Let’s face it… understanding lens names is difficult at best and downright frustrating sometimes.
For instance, take the new Sigma APO 70-200mm F2.8 II EX DG MACRO HSM. EX? DG? HSM? APO? What the heck does all that mean? And how does it differ from the Canon Canon EF 70-200mm f/2.8 L IS USM? What about the Canon EF 70-200mm f/4 L USM?
Ultimately, lens naming comes down to one basic idea:
Every letter counts.
Manufacturers try to cram a lot of information into lens names and that can make them very confusing. The key is to realize that small distinctions and differences in name can mean a very different lens. Not always, but sometimes.
The first method of distinguishing lenses is focal lengths, an optical feature of the lens that determines how the world will look through the viewfinder. Anyone familiar with photography knows that longer focal lengths means more ‘zoom’. More recently, lens makers have been specifically choosing focal lengths to help distinguish lenses. For instance, the Canon EF 100mm f/2.8 USM Macro, Sigma 105mm F2.8 EX DG MACRO, and Tamron SP AF90mm F/2.8 Di 1:1 Macro all are meant for the same purpose (medium telephoto 1:1 macro lens), but have been given slightly different focal lengths to prevent confusion.
The second major way to tell lenses apart is aperture, or the amount of light a lens allows onto the film or sensor. Typically, larger apertures (smaller numbers) mean a more expensive and high quality lens. For instance, the Canon EF 50mm f/1.8 II costs about $85 on the used market while the Canon EF 50mm f/1 L USM is rare and sought after and goes for more than $4000! Two lenses with the same focal length(s) and different apertures are going to be very different lenses.
Furthermore, lens makers usually have a ‘pro’ line that has better optical and build quality and fetches higher prices. Canon has the “L” series, Sigma has the “EX” series, Tokina has the “Pro” series, etc. Usually, these designations means something specific about the lens; for instance, the Canon “L” designation means that the lens has aspherical elements, but often the “L” lenses include other premium features like USM, weather sealing, etc.
Other letters in the names of the lens can designate specific mounts/compatibility (DI, DI-II, EF, EF-S, FD, etc.), autofocus motors (HSM, USM, AF-S, etc.), image stabilization (IS, OS, VC, VR), focus construction (IF, RF), optical elements (APO, DO, ED, UD, ED, SLD, etc.), coatings (S.C. and S.S.C. for those Canon FD enthousiasts), and even the number of optical elements (early Nikkon lenses had a dash and a letter (see table) that denoted the number of optical elements). It pays to pay attention to the specifics, because typically a little difference in letters can either mean a different lens or a mistake when listing the lens model. This can mean a huge difference in value, and you don’t want to be on the wrong end of an eBay listing mistake!
Finally, it should be noted that some manufacturers release different versions of the same lens and designate the difference with roman numerals, sometimes preceded by Mk or Mark. For instance, the Canon EF 28-80mm f/3.5-5.6 V USM is the last lens of somewhere between 6 and 8 versions! Sometimes the new version is better and fixes problems with the previous version; sometimes the new version is worse that its predecessor. For instance, the Canon EF 50mm f/1.8 I sells for 60% more than the Canon EF 50mm f/1.8 II because the first version has a metal mount and sturdier construction than the second version. Typically, budget lenses tend to have more versions because sales volume is larger and lens makers are constantly striving to reduce costs. So, it pays to be aware of differences in versions!
Your best defense when looking for a used lens is to be aware of what versions exists, what the differences are, what typical prices are, and what they look like. Especially what they look like. I have gotten some deals on lenses since they were mis-listed and the seller did not know what they were selling, but I’ve also seen budget lenses mistakenly listed as more expensive lenses!
The UCDB Lens Database is a great way to compare lens prices and versions. Currently, there are 135 lenses in the database, every single Canon-made EF and EF-S lens I could find. So if you’d like to compare versions and prices, the database is a great place to start!