DSLR Lenses: 7 questions photographers must ask about their next piece of glass

"I’m using the zoom lens that came with my camera, but I’d like to add one or two more. Where do I start?

Lenses can be split into three basic types: wide-angle, standard and telephoto. The zoom lens that came with your camera offers a focal length in the region of 18-55mm, giving a wide view at the 18mm end and a standard-ish view at the other – depending on the camera it’s attached to.

A good tip for choosing which new lens(es) to go for is to avoid duplicating your existing focal length range where possible. Take a look at lenses that offer either a wider or longer focal length, depending on the type of things you mainly take pictures of.

But do upgrade your kit lens as soon as you can afford to. It’s been built down to a price so that it can be bundled with cameras, and doesn’t feature the sharpest or brightest glass around.

You’ve mentioned focal length three times. That’s how long a lens is, isn’t it?

Not exactly, but without getting caught up in the magic of physics, think of it this way: focal length has an effect on how much of a scene you can see through the lens. Wide-angle lenses have short focal lengths (such as 10mm) and a wide-angle of view – hence the name.

Some of the widest lenses available can take in almost a 180° view of the scene, and with a lens this extreme you need to make sure you don’t accidentally include your own shoes in the frame.

As the focal length of the lens increases, this viewing angle decreases, to the point where long telephoto lenses of 400mm and above only take in a very narrow view of a few degrees.

OK, so how does a standard lens fit in?

A standard lens for a full-frame digital SLR is 50mm, and it captures roughly the same view as the human eye. However, the smaller ‘APS-C’-sized sensor inside the majority of DSLRs is around 1.5x or 1.6x smaller than a full-frame one.

Subjects appear bigger in the picture, a bit like if you’d cropped a shot in Photoshop, and to get a similar result with a full-frame DSLR you’d need to use a longer lens.

So, a 50mm lens fitted to a regular SLR actually gives you a ‘full-frame equivalent’ focal length of 75mm (50mm x 1.5).

To get the standard angle of view that a 50mm lens gives you on a full-frame DSLR, you need to use a 35mm lens on most other cameras (35mm x 1.5 = 52.5mm).

Nice maths. So, what’s the point of a standard lens?

Pictures produced by standard lenses have a natural quality that lets a viewer concentrate on the subject of the photo. A 50mm prime – the ‘nifty fifty’ – is a great value all-round lens, ideal for portraits, street photography, still lifes and more."

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