Buy a compact camera or a lens for an SLR and there are numbers involved which have little to do with the price. I was surprised to learn that many amateur photographers don’t actually know what "24-70 mm" means in real terms, so I thought I’d start this thread with some explanations in the hope that other, more knowledgeable PB members can add to it.

(By the way, this post is for people with normal salaries looking at compacts and SLR photography. If you’re in the rangefinder / medium-format stratosphere, you can probably hire someone to explain their values to you.)

What it does

Focal length determines the angle of view. The longer the focal length, the narrower the angle. Longer focal lengths also give the impression of compressing distances between subjects at a distance (known as “foreshortening”), which is why portraits are usually more successful with longer lenses.

In contrast, very short focal length lenses (fish-eye, ultra-wide and wide-angle lenses) distort the edges of the shot (and make portrait subjects look like Homer Simpson).

What it is

Simply put, focal length is the distance in millimeters between the focusing part of the lens and the film or sensor in the camera. Most of us now use zoom lenses which vary that length. (Incidentally, avoid that spawn of the devil, “digital zoom”, which merely amplifies a magnified optical zoom image with crap quality results.)

Below is a vague scale that might help.

Under 24 mm: fish-eye and then ultra-wide angle - good for interiors, architecture, single-shot panoramas, etc.
24-50 mm: wide angle – like the above, only less wide
50 mm: “normal” – equates approximately to what the human eye sees and is a very popular focal length
50-70 mm: slight tele – good for three-quarter portraits
70-130 mm: known as the portrait length
130-200 mm: standard telephoto
200-500 mm: long telephoto – often used by nature photographers and sports photographers.
Above that lie the custom order lenses which cost more than cars.

There are, of course, fixed focal-length lenses (known as “prime” lenses) which usually offer slightly better image quality and greater apertures in the same price range, but don’t have the flexibility of zooms. Good macro lenses, for example, are always prime lenses.

Most compacts offer a range between 28 and 130 mm that covers most photographic purposes. Some “bridge” cameras offer a “superzoom” range of 24-400+ mm, although I doubt the quality is very good at either extreme.

The dreaded crop factor

These values are given as if your compact / DSLR used 24x36 mm film. Read the sales literature carefully and you find the word ‘equivalent’ or ‘rated’ somewhere. Unfortunately, very few DSLRs and almost no compacts have a digital sensor as big a film exposure (they’re known as “full-frame” cameras and tend to be bought by professionals). Usually, the sensor tends to be 1.3, 1.5/1.6 or 2 times smaller than the film equivalent on a DSLR. Compacts tend to have sensors the size of a thumbnail.

The main effect of a smaller sensor is to multiply the focal length by its “crop factor”. For example, most Nikon DSLRs have sensors with a 1.5 crop factor. This means a 50 mm lens on a D60, for example, behaves like a 75 mm lens on a film or full-frame camera. The practical upshot is cheaper and longer telephoto lenses and harder and more expensive wide-angle ones. There is little point discussing the crop factor on compact cameras because (with very few exceptions) you can’t change the lens anyway.

Mutatis mutandis

You can change a lens’s focal length by placing something between it and the camera: an extender, an extension tube or bellows.

An extender multiplies the focal length but reduces the maximum aperture. If you add a 2x extender to a 200 mm lens, it acts like a 400 mm but requires four times as much light and image quality usually suffers. This is a lower-cost way of getting longer lenses.

An extension tube reduces the minimum focusing distance and increases magnification (also at the cost of image quality). This is a cheaper way of taking macros.

Bellows do the same as extension tubes, but you lose focusing, aperture and exposure control.

Tentative buying guide

The kit lens that comes with many DSLRs and compact cameras usually offer a film/full-frame equivalent range of 30-130 (with exceptions), which covers most situations.

Remember always to think in film focal length terms.

Remember also to look at the maximum aperture (the other string of numbers). A long, very slow (i.e., one requiring a lot of light) lens is practically useless.

Avoid any lens that promises to do everything. If it does honour its promise, it does so poorly. A good rule of thumb is that a zoom lens with a zoom factor greater than 10x is going to disappoint (by being slow, soft and prone to flaring, distortion and colour fringing).

Camera manufacturers try to lock you in to their brand by making it (almost) impossible to fit, say, a Canon lens on a Nikon body. However, some lenses are incompatible within the make; Canon EF-S lenses do not fit full-frame cameras.

Third-party manufacturers usually offer cheaper alternatives to name brands, but you usually get what you pay for. Unless you’re very dissatisfied with you kit lens (and need to change that first), I’d start with a good wide-angle or telephoto and then consider a macro lens.

A bit of nit picking, but it just caught my eye ;-)

First of all, regarding the focal length: in description of "50-70 mm: slight zoom" or "130-200 mm: standard zoom" I would use "tele" instead, as "zoom" is wrong word here. Also I'm not sure if "24-50 mm: wide angle" is correct, usually 35-50 mm is considered as "normal"? Another thing - 2x extender loses 2 aperture steps, therefore it requires four times more light.

Now, I would like to add a couple of small things if I may:

1. Crop factor affects DOF the same way it affects the focal length, so 50mm f/1.4 lens mounted on Olympus camera will have 100mm focal length equivalent and f/2.8 DOF equivalent.

2. With extension tube lens loses ability to focus to infinity, so it will become "macro only" lens.

@ Rokas:

Thanks for correcting my careless mistakes and typos and making this thread better. I appreciate it and the additional information you've been kind enough to add. I've edited the OP to take account of your comments.

A good introduction Stephan, just a philosophical point. Is it relevant to a young generation who might conceivably never have seen 35 mm film much less handled a 35 mm SLR to learn these archaic rules of thumb instead of the rules for the camera that they are actually using? Or is it just us old dinosaurs pontificating as we are inclined to do? Just wondering )

@ Brontosaurus from Diplodochus Stultus

When I'm lucky, I get questions like "what are those funny numbers on the front end for?". When I'm unlucky, I don't get those questions...

As you've noticed from the general content of my forum posts, I'm trying to pitch the level of technical information and thought I'd try here: amateurs and enthusiast compact users.

How am I doing?

Must go, have to rip a live goat asunder.