Let's face it, we're all here on eBay looking for a bargain. We want the best for the least, or at least better for less (than retail). And every seller out there is trying to convince you they have just what you need. Unfortunately, not every older, used lens is a bargain, even though it may be billed as one. I'll go over some situations where older isn't better, and then offer some specific examples. As with every guide (especially mine) this advice (OPINION) is mine only, there will be another guide that tells you the opposite, so your job is to weigh the facts and make your own decision.

First, consider the age of the item. The Canon EOS system of cameras and lenses dates back 20+ years, which is a long time for modern technology. How happy would you be with a PC made in 1987? Of course, a lens isn't a PC, but the point is that technology is constantly improving, and offering more bang for the buck. In lenses, that translates to new optical designs, better manufacturing processes, newer materials, and more consistency between samples.

In addition to changing technology, what about the effects of aging on a lenses' parts? Moving parts means friction, wear and tear, invasion of foreign material like dirt and dust, plus decomposition of some materials. Ever see a 10 year old rubber band? Or a 15 year old tire? Not quite the same as new, are they? Over time, some flexible materials just decompose. Another problem common to older lenses (although not EOS lenses as much) is that the internal lubricants start to break down and spread into areas they don't belong (like the aperture blades).

Sometimes, evolving technology translates into newer, lighter, better materials. Polycarbonates and other high-strength plastics have replaced may parts once made from metal. Folks selling older lenses want you to believe metal is better and stronger. Conversely, though, these modern plastics are often many times stronger than steel  in comparable weights (by weight, the plastic in a lens mount is 5 times stronger than steel), and so the resulting lens can weigh much less than the old model. Newer optical materials can lead to similar savings in weight and better designs through the use of more refractive materials and potentially fewer elements. Recognize that every glass surface inside a lens causes a tiny amount of light loss/dispersion, and the value of reducing the number of surfaces actually starts to make sense!

Finally, as things age they wear, eventually to the point of failing. The focus or zoom mechanism of a lens wears a bit every time it's used, and will eventually lose tolerances. Electrical components like capacitors and motors have limited lifespans. Couple that with the limited period of time that spares are kept, and a lens' age can become a liability. There are many, many lenses that Canon no longer repairs because there aren't just any parts for them. Go down to your local Honda dealer and try buying a new seat for a 1990 Civic!

Enough gloom and doom. Let's talk about some specific lenses.

100-300mm f/5.6 - often compared to it's big brother (100-300L), this lens is not fast (f/5.6 throughout), and its' focusing speed is 3 to 8 times slower than its' modern counterpart. The newer lens is a stop faster on the short end, 37% shorter when fully extended, weighs 20% less, has 2 fewer elements (4 fewer glass surfaces), and costs 10% less than the original. The newer version came out in 1990, so any copy of the older lens is at least 17 years old. Oh, and the old lens is a push-pull design, so it works a lot like an air pump, efficiently sucking in dust particles every time you zoom out (and although dust seldom effects image quality, it does get trapped in tight places and lubricants). And the newer lens has internal focusing and a non-rotating filter ring, useful if you use a polarizer or graduated ND filter. Plus, it focuses to within 54 inches instead of 72 inches for the old one. And finally, the new version has ring type USM, which mean you have full time manual touch up, even in AF mode.

50mm f/1.8 - the almost legendary "plastic fantastic". The older version had a metal mount, distance scale, and a wider focusing ring. Amazingly, replacing that metal cut the weight of the lens by a full third. Today, you can spend $150 or more for the older one (discontinued in 1990), or buy a brand new one for $65-75. Same optics, same resulting image. But if you plan on needing a really strong mount for a lens that sticks out a whopping inch and a quarter from your camera body, then maybe the metal one is for you. In any form, I highly recommend this lens, either version. It's by far the best value in Canon's line.

70-210 f/4 - nice constant f/4 aperture, and no real modern replacement unless you want to spend $500+. Like the old 100-300, slow to focus, push-pull design, and old. Given a choice, go for the later 70-210 f/3.5-4.5 USM version, which overcomes most of those problems, except it has 3 more more elements. And when you hear somebody saying how this lens is so much like the modern 70-200 f/4L, just close your eyes and chant "there's no place like home", cause it's nothing like the L lens, except superficially on paper. Sadly, some of these guys actually believe their own ad copy.

35-105mm - this lens came in two versions, and older push pull and a newer USM. Don't get suckered into to paying as much for the older, slower one because somebody tells you it's better. It ain't!

24-85mm - this is a great little lens, well worth the $200 or so it will cost you used. Don't get fooled by claims that the "champagne" version is better, it's just painted silver to match the Canon IX APS SLR it was sold with. Same lens as the black, exactly. The only drawback to this lens is that it's one of only 2 or 3 Canon lenses that uses 67mm filters, so those 58mm ones won't fit. This is also a pretty new lens, dating back to the late 90's.

Kit lenses (28 or 35 to 70, 80 or 90) - Folks, these are the lenses that they box with the cameras when you buy them. There are no secret Roman numeral combinations or models that are significantly better than any other. They all have inexpensive arc-motor or micro-USM focusing, 5 aperture blades, and 9 or more elements. They are suprisingly decent lenses, but don't get wrapped up in looking for one with a flower on it or a Roman number IV, it don't REALLY matter!

In closing, before you buy an old lens, do your research. Look at reveiws on sites like photo.net or fredmiranda.com. Research the specifications in the Canon Lens Museum (google those three words and you'll find it). Make your buying decisions based on facts, not eBay auction descriptions. And remember, in the end your pictures will improve a lot faster if you spend time taking them instead of collecting equipment (like me!).