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The difference between fast and slow digital SLR camera lenses

When you buy a digital SLR camera lens you will notice it has an f number on the box. For example, Canon EF 75-300mm f/4-5.6. It is the f number (f/4-5.6) that determines whether the lens is considered fast or slow. The f number is also know as its aperture value.

A slow lens has a maximum aperture value (f number) of 3.5 to 5.6 or higher. The higher the f number the slower the lens. Here is another example using two of my lenses. One is a Canon EF-S 17-85mm f/4-5.6 lens and the other is a Canon EF 100mm f/2.8 Macro lens. The first lens that has a maximum aperture value of f/5.6 is considered slower than the second one that has a maximum f/2.8.

Advantages of a fast lens

Faster lenses let in more light at faster shutter speeds. This allows you to photograph more easily in low light without the need for a tripod or flash. It also lets you take photographs long into the evening without raising the ISO. The lower you can keep your ISO setting, the better the photo quality will be. Furthermore, with a fast lens you can shoot subjects at closer range, for example portraits, while blurring the background. Whereas with a slower lens your background needs to be further away for it to be blurred.

Disadvantages of a fast lens

A faster lens is usually more expensive, larger and heavier than a slower lens. Buying a f/2.8 lens will cost you more than buying a 4.0 or 5.6 lens. It will take more room in your bag and be heavier to carry around.

Which lens should you buy, fast or slow?

If you can afford a few extra bucks and don't mind the larger size or heavier weight, then go for the fast lens. There is a saying that you get what you pay for. A fast lens is more expensive due to its ability to take better photographs more easily in all conditions, including low light.

If you have limited funds and you don't mind using a tripod or flash in lower light, then buy the slow lens.

UPDATE: Our tutorials for photographers are now being continued over at SLR Photography Guide.