One of top-gifts the Christmas-2007 was several units from same group: the DSLRs. I mean, you know why. In this year prices for entry-level DSLRs cross a border of $500 (downsides) and this fact extends a number of happy DSLR-novices. A big advantage of using a SLR or DSLR camera is the ability to swap lenses to fit the job. Almost as sure as a fate a DSLR-newcomer understands them he needs something else then standard 18-55 mm lens from a kit. Some wants “faster” lenses, some needs more sharpness, etc. It is not a secret, lens’ quality vary in different samples. Canon and Nikon (together with third-party manufacturers like Sigma or Tamron) are now making cheaper lenses available. This is good news because you can now buy these lenses at a lower price. The bad news is these cheaper lenses generally have a poorer build. They made from plastic to reduce their weight and price, but lost some part of quality. For most of amateurs such quality is enough good, but what to do when you want move up to [almost] professional grade?
Another hand, a typical photo amateur has a limited budget and therefore hi-class new lenses are inapproachable because of their price. On eBay or in professional shops you can found good used lenses from old film cameras. May be they nave no auto focusing and embedded shake reduction, but they was produced from glass and steel (not from polycarbonate!) in Japan or Germany (not in Vietnam or Turkey!). Out of them, these lenses are unreally cheap. But second-hand devices may have any condition from “like new” to “awful”. How to found a lens without problem? How to check a lens before buying? I will try to answer these questions in an article.
External viewFirst of all, evaluate a lens outward. ScratchesFirst of all look on contacts backside. If they have some scratches, then this lens was in use. 
Such view have contacts after about 2 month. Contacts looking "like new" 
Such lens after about 1 year. Contacts have good visible scratches. But clear contacts are not a warranty of a new lens: when a lens was rarely or never detached from a camera, these parts, naturally, stay intact. Then look on focusing and zoom rings. Scratches and chafes say enough. Impacts?Just looking for accidents: indents on metal parts, especially on a filter ring, flaws on plastic details etc. 
A filter ring. A downfall calls elements shifting and such lens needs at least a good repair, or, in worth case unworkable at all. In lenses which have taken a good knocking, the zoom action may be damaged. In such cases, the lens may not be able to achieve its full zoom range. Just twist the zoom ring. Make sure the lens is capable of achieving its complete zoom range. If it cannot do so, then the zoom action is damaged. Repaired?Good and qualified service is not a problem. But scratches around screws and especially broken screw slots say: this device was in non-professional repair. 
This screw has never been removed 
This lens defenitely was repaired with wrong hands Missing parts?A lens usually comes with the following: Front lens cap Rear lens cap Hood Manual/documentation Warranty card Box In some cases, the package even includes a hard and/or soft lens case. The most common items to get lost are the manual/documentation, the warranty card as well as the box. When a lens is 10 years old, you don’t need a warranty card anymore. The lens caps as well as the hood are also common items to go missing. Missing caps may be a problem because they protecting front and back lens elements. Naturally, no cap – no protection. You can buy generic lens caps but generally, they do not look quite as nice or work as well as the original lens caps. Replacing it will not be cheap as original front lens caps can set you back anywhere from $10 to over $100, depending on the size and model. The rear lens cap is actually less of a problem than a front one. Yes, it is important to protect the rear lens element from scratches. But because you will always have one lens attached to the body, you need for N lenses (for the same mount) just (N-1) rear caps. Glass componentsLook on glass surfaces for scratches and spadings. For better viewing you will need a bright lamp. Place a lens on an angle to getting a dark background. Probably, you will found enough bullshit: dust particles, air bubbles inside glass parts etc. Don’t worry: not all defects are really fatal. In fact, small scratches and air bubbles on a front lens almost not affected on image quality. But same defects on a back lens much worth. Why? They are nearer to sensor and affected more. If the lens has scratches that affect picture quality, you should think twice about buying it. Repairing it may cost you quite a bit of money, unless you can buy the replacement element cheaply in online auctions. If the scratches do not affect picture quality, you can still use that as a tool to help you bargain down the price of the lens. 
Some dust inside is an unavoidable evil: even in hi-end lenses with sealing this is just a question of time, especially with telephoto zoom lenses and push-pull zoom lenses. You can check for it by looking through the lens at a bright source of light. You will need to flick the aperture pin to open up the aperture. But before you check for dust inside the lens, please clean off the dust on the front and rear elements! Otherwise, you may mistaken those for dust inside the lens. Dust near the front element are less of a problem than dust closer to the rear element. But this is normally not a problem unless there is a lot of dust. Then, you may notice a loss of contrast. The Lens Fungus is the death verdict for lenses. It is hard to clean off and will often recur. Even if it is successfully removed, the lens elements may be irreversibly damaged. Fungi have been known to destroy the glass coatings. So, even a good cleaning may not restore the lens to its original condition. 
Lens fungus may escape detection for a long time because it usually starts from the inner wall of the lens barrel. Even when it infects the glass elements, it starts from the side and grows inwards with a fine mesh-like growth. Image quality will not be affected initially. In fact, unless you look closely with the aperture wide open, you may not notice it. 
Mechanical partsFocusing and zoom rings must moving smoothly and without of any jams. But when the rings moving too lightly, the focusing may be displaced. In some lenses, the focus and zoom rings may become a little tight, especially if the lens has been put in storage for a long time. Dust can also get lodged in the threads. Then you will get a gritty sensation when you turn it. In these cases, you will need to send the lens in for cleaning and lubrication or, (IMO) forget about this lens and looking something else-. When a lens is placed upright (in both directions), the optical block must not be moved yourself under its own weight. Some “long” telezooms have special latch for this purpose. Pay attention on a bayonet: a lens must sitting on a camera firmly. Then you must to check aperture blades. The aperture blades to stick to each other. As a result, the aperture blades will either open or close slowly, or part of the aperture may not open or close at all. The end result? Wrong-exposed photos. You can take a good look at the aperture blades through the front elements. Just set the aperture as small as possible and press and release the DOF Preview button. You can see the aperture blades snapping close and open again. They should all snap quickly and simultaneously. If they appear to be sluggish or if some blades do not open or close properly, then the lens has sticky aperture blades. Electronic First of all: when a lens has auto focusing and/or other hi-tech features which connected to the camera body with contacts, check these components with your camera. Sometimes (not often, fortunately) lenses are not recognized with camera and on attaching you will see an error message. It may be a system failure in camera’s software or problem with contacts. You must check: Auto focusing in automatical mode (AF) Auto focusing in manual mode (MF) – focusing confirmation Aperture In the next future I'll prepare more articles about this theme: "Lens' Runtime Tests" Last update: 12-03-2009 16:44
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