Looking at my lens cabinets, I’ll say that about 70% of the lenses were brought used. Like anyone sane, I much rather buy things new as it minimizes the risk associated to getting used gear, especially when it comes to lenses. Lenses are complicated and delicate pieces of glass with mechanical parts, that have to be stored correctly, it doesn’t take much to damage a lens, so my first tip is to buy new when you can afford to. The are a couple of valid reasons to buying used lenses; there is often a substantial discount in used gear and in many cases the lens has been discontinued, often for decades, so you’ll have no option but to buy lenses used. Apart from risking buying damaged goods, you are also opening yourself to fraud if you buy online. I always prefer the local option where you can check the lens out physically when possible, but there have been many occasions when the lens I am looking for is simply too rare to be found locally and I have to look abroad. It is essential to buy only from reputable sellers overseas, in fact I usually try to restrict myself to buying things from well recognized business only and try to avoid eBay as much as possible. Come to think of it, I don’t think I’ve ever brought any lenses directly from eBay, lenses aren’t that hard to find from safer sources.
Always check the market for the going price of lenses before buying, don’t just go by how much the lens cost when it was new. Don’t just check your local forum, always check prices from as many sources as you can before setting a base price that you are willing to pay. Local forums or even eBay, especially active forums can easily be manipulated by clever sellers to make items seem more expensive than they are really worth and sometimes hype up items for a little while to make them more attractive than they really are. Asked someone with experience for their more objective advice.
So I have organized a deal locally and have agreed on a basic price and I am meeting the seller face to face. What do I look out for in a used lens? First thing is to do the examination somewhere with good lighting, hard to see dings and scratches in the dark. I start with checking if there is anything missing, in the perfect world, there should still be the box, all the original packaging, instructions, warranty card, original receipt. I have known savvy lens owners actually sell the original advertising leaflets with the lens, this prudent move could pay off in the long run. Obviously, none of these things are necessary to the user, but will help maintain the value of the lens in the long run. Five years down the line and you want to sell the lens, having all these little details will make a lot of difference between an easy and quick sale at premium prices or having to wait for months for the right buyer.
Apart from the packaging the front and rear lens cap should be present and also the hood. If you need to replace these with originals they can be hard to find or expensive to buy. Hoods tend to wear out quicker than the lens and its condition can be telling on how rough the lens have been used. Hoods from discontinue lenses are also difficult to replace so make sure there is a hood.
Now that everything has been accounted for, I look a the lens itself. Start with the glass, dust off the surfaces with the blower that you brought with you and use a pen torch and look inside, first thing to look for is lens fungus, if there are any fungus on any of the elements, it should result in a massive decrease (I’ll say 75%-85% drop) in the price or just walk away. This is probably the biggest death nail in a lens’ coffin as its just going to get worst. If the lens was advertised with fungus and can be brought for next to nothing, then it could be okay, just two things to bear in mind: If the infection is severe and much of the lens elements is cloudy and etched, these lenses will be useless anyway so don’t buy, but if there is just a little on an edge and it doesn’t look etched into the glass, that maybe acceptable with a discount. The fungus can be cleaned by a technician but it often leaves a scar and the fungus will likely grow back sometime, but you may have a few years before that happens. The other thing to bear in mind, is fungus sporulates and I will keep known lenses with fungus in a separate box away from your lenses.
Dust isn’t a problem usually, there is always going to be dust in lenses, even weather sealed ones, so no need to be too anal here. Only if there is severe dust problems with dust clumping inside would I be concerned. But it can usually still be cleaned easily by a technician for a moderate cost.
Large scratches or chips on the front and rear elements should be obvious and may decrease the contrast of the picture in the area affected, so the price of the lens should reflect that. I think a 30%-50% discount to the price is appropriate to reflect the severity of the damage. Small minute scratches, even numerous is rarely problem unless its very severe.
Shine a light on the surface of the lens at different angle to study the state of the lens coating, mild abrasions shouldn’t cause any problems but deep and large ulcers should be avoided as re-coating the lens is a very expensive procedure and will change the nature of the lens image. There are some people that intentionally buy (often antique) lenses with severe coating wear and scratches at discount prices and get them re-coated with modern coatings. This is one way to buy a bargain.
Another show stopper glass problem is lens separation, there are glass elements in side the lenses that should be stuck together with optical cement and if these are separating, I would walk away. How to see it is easy: put the lens up to an even light source, look through the lens barrel, it should be all clear, but if there is lens separation, it usually starts from the edge and move into the center and the area where there is separation will appear less transparent or grayish as it will have a different refractive index and angles. I wouldn’t buy a lens showing separation.
Next I check the aperture blades for obvious damage. Apertures can have a couple of problems, oil on the aperture blade is a common occurrence, it may not affect the lens’ performance immediately but it is inevitable that the oil will cause the aperture blades to stick to each other leading to a sticky aperture blade that don’t close properly. On manual rangefinder lenses, this is easy to check as its all mechanical but on autofocus lenses it isn’t as obvious and to stop down the aperture blades you will need to bring a camera to check. With autofocus lenses I will press the DOF preview button and look through both the front element and the viewfinder to check for uneven lighting throughout the field, if there is a problem the lens should be discounted by %15-20. This problem is fixable but it can get expensive. If there is oil on the shutter, make time and get it clean before it gets worst.
I then rattle the lens gently to see if there is anything loose, if it is rattling it will mean that it needs a look at by an expert and extra cost. But if everthing else seems fine that it is rarely a show stopper and should be fixable. I would take %10 off the price.
The next thing I check is how smooth is the focusing, with manual lenses, rotate the focus knob and feel for stiffness, stiffness is sometime cause by dirt getting into the gear and often just needs a quick clean and lubrication that is a cheap fix. Looseness or unevenness can usually be adjusted by a technician at extra cost. With autofocus lenses there is a lot more that can go wrong, I usually switch the lens to manual and put it through its whole zoom range, as with manual lenses stiffness is usually easy fix but looseness is more involved so I would ask for a discount if its loose. If the zoom action is damage you may feel some resistance while manually operating the zoom and the zoom action may get stuck at either end of the zoom range, this is usually an expensive repair job. Turn it back on to autofocus and focus at various distances including close and infinity to make sure it drives well and can focus at all the distances.
The last thing I do is check the body barrel condition, look at the filter ring first, it is usually the first thing that hits the ground or get scratched, if it is damage, see if you can still screw a filter onto it. I then check the rest of the barrel for signs of wear which should be reflected in the price. I usually check the screws on the body and base to see if they are worn, I will be very suspicious of a lens if it has worn screws which means that the lens have been opened up. If whatever problem it had was fixed, why is he selling it to you? For auto-focus lenses remember to check the pins that are used to communicate with the camera body, make sure they are not damaged and not rusty.
In the case of autofocus lenses, if the various buttons are functioning and is not too loose. If a crucial button is very loose it may mean an expensive repair, so talk down the price to cover the cost.