I’ll use this as an example lens:
Nikon 55-200MM F/4-5.6 AF-S VR DX
First of all, compatibility.
The good news is, you don’t need to know anything technical or memorise lists of codes – nine times out of ten, the lens name will tell you. Canon and Nikon don’t make lenses for each other; you don’t get lenses made by Canon for Nikon lens mounts, or vice versa. So if it says ‘Canon ...’ or ‘Nikon ...’, it’s for their own cameras. Third party manufacturers make lenses for all mounts; however, they will specify ‘Canon fit’ or ‘Nikon fit’. AF-S is the mount code, but as I said you don’t need to memorise long lists of them. Nikon lenses are easy – if it says Nikon, it will fit. Canon is trickier, but only slightly: you need to remember EF. There are two variants, EF (full frame mount) and EF-S (crop sensor mount since 2003.) EF lenses will fit EF and EF-S mounts; EF-S lenses will only fit EF-S mounts. If you go for a Canon, it will in all likelihood be an EF-S one and therefore any lens that has EF in the title will fit.
List of Canon EF-S cameras
This is focal length; I could go into the physics and optics of focal lengths ... but you need a working knowledge, not theoretical physics knowledge. Put simply, the further away the thing you want to focus on is, the bigger the biggest number has to be. It gets a little bit complicated by size of the object – far away from you but huge needs a shorter focal length than far away and tiny – and landscapes mess it up completely (you actually use short focal lengths, I often use around about 18mm, because the subtle difference is you’re not focusing
on something far away, you’re just trying to keep a long distance in focus
. It’s a depth of field task instead, so short focal length and small aperture is best. The point you actually focus is often quite close to you.) Lenses with long focal lengths are telephoto lenses, lenses with short focal lengths are wide angles – the exact point where they become telephotos or wide angles is not entirely clear.
Focal length has an effect on object size and field of view. Telephotos magnify images relative to where you were in relation to the object – the photo of the little bird won’t be the dot that you saw, it will take up the photo if you used the right focal length. Wide angles will minify – if you took a photo of an object with a wide angle, it will appear as though you stood further back when taking it. Telephotos have a narrow field of view; it can pick out the bird, but won’t capture much of the tree surrounding it. Wide angles do the opposite, they get a lot of the scene, sometimes more than we can actually see – I’ve take a few images where I was unable to see my feet in reality, but in the photo my feet are at the bottom of it.
The smaller number listed is the minimum focal length – in this case, 55mm – and the larger one is the maximum focal length – 200mm here. If you are using a camera with a crop sensor (basically not a full frame camera), you get something called crop factor, which means that whilst the actual focal length is as stated, the effective focal length (what it appears to be, and the focal length you would need to reproduce the photo on a full frame camera) is larger – in fact, the crop factor larger. For example, Canon consumer models have crop factor 1.6; if I was using an actual focal length of 100mm, it would appear to be 160mm – 100 x 1.6.
Where two numbers are listed, it is a zoom lens; to get the amount of zoom, divide the larger number by the small one – here, the zoom is 2.6. Some lenses are prime lenses – they have a fixed focal length. In that case, only one focal length would be listed (50mm is quite a common one, there are plenty of others) and obviously there is no zoom.
These are apertures, or at least they are the biggest apertures that can be achieved – bizarrely, a small numbers means a big aperture, and vice versa. The lens only affects the biggest aperture, you will be able to get the smallest aperture the camera is capable of on any lens. Where there are two numbers listed like this, it means the biggest aperture varies with focal length – you can get an f4 at 55mm, but only an f5.6 at 200mm, and some number between the two at focal lengths in between. Some lenses are called constant aperture, or the biggest aperture does not vary with focal length.
Maximum aperture size can be a reasonable indicator of how good a lens is in low-light. Say I am able to hand hold a camera at 1/50th of a second (just written as 1/50, commonly) – this is the slowest shutter speed I can use without getting motion blur. If I can have an aperture of f2.8 at this shutter speed, I need less light in what I’m photographing than if I were using it with an aperture of f5.6, because f2.8 is bigger and lets more light in. For event photography, this may matter to you. Good low light, you need f2.8 or better really. An f4 is average – not the best, but you could do worse.
The extra bits.
These are very variable between lenses, and also between seller – some list things that others don’t. They are numerous and I won’t list them all, but I will go through a few of the common ones:
VR = vibration reduction. It’s Nikon’s name for what Canon calls image stabilisation (IS), Sigma calls optical stabilisation (OS) and Tamron called vibration control (VC); it means that the lens can compensate for some shaking when in use. Whether or not you need it depends upon what you’re photographing (if everything you shoot will be at 1/1000 shutter speed, it’s not going to matter; conversely, if it’ll always be mounted on a tripod, IS can actually be detrimental) and how steady you are (the general rule is you can hand hold with the shutter speed of at least the effective focal length; for example, if you are using an effective focal length of 50mm, you can hand hold at 1/50 at the slowest – the knock on effect being shaking is more important for longer focal lengths. But that is only general; I have very shaky hands and stabilisation is important to me, unless it’s in a tripod lens. Some people are very steady and can hand hold at slow shutter speeds.)
USM = ultra-sonic motor. You may see this on some Canon lenses; it basically means the focus motor is quieter than normal. Can be useful if you’re photographing skittish animals and so forth.
L = Canon’s ‘luxury lens’ range – very good optics, but also very expensive, I wouldn’t worry about this yet.
DX = Nikon incorporate this to signify a lens that was designed for a crop sensor, so that people buying for full frame (FX) will know that it may cause some problems. EF-S is used to signify this in Canon, with the addition of it actually won’t fit a non-crop camera.
If you ever come across one you don’t know, just Google ‘[code] lens marking’.