengths of 5.8-24mm. Believe it or not, this is the equivalent of 28-116mm zoom lens on a 35mm SLR camera! (The Coolpix 5400 no longer seems to be available although accessories for it are, but you might track one down for a good price on eBay.)

Nikon 24-85mm f/2.8-4 AF-Zoom Nikkor	Nikon Coolpix 5400: 5.8-24mm (28-116mm, 35mm equivalent) f/2.8-4.6 Zoom Nikkor ED

Because the image sensor size used in digital cameras are of different sizes, the same focal lengths may be expressed using different numeric values. Smaller image sensors require smaller lenses; larger image sensors require larger lenses to ensure all the surface of the image sensor is covered. Other factors, such as the amount of optical zoom provided, may further affect the distance between lens and the image sensor.

It’s more convenient to use the SLR equivalent as a handy reference point. Most, if not all, camera manufacturers will list the 35mm SLR equivalent in the specifications of a digital camera. Interestingly, some digital cameras are now even engraved with the 35mm equivalent on the lens barrel to express the focal length of the lens. It makes more sense in a way, though it is important to bear in mind that 35mm equivalent does not mean 100% compatibility with the real SLR lens. This is especially true in the area of depth of field.

If you use a digital SLR (dSLR) that uses lenses made for 35mm film cameras, you need to be aware that the focal lengths expressed on the lenses must be multiplied by a factor. That factor, the Focal Length Multiplier, depends on the image sensor size used (that will be in the Specifications section of your camera manual).

Of course, if the image sensor is full-frame, i.e. it is the same size as 35mm film, then the multiplier is 1, and the focal length of the lens is accurate.

However, only a few dSLRs use a full-frame image sensor (such cameras are beyond the budget of hobbyist photographers, being in the range of several thousands of dollars), with most using a smaller image sensor, usually APS size (or roughly half-frame). That’s why you’ll read that a focal length multiplier of, say, 1.6 needs to be applied to the focal length of the lens to obtain the true focal length.

So, suppose your dSLR has a Focal Length Multiplier of 1.6 and you use a 50mm lens with it, the actual focal length of the lens when used with your dSLR is 1.6 x 50mm = 80mm. This is both good and bad. The good news is that you can now get super telephoto focal lengths on your dSLR without buying costly and unwieldy dedicated lenses. For example, a 100-300mm zoom lens, with a focal length multiplier of 1.6, becomes approx. 160-480mm. The bad news is, of course, that super wide-angle lenses are equally affected and a 28mm lens becomes a 45mm lens. One solution is to buy a 16mm fisheye lens which, when factored up by 1.6, becomes a 26mm lens.

To confuse matters even more, ‘digital’ lenses are appearing (or sold along with digital camera bodies) which show the digital focal length range rather than the SLR equivalent. Canon provide an 18-55mm lens with their EOS digital range of cameras. While this is equivalent to a 35mm SLR range of 29mm-56mm, that equivalent range isn’t printed on the lens. A little experience will let you identify 35mm SLR lenses from lenses specifically designed for use with dSLRs.

Finding Out More:

• Using Your Camera, A Basic Guide to 35mm Photography Revised and Enlarged Edition
• Creative Camera Control, 3rd edition
• Understanding Exposure, 3rd Edition: How to Shoot Great Photographs with Any Camera
• “The Camera” By Ansel Adams