Having made it through a, c, d, and e (we skipped b), the photography fundamentals cruise ship's next port-of-call is f, where we'll be exploring focal length. There's a lot to take in here, so I hope that you're wearing comfortable walking boots.

Your camera's lens works by refracting the light from what it can 'see' and converging it at a single point, the focal point, to create an image. Just how much the lens is able to 'see' depends on its focal length.

The distance between the lens and the imaging sensor (or film, if you're old school) is what determines focal length. It's measured in millimetres.* A shorter focal length means a wider angle of view, so the lens 'sees' more. These are called, somewhat unoriginally, wide-angle lenses. By the same token, increase the focal length and the angle of view decreases and pulls you closer to your subject. Lenses with larger focal lengths, roughly over 135mm, are known as telephoto lenses. Those in between, around the 50mm mark, are 'normal' lenses.

Here you have a wider angle of view that takes in more of the scene.

A narrower angle of view draws you closer to your subject.

* Long lenses don't really measure 20, 30, or 40cm; they create the telephoto effect by a complex series of elements thatwork together to create that same effect.

The crop factor

When we talk about focal length, we're usually doing it with reference to a 35mm, or full-frame, sensor camera. If you're using a camera that has a sensor smaller than full-frame, for example a Nikon D5100 with an APS-C sensor or an Olympus PEN with a Four Thirds sensor, it will have a magnifying impact on the focal length of your lenses. The smaller sensor is only able to detect a portion of the entire field of view, making it appear as if you're closer to your subject.

This is known as the crop factor. Multiply the sensor's crop factor (for example 1.5 for a Nikon APS-C sensor) by the lens' given focal length and you'll have the effective focal length. (It doesn't actually change the focal length, but it looks that way.)

This can be very useful when you use a telephoto lens, because your 200mm lens on your Nikon D5100 suddenly becomes the equivalent of 300mm; but it can be a bit of a pain when you're using wide-angle lenses as they will lose some of the impact of their increased angle of view.

So that we're all straight, I'm going to be referring to focal lengths as if they apply to a full-frame sensor.

Rough-and-ready guide to focal lengths

22-35mm - Wide-angle - Landscapes
35-70mm - Standard or normal - Street, documentary, some portraiture
70-135mm - Medium telephoto - Portraiture
>135mm - Telephoto - Sports and wildlife

Wide-angle lenses are good for...

Wide-angle lenses might be the go-to lens for landscapes, but there's more to them than that!

Emphasising the foreground

You’ve probably noticed that if you take a portrait with a wide-angle lens, the subject’s nose looks comically large and her or his chin juts out further than Brighton pier. Why? Well, the lens' wide angle of view means that in order to get in close enough to fill the frame with your subject, you have to get in very close. The closer that you get, the larger whatever is in the foreground will appear. As those objects are likely to be a nose and a chin, in a portrait, they are going to look huge compared to the rest of the face.

If you were to photograph a building close-to and from below, this same foreground-emphasising effect would make the base of the building look stupidly large and the entire structure ridiculously tall. Being close, the foreground is emphasised and the lens' wide angle of view gives the entire scene the illusion of somehow being stretched or exaggerated.

Enclosed spaces

When you're taking photos in an enclosed space, using a wide-angle lens will ensure that you can include everything in your scene that you want to be there. This is why estate agents use wide-angle lenses to photograph inside properties, so that the rooms can be shown in their entirety.

Converging verticals

Whatever lens you use, including your own eye, perspective means that parallel lines will appear to converge eventually. Just think of train tracks coming to a point as they streak away into the distance. When you use a wide-angle lens, this convergence happens faster than it would with a normal or telephoto lens.

You can use this convergence effect creatively, to envelop people in trees or give buildings an odd look. But of course, you might not always want your buildings to tip backwards or their parallel walls to converge. Standing further back from your subject and not tilting your camera upwards can help to overcome the problem, but sometimes this isn't always possible. I don't know about you, but I find that standing in the middle of a busy road to take a photo doesn't do me many favours. Thankfully, there are lens correction effects available in most editing suites!

Telephoto lenses are good for...

You might associate telephoto lenses with hardened sports photographers and weather-beaten wildlife photographers, but they're good for a whole lot more.

Normalising

Having a narrower angle of view means that the subjects fill the frame more; giving the impression of being closer together and closer in size.

Instead of objects in the foreground looking larger, as would happen with a wide-angle lens, telephoto lenses make their background objects appear comparatively larger. It's because of the narrow angle of view: things in the background take up proportionally more of the frame, which makes them look closer in size to things in the foreground. It's called the normalising effect.

This then is why portrait photographers prefer longer lenses, often around 85mm.

Density

By giving the impression of compressing scenes and bringing subjects closer together, you can use a telephoto lens to make it look as if more is happening in your image, and that it is somehow more dense. For example, a moderately busy street scene could become bustling when shot through a telephoto lens.

Layering

Classically, it's a wide-angle lens that is used for landscape shots, so that everything in a sweeping vista gets in the frame. However, I've taken some of my favourite landscape photos with a telephoto lens. They have this beautiful stacked look that can emphasise the different layers in the environment.

Depth-of-field

You often hear it said that telephoto lenses have a shallower depth-of-field. This isn't strictly true. It would be more accurate to say that because telephoto lenses are mostly used to magnify subjects, and the subject will then fill more of the frame, the depth-of-field appears to be shallower. Not that there's anything wrong with that; it allows you pick out your subject from a beautifully blurred background.

TL;DR

Focal length is the distance between the lens and the sensor
It is measured in millimetres
Shorter focal lengths produce wider angles of view, that capture large scenes
Longer focal lengths bring you closer to your subject and have a magnifying effect

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