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Is this a blemish or not?

Posted: 25 Aug 2013, 18:54
by Waltzing Paul
I recently aquired a used lens which has a rather interesting blemish.
It's a 240mm Linhof lens by Schneider Kreutznach and is in very good condition.
I took some polaroid photos to see if the blemish has an effect on the picture quality.
They came out very good with no sign of any issues although I couldn't produce a large image to see the fine detail.
The blemish is about 2mm in diameter and is within the glass (not on the surface).
You can only see it in certain light by angling the lens.
Looking straight through the lens it is virtually invisible.
Can anyone shed any light on this?

Lens 3 small.jpg
Lens 3 small.jpg (295 KiB) Viewed 1630 times

Lens 1 small.jpg
Lens 1 small.jpg (224.88 KiB) Viewed 1630 times

Re: Is this a blemish or not?

Posted: 25 Aug 2013, 19:05
by Andrew Nichols
often air bubbles and such are ok, as they some how dont effect image.
it has to be a big blemish to cause an effect.
i have an air bubble and a chip of 1mm no probs
sorry i cant give you a technical reason.

Re: Is this a blemish or not?

Posted: 25 Aug 2013, 20:37
by Geoff Roberts
I have never seen anything like that before, no idea what it is. If you're happy with the pictures I wouldn't worry, unless it's something that can deteriorate over time.

Re: Is this a blemish or not?

Posted: 26 Aug 2013, 06:45
by Lachlan717
"Perfect" glass is a relatively modern concept in photography. It seems to coincide with multicoating.

As Andrew wrote, it takes a pretty substantial issue to show up in the image. I recall seeing a website quite a few years ago where a cracked (35mm) lens was tested. http://kurtmunger.com/dirty_lens_articleid35.html
So, I wouldn't worry a out this.

And, sometimes it can work in your favour: I bought a 14" Verito that has 3 tiny chips in the rear glass for $150. It's a soft focus lens FFS.

Re: Is this a blemish or not?

Posted: 26 Aug 2013, 11:01
by Maris
Yes that is a blemish It's a failure in the interlens cement causing a lens element separation at that point. The separation may stay small or it might grow (over years or decades) causing the lens elements to separate completely. Only time will tell.

While the separation is small it's virtually impossible to find a degradation in image quality. One thing for sure, the resale price of such a lens drops dramatically. When I'm buying used lenses I look for flaws like this to cadge a great lens at a bargain price.