An Introduction to Lenses

Focal Length

The first thing you’ll notice about lenses are their focal lengths. In physics terms, this means how far the lens comes into focus from the image sensor. You don’t need to know this, but it might impress people! What you need to know in practice is that a smaller focal length will give you a wider view, and a larger focal length will give you a tighter view. Seen here with the same scene:

Lenses - view from a 35mm lens
Scene taken with a 35mm focal length lens.
Lenses - shot taken with a 200mm lens.
Same scene, from the same viewpoint, shot with a 200mm lens. Both taken on a full frame camera.

Crop Factor

Focal length can be a bit misleading though, as it differs depending on the sensor size. The smaller the sensor the smaller the focal length needed to achieve what looks “normal”, and vice versa. E.g. on a 35mm/full frame DSLR – “normal” is 50mm (roughly) on a 5×4″ large format camera, it’s around 150mm, and much smaller for cropped sensors and camera phones. For the compact we’ve been looking at with it’s tiny sensor, “normal” somewhere around 12-15mm. 

In practice this means that although, say, the same lenses can fit a full frame DSLR and a smaller sensor camera from the same manufacturer, if you put a 50mm lens on a smaller sensor camera, depending on the size of the sensor you’ll crop into the image a certain amount. This is called the “crop factor”. In this case, with the Nikons I’ve used in the past, it’s 1.5x, meaning that a lens designed for a full frame camera becomes effectively half as long again when put on the camera with the smaller sensor. It’s very important to understand though that doing so doesn’t actually make the lens longer, it just crops into the centre of the image the lens projects:

Crop factor
What happens when you put a lens designed for a larger sensor on a camera with a smaller sensor. The sensor only “sees” the middle part of the image, and therefore it appears that you’re shooting with a longer lens. The image is “cropped in”.
Image Sensors
Different size sensors – “full frame” top left, and “cropped” bottom right.

This effect can make it appear as if you’ve put a longer lens on, and in practice a 200 mm lens will now look like a 300 mm lens (for example), but in true scientific terms you haven’t actually made the lens any longer. This is vital to know if you ever get drawn into an argument on an internet forum, not that I’d ever advise you to get into an argument on an internet forum – you’re far too sensible for that!

Prime and zoom lenses

Sticking with focal length, there are 2 main types of lens – a prime lens and a zoom lens. A prime lens has just one focal length, and a zoom covers a range of focal lengths. Common zoom ranges are 24-70mm, 70-200mm, and 18-35mm, although they can be made to cover almost any range. At first glance you may think “I’ll just buy zooms, they’re much more versatile”, and I use them all the time, but it’s not quite as simple as that – it never is, is it? Here are the pros and cons:

Prime – faster maximum aperture. Highest image quality. Generally smaller and lighter. Often cheaper than a decent zoom, although fast ones can get pricey

Zoom – range of focal lengths, so more versatile in one package. Decent ones can be expensive, and also very big and heavy. Optical quality great, but usually loses out to primes under testing conditions.

So there’s a place for both in your bag. As a guide, speaking as a working pro, I use 3 zooms regularly, with another for very occasional use, and 3 prime lenses as often as I can. Personally I prefer shooting with primes, for all the advantages listed above, but in practical terms when I’m on a job and am short on time, zooms are often a better option. If you’re just starting out, I’d get a standard zoom (one probably came with your camera) and then look to buy a decent prime when you can. A 50mm is usually a good start – f1.8 versions cost around £100, and will be much faster (we’ll come onto that in a bit) and better optical quality than your zoom lens.

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