Saw the tips from the website
http://www.nelsontan.com/articles/2ndhand.html and do a cut and paste on the lens section.
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How to choose a lens
Lenses are much easier to check than camera bodies, although the optical aspects can only be determined by shooting film. To check the optics, hold the lens towards a bright light source to check for dirt and debris. If the shop does not have bright lights, tell them you want to step outside the shop for a look. A bit of dust and dirt will not affect the optical performance much, and you can use it to drive the price down. Avoid lenses that are too dusty or dirty. Hold the lens progressively nearer and further away to check, because at too close a distance, you may miss seeing some of the dirt.
Check for fungus by viewing the lens at a area near a bright light source, not directly at the light source. Fungus are web-like organisms that thrive on the lens coating. Check the corners carefully to spot for any signs of fungus. If there's fungus, you should forget about buying the lens. Fungus permanently damage the lens, and are likely to return.
Check the front and back elements are scratches and smudges. Like dirt and dust, a little scratch and smudge will not affect the image much. But you can use these flaws to bargain the price down. If scratch is long and deep, forget it.
Next, when you're happy with the optics, check the mechanics of the lens. Start with the aperture. The aperture should turn smoothly, while clicking at the various stops. Check the aperture blades while they open and close, to see if they are symmetrical. If they're uneven, do not buy the lens. Then check to see if there's any oil residue on the blades. On the older lenses, some of the oil might have seeped from the aperture mechanism onto the blades. This may affect the smoothness of the aperture working, and thus the final image may suffer from over-exposure.
Check the smaller components like minimum aperture lock (if there is) and the filter thread. If the filter thread is damaged, you may not be able to use filters.
Finally, check the zooming and focusing ring to ensure that they are smooth. If the rings are stiff or feel grainy, there may be problems because some debris may have gotten inside. Sand and bigger pieces of debris will slowly grind away the interior every time you turn the rings. So unless you're willing to pay for the repairs of the lens, avoid lenses with stiff rings.
For autofocus lenses, fit them onto a camera body and check if it autofocuses correctly and smoothly. Also ensure that the readout on the camera corresponds to that on the lens. But remember that variable-aperture zooms will not show the same aperture read-out on the lens and the viewfinder if you zoom out !!
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