zoom out, focus at one spot. zoom in, focus again. if focus changes then the zoom alignment is off
2. distance scale
where-ever distance u focus at must match the reading at the scale. for 10m to 20m and more it will register as at infinity
3. distance limiters
verify that the AF obeys the different kinds of limits. dont use your hand to manually turn the focussing ring since u may come back with a erroneous conclusion that the limiter is spoilt
4. AF response must be normal ie not sluggish
5.for variable aperture zoom: verify the max aperture limits vary as stated when zoomed
6. flash+zoom: mount a body+flash+the zoom lens (that is, if its a zoom) the zoom range should register in real time on the flash LCD
7. stiffness/looseness during zoom
shouldnt be there for minty stuff. for well used ones..maybe to some extent
8. electronic contact pins must all be untampered with
Some older lenses may have vaporised grease deposit on the lens.....you may not see it in very bright light source......open the aperture wide with a pen...look into the lens at wide angle (if its a wide to tele zoom).....slightly tilt the lens away from the gright source....and if the lens is foggy with grease deposits....then you may see that the inside actually glow....a little.....
yupz its the zoom alignment axis thingy. the axis shouldn't change when zooming. eg say foe a 80-200/2.8 . say at 80mm u focus (AF) at one spot. all other factors being constant, say u zoom to 200mm. then focus (AF) again. it shouldn't change to left, right, up, down, somewhere else, etc. for lenses that extend when zooming eg the consumer type 28-105/f3.5-4.5, with more rough handling, the zoom alignment is more prone to be off. thats why i have a PURELY PERSONAL issue with the canon's EF 28-70/2.8L
yeah one more thing is to physically check the aperture blades when they open at all given apertures: the openign should be symmetrical and not lop-sided . for nikkors this can be easily checked. for canon EF u got to use the DOF preview+dismount method to check individual aperture
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 !!
actually, i'm not so sure about this one. like surely the axis should not ideally move left to right, up or down, but that wouldn't be the only thing that would cause such a problem. simply, the elements inside the lens, while zooming, will change their relative distances and affect the focal point just like how focussing usually affects zoom (called focus breathing). so, while a lot of people will practice the trick of focussing while at wide angle and then zooming in to take the shot, it's not perfectly accurate. pretty much all zoom lenses will need refocusing, ever so slightly, after a change in zoom.