How do you check a lens for fogging?

Can someone tell me the best way to inspect a lens instore for

fogging? This relates to the collapsible 50 Summicron I asked about a

while ago. What else should I look for? Do a few cleaning marks really

make a big difference?

David,

<p>

Best way is to shine a torch thru it. Just holding it up to the

light and looking thru is not enough. With a torch, if its fogged,

it will become very apparent as a white cloud on the glass surfaces.

<p>

Minor scratches and slight fogging wont affect the image, but will

def affect the resale value.

Erwin Puts did a test which included both a collapsible with and

without coating defects: <a

href="http://www.imx.nl/photosite/leica/mseries/testm/scronhist.html">

http://www.imx.nl/photosite/leica/mseries/testm/scronhist.html</a><br>

He concludes: <br><i>"The differences between the C with defects and

the clean one are remarkably small. A drop in contrast, a higher

level of flare and a loss of micro contrast when recording very fine

details are visible. These differences might or might not be

important depending on user-expectations and shooting

circumstances."</i>

Slightly OT, but I am always shocked at first when someone proposes

shining "a torch" through a lens. It always takes me a moment to re-

realize that you speakers of non-American English are talking about

what we call a flashlight. To us, a "torch" is a long stick with a

blazing fire at the end...hardly something you'd want near a Leica

lens, even if it is supposed to withstand extremes of temperature;>)

Actually, Jay, the torch test is used to separate inferior plastic

immitation lenses from real Leica lenses: the plastic ones burst into

flame but the real Leica lenses just become incandescent. However,

don't overdo it or you get some unintentional aspherical elements

when the glass melts.

<p>

P.S. Just kidding - don't try this at home!

For speakers of British English: "Olympic flashlight"? "I'm carrying a

flashlight for you"?.

Anyway, I tried the torch test. Didn't SEEM to be fogged, a few

scratches on the front element, but the price - about $US150 seemed

good. No-one here seems to ant old lenses, so I rescued it. How does

that price compare?

One thing to remember when doing the flashlight test is that the

coatings on those old lenses aren't as effective as modern

multicoatings, so even if the lens is in excellent shape, some degree

of flare is normal. If there's haze or fungus, it's VERY obvious.

Just the few scratches. Reading Erwin Putts you'd think the best place

for those old lenses was the rubbish bin. I also have a Summar, and

like it for portraits and landscapes. At smaller aperturs its quite

sharp. But why do lenses have to always be razor sharp anyway!?