Hopefully, if you're lens shopping you have a purpose in mind. Make sure you really think about what it is about your current lens(es) that is making you look for a new lens. What will the new lens be used for the most? Will there be other uses? What kind of lighting situations will it be used in? And are there features you want in a new lens?
This is usually the first part of the lens description followed by "mm" as in the bolded part here:
Canon 75-300mm f/4-5.6 USM IS AF
The smaller the number, the farther away your subject will appear and the more of your surroundings will fit into a shot. "Wide" lenses (10mm to 30mm) are often used for landscapes, "Normal" or "Standard" lenses are 30mm to 50mm and come close to what we see normally with our eyes. For this reason, they are often used for portrait work and as general walk-around lenses. Telephoto lenses are anything 100mm and up. They are used for wildlife, candids, and other situations where you want or need to take photos at a distance.
When you see two numbers like you do above "75-300mm" that means it's a zoom lens. These are the lenses most of us are familiar with. They'll make the subject appear closer or further away as we turn a ring on the lens.
Primes / Fixed Lenses:
If you see just one number, the lens is a "prime", otherwise known as a "fixed" or "standard" lens. These lenses don't "zoom". In order to make the subject appear closer or further away, you'll need to "zoom with your feet", in other words, move yourself closer or further away.
People choose primes because you can usually get better quality and "faster lenses" for much cheaper this way. (We'll discuss "fast" lenses later.) Usually people purchase something like a fast 30mm or 50mm prime for portrait work.
After the focal length there's usually a "f/" followed by a number or two numbers. The maximum aperture is bolded in the example below:
Canon 75-300mm f/4-5.6 USM IS AF
If you aren't using manual mode yet you can see some of my other articles for the specifics on aperture, but here's the quick and dirty -the lower the numbers after the "f/" the "faster" the lens is. f/4-5.6 is not all that fast, the example lens is fairly average. When you get into f/2.8 or even all the way down to f/1.0 with some specialty primes, those are what are considered "fast".
"Fast" lenses will give you 2 advantages. First, the lower that number, the better low-light performance you'll get. This is really why they are called "fast" -the lower your aperture number, the faster shutter speed you'll be able to achieve, so if you take photos of, say indoor sporting events or dance recitals a fast lens can help you avoid blur due to camera or subject movement.
The side-effect from using really low-numbered apertures like f/2.8 or 1.4 is a really shallow depth of field. So, if you like those photos with really smoothly blurred backgrounds and a sharp subject, those are done with small numbered apertures. This is very nice for portrait work, but when you are shooting indoor sporting events this will mean your focus has to be EXACT in order for you to get a sharp shot. Focus is very important because at these low-valued apertures your depth of field is so shallow.
This usually comes first in the lens description. In the examples above it was Canon, but I included a Sigma lens in the first paragraph of this article as well. Nikon, Olympus, and other camera makers obviously make lenses for their systems as well, but Sigma, Tamron, Tokina, and others are 3rd party lens makers that make lenses for at least Canon and Nikon.
Personally, I consider Canon and Sigma whenever I go to buy a lens. I own both makes of lenses and feel that in some cases Sigma is just as good as Canon, but it does depend on the specific lens you are looking at. I know other photographers who are happy with their Tamron lenses, but I've never found one of quality for my own use. No matter what the make, you'll need to do your homework before buying a lens because quality will differ greatly depending on the lens you are looking at -even with Canon! (...unless you buy all "L" glass.) If you are on a budget, though, 3rd party lenses are worth considering!
The Alphabet Soup:
After the basic lens description ("Canon 75-300mm f/4-5.6") comes a bunch of letters. I'll try to define as many as I can here:
- AF -AutoFocus
- EF or EF-S -"EF" means it'll fit any Canon camera wether it's full frame or a crop sensor. These lenses are required if you have a full frame camera (a Canon 1D or 5D). "EF-S" lenses fit only crop sensor cameras. These include Canon's Rebel lineup (XT, XTi, XSi, XS), and the 10D, 20D, 30D, 40D, and 50D's. The "EF-S" lenses are usually a little cheaper, but if you decide to move to a 1D or 5D someday, you won't be able to use them on your full frame camera.
- HSM -Sigma's version of "USM", it's an autofocus mechanism that's faster, focuses better than non-HSM in low light, and it's quieter than non-HSM lenses as well.
- IF -Internal Focus, these lenses will not have rotating front elements and they won't change length while you are focusing. This is a good thing for macro photography where if the lens moves while you focus there's a risk of the front of the lens hitting your subject as you focus.
- IS -"Image Stabalized", this means the lens will compensate for some movement as you take photos. This helps in low-light situations when camera shake often ruins photos. It's not magic, but it'll help give you a stop or two advantage depending on the lens. Some lenses have dual IS motors so you can turn on just vertical IS or both vertical and horizontal compensation. The ability to turn off horizontal compensation is nice for automotive photography since it can help keep panned shot sharp.
- "L" -If you see an "L" tacked on to the end of a lens description like "Canon 70-200mm f/2.8L" it designates the lens as one of Canon's pro line lenses. If it's a telephoto, it'll be white like you see on the sidelines of many sporting events. If it's a wide-angle lens it'll just have a red ring that designates it as an "L", but either way "L" lenses are generally of excellent quality, but are also very expensive.
- Macro -These lenses will focus closer than non-macro lenses. True macro lenses will give you 1:1 size ratios although some "macro" lenses won't do 1:1, so if you really want to get into macros, make sure you verify whether the lens you're looking at will do 1:1.
- MF -Manual Focus
- OS -"Optically Stabalized", Sigma's equivilent of the "IS" designation for Canon lenses
- Rotating front element: One thing to watch out for on cheaper lenses is a front element that rotates. This will make it difficult to use a circular polarizer filter on the lens.
- USM -"Ultra Sonic Motor", this is an autofocus mechanism made by Canon that's fast, focuses better than non-USM in low light, and it's quieter than non-USM lenses as well.
- VR -"Vibration Reduction", Nikon's designation for image stabalized lenses
More Sigma lens definitions:
Lens Test Data:
The final step in lens shopping is to look at comparisons between similar lenses. Google searches may result in many test shots, searches on Flickr can show you samples of what others have done with the lens, and these are really good things to do, but the best way to compare lenses is to look at the sites below: