Tips on buying used lenses

[dpalace@sunland.gsfc.nasa.gov (Dick Palace)]

Over the years I've developed a general method to determine
the condition of a used lens. It works practically alway, provided
that you take the time to inspect the lens.

I break it down into mechanical and optical. Do the optical first,
if you find something wrong then you just stop, no need to continue.

Optical:

First off, if you buy mail order, make sure that you can return it
if you find something wrong. I normally go to the photo shows where
I can get my hands on the lens. Now my opinion is that most people
are honest, but it seems that a lot of bad stuff tries to get pawned
off at the photo shows. Be aware and inspect carefully.

Open the diaphragm all the way, to get a better view and
Have a magnifying glass of at least 5 power and of reasonable good
quality. Focus on the surface of the lens and look at it very
carefully. You may or may not see scratches, or missing coating or
fungus growth or other. A few cleaning marks should not effect the
lens, but that you will have to decide. Reject it if some of the 
coating is missing or there is fungus growth. Now very slowly move
the magnifying glass closer to the lens. The idea is to find where
the first element and the second are cemented together, and focus
on it. We are looking for lens separation and it has a texture like
lace. That my best description, you will know it when you see it.
It helps to see it sometimes if you set up a flare condition, where
you look at a dark area with the lens, but angle it in such a
direction so that the light shine at an angle onto the lens. The story
that I get is that Leitz used some bad balsam (material used to cement
lens elements together) back in the 50's and a lot of the lenses from
that era show seperation. If you are offered a lens from that era at
a good price, beware, and inspect it thoroughly. If it looks good so
far then look at the rear elements in the same way. Passed so far,
good. Now, holding the lens in one hand give it a sharp wrap into the
palm of your other hand, not lens first but the side of the lens. Look
through the lens. Do this a couple of times. We're looking for flakes
of black that may or may not be floating around inside the lens.
Decision time. For a user lens you don't want missing coating,
excessive scratches, fungus growth or lens seperation. All of these
defects are too costly to repair. A few black flakes of paint will
probable not hurt as gravity will cause them to settle to the bottom
of the lens. If need be, you can have them removed at a reasonable
cost. If you made it this far then we can go to the mechanical.

Mechanical:

Look at the lens, look at it good. Scratches are ok, but beware of
dents. Dents tell you that the lens has been dropped or banged into
something. I automatically reject a dented lens, you have to decide
for yourself. Rotate the "f" stop ring and look into the lens at the
diaphragm. All the blades should open and close in a symmetrical
pattern, and you should not feel any binding or excessive looseness
in the rotating the ring. If the diaphram blades have oil on them, then
someone has played do-it-yourself on the lens and may have or may have
not caused other damage. I would not reject the lens for oil if
everthing else checks out ok. I normally send any new used lens that I
buy in for cleaning and check out. Now rotate the focusing mount
through it's full range several times slowly, feeling for any
roughness or binding, which could be trouble. Ok, then check the lens
mount. Normal wear from taking the lens on and off is ok, but any kind
of deep scratches or gouge marks should be grounds for rejection. 
Now with your 5 power magnifier take a look at the screw heads holding
the lens together. If they are chewed up and ragged and maybe some 
missing, then I would again assume that some do-it-yourselfer had been
at it again and you will have to decide if you want to take a chance
on buying it.
Decision time again. Reject the lens if it has a dent or a rough or
binding diaphram ring or focusing mount, or a damaged lens mount.
One last thing on mechanical. When checking out one of the screwmount 
collapsible lenses, pull the lens out and lock it into place. Now
wiggle the collapsible part and it should be solid. Here again,
sometimes things like this can be fixed and then again maybe not.

Now I know that dried lubricants can cause the focusing to feel tight
and also the diaphram ring. But, is it the lubricants? It's up to you
if you want to take the chance and buy the lens and then send it to
be cleaned and relubricated. I've been to many photo shows and seen
some of the junk that was trying to be sold as good, so beware. 
ALWAYS, and I repeat, ALWAYS make sure that you can return the item
and get your money back. Two times I got a lens home and found
something that I missed that made the lens unacceptable. Even
if you have to pay a restocking fee, you will be better off.

That's all I have, hope it helps. Anyone care to add to it?

Dick