The most obvious number when looking at lenses is the focal length, given in millimetres. It tells you what field of view the lens will have. A shorter focal length will get a lot more in view, usually good for landscapes or some street photography. A longer focal length gets you ‘closer’ to things that are further away, usually ideal for sports and wildlife, or anywhere you want to pick out details in a scene.
Things are fairly simple when you’re thinking of a ‘full frame’ sensor, but if the sensor is smaller (or bigger, if you’re spending a lot on the camera!) the same focal length will give a different result.
Cameras that aren’t full frame, and have a built-in lens, will often show the focal length as ‘equivalent’, which isn’t really the focal length of the lens. But it does tell you what field of view it gives compared to a focal length on a full frame camera. And most of the time, that’s what you really want to know - how wide is the view? You need a number you can compare with other cameras and lenses, not a heap of complicated calculations to do based on the focal length and sensor size.
Common Focal Lengths and What They Mean
There’s no reason for lenses to use any particular focal lengths, but they commonly do. Certain numbers are commonly used, others not so much. And they’re usually rounded a little. So you often see lenses with focal lengths of 24mm, 28mm, 35mm, 50mm, 85mm and 135mm, but you don’t tend to see 33mm lenses or 51mm lenses.
If we’re talking about a full frame sensor (we’ll talk a bit more about how sensor size changes things soon) a 24mm lens is quite wide angle. You get a lot in the shot. 50mm is often referred to as a ‘standard’ lens. People often say it’s closest to what our eyes see, though I’d tend to think of that as a bit wider, perhaps more like 35mm. Years ago, most cameras came bundled with a 50mm lens, and they’re very commonly available. They’re relatively easy and cheap to make, too, which makes them cheap to buy, new and used. Most phone cameras are around 24mm equivalent, so people are much more used to wider angle shots now than they were years ago.
The Effect of Sensor Size
If the sensor is full frame (the same size as a 35mm film frame (and the 35mm there is to do with the size of the film, not anything to do with focal length!)) things are simple - it’s how we normally think of lenses. 24mm is wide angle, 35mm fairly normal, 50mm either ‘standard’ or a little long, and 85mm and up are a bit ‘telephoto’.
Things get more complicated when you use a smaller sensor, though. Most lower-cost cameras, even those with interchangeable lenses, like lower end mirrorless cameras, most Fujifilm cameras, or Sony A6000 series cameras, use smaller sensors. Those use a sensor size called APS-C. It’s for pretty much historical reasons, but then so is the size of the ‘full frame’ sensor.
Full frame comes from the size of 35mm film, as mentioned, and APS-C comes from APS film, which was intended to replace 35mm for a while, but didn’t really catch on all that well.
The simplest way to compare sensor sizes is with the ‘crop factor’. An APS-C sensor has a crop factor of 1.5. Or 1.6 if it’s Canon, because they use a slightly different version of the APS-C sensor that’s a little bit smaller, though not by enough to make much practical difference. If you know the crop factor, you’ll know the effect it has on the focal length.
Put a 50mm lens on a camera with an APS-C sensor, and it’s still a 50mm lens. That’s a physical feature of the lens, and can’t change. But the field of view you get through it is the equivalent of what you’d see through a 75mm lens (50mm x 1.5). The lens doesn’t change, but the sensor is smaller, so you see a smaller part of the view through the lens.
It’s called a crop factor, because it has the same effect as if you’d cropped the image from a full frame sensor.
Another common sensor size is Micro Four Thirds - they have a crop factor of 2, so the same 50mm lens is now the equivalent of a 100mm lens. A ‘standard’ lens has become quite ‘telephoto’.
For smaller compact cameras, without interchangeable lenses, they focal length they quote is often the ‘35mm equivalent’ length, because otherwise, it only makes sense if you know the sensor size, and then do some maths. Nobody wants that.
The word ‘telephoto’ usually just means a lens with a long focal length. 85mm could be described as a telephoto lens, but at the short end. A 200mm or 300mm lens is a relatively long telephoto.
Technically, telephoto means a lens whose physical length is less than its focal length. My Leica Tele-Elmarit lens is a 90mm focal length, but the lens itself is shorter than that. It’s not how the term is used now, though, so there’s no point in using it ‘correctly’. Nobody cares (well, maybe Leica do) and you’ll just end up sounding like the annoying kid who tries to correct people who say ‘battery’ by explaining that a single one is actually called a ‘cell’. Yes. That was me. I was that twat. Sorry. I got better.