Nikon gives me the advantage to use some really old glass. Most of the lens are compatible with even the newest of the DSLRs. User Manuals of all the cameras also list the lens that are compatible with a camera and which ones are not. Apart from old Nikkor glass, many used / old lenses from various other manufacturers are also available in the market. People keep upgrading and selling old lenses.

Some important things to look for when buying a used / old lens –

The outside of the lens matters as much as the optics. A well looked after lens will also have a good exterior. Dents, scratches, peeling paint – all show that the lens has been extensively used and not always carefully. Once in a while I do come across a lens with poor looking body but good optics but it is very rare. My first rule in buying second hand lens is that the lens should look good even to an untrained eye.

The body should have no signs of being opened. Common tell tale signs are scratches on the screws on the mount, peeling paint at the corners of lens holding rings indicating an attempt at unscrewing or any screwdriver marks.

(Scratches on the lens holding ring showing attempts at opening the lens)

Mechanical functions should be as smooth as can be expected by looking at the lens. Focusing should be smooth, without any jerks and no variation in friction across the focusing range. If the focusing gives a gritty feeling, the lens is a no go zone. Autofocus lenses (like Nikkor 18-55 lens for DX cameras or the famous Nikkor 50mm f/1.8 D) sometimes do not have very smooth manual focus which is not a problem since they were meant to be used with autofocus. Aperture rings should ideally have firm but easy to change ‘clicks’. Zoom should also be as smooth as the focusing mechanism. Some of the telephoto zoom lens tend to have ‘zoom creep’ or ‘focus creep’ when pointed up or down due to the weight of some parts of the lens. This is quite common and can be seen in even new lenses. If your use is not going to involve shooting straight up or down or if the creep is not too fast, you can go for it. I have had a few long zooms that had lens zoom creep even when they were new.

Aperture ring should be smooth in opening and closing. One important thing to check is the consistency in the size of aperture. This can be done by checking the size of the aperture at various f-stops while going up and down the aperture scale. For example in a Nikkor 50mm f/1.8 D lens, move the aperture from f/1.8 towards f/8. Take a close look at the actual aperture opening size at f/8. Now go all the way to f/22 and then open the aperture back to f/8. Now look at the aperture opening once again. It should not be very different from the earlier f/8 aperture opening. The aperture blades should also be free of any oil marks. Check on both the sides. One common question I get from photographers is about the aperture function check in ‘G’ Nikkor lenses and similar other lenses with electronic aperture control. In my opinion these lenses are so new that there should not be any issues with the aperture. Either they work or they don’t. Aperture blades should still be checked for any oil marks.

Now comes the optics. First open the aperture and raise the lens to a light source. Scan every optical element that you can identify and look for any sign of fungus or mold. These are easily visible as points with a cobweb like fibres around it or as branching growths. Any such lens is again a no go zone for me. They can be used to create perfectly fine photographs and the growth may not have any significant effect on picture quality but then for me these are a deal breaker. When the growth is excessive, the images turn out soft and even blurred depending on the intensity of fungal invasion.

 
(Lens fungus filaments seen at the periphery of the optical element in this photograph)

Use a penlight (torch/flashlight) for checking the optics and coatings of the lens elements. Shining the light into the lens easily shows any dust particles or artifacts on any of the lens elements. Some amount of dust is common in even the best preserved lenses and can be easily ignored. Excess amount of dust is again a no-no. Some old lenses also have precipitation of lubricating oils on the optical elements which can indicate that the lens that has seen extremes of temperatures. This can happen even in an unused lens and causes softening of image.

Moving the penlight around shows multiple colored reflections inside the lens. A good trained eye can easily check almost all the elements of a lens using these colored images. By carefully moving the light around, optical elements can be examined in detail. The colors are due to various coatings.

(Multiple colored reflections of three lights under which the lens was kept)

An interesting fact – Eye care people occasionally use this method to scan the surfaces of optical elements in eyes (cornea and lens). These reflections of the penlight in the eyes are called Purkinje Images.

A good detailed examination can bring out a lot of information about the lens but as the saying goes, the proof of pudding is in the eating….. so go ahead and mount the lens on your camera. Take some test shots in various kinds of lighting conditions. If you are satisfied, go for the lens.

Good glass is an investment and my oldest lens has been with me since my first SLR. The pace at which the improvements are happening in photography electronics, everything becomes obsolete even before you know it, but lenses last a long time.

One small recommendation – do consult your camera’s user manual to see the compatibility and functionality of the lens you are considering with your camera body. Some new DSLRs do not permit the use of very old lenses.

Lens fungus image – Smial / Wikimedia Commons / GFDL / CC BY-SA 2.0 DE

Further reading –
Next lens to buy?
Prime Lens or Zoom Lens?