[pure-silver] Re: Slightly OT: Lens Scratches
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Tue, 16 Nov 2004 17:00:48 -0800
----- Original Message -----
From: "Charlie Thorsten" <charlie_thorsten@xxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Tuesday, November 16, 2004 3:46 PM
Subject: [pure-silver] Slightly OT: Lens Scratches
> Things have been a little quiet so I've got
> a question for all the lens/optical/LF gurus
> out there. How important are coating scratches
> on the rear element of a large format lens?
> Will they affect sharpness more than on the
> front element? I'm not talking about deep
> scratches into the glass...more like coating
> scratches that won't clean off. I've seen
> many lenses for sale that have this "flaw".
> But is it really that serious from a practical
> image-quality standpoint? Thanks!
>
> -Charlie
>
>
Scratches scatter light whether they are in the glass or
just in the coating. The effect on the image depends on how
many there are. A couple will make no practical difference
but I've seen lenses, actually I have a couple, where the
front element looks almost like ground glass. These are
sharp but have very low contrast as might be expected.
While its often said that scratches on the rear element
have more effect than those on the front it really depends
on the lens design. I think the idea is that scratches have
less effect where the light is less convergent but in
practice it makes little difference.
I am not sure how to tell if a scratch goes through the
coating into the glass, the coatings are very thin. In any
case anything visible to the eye is going to have an effect
on the image.
I would be careful of scratched lenses because it
indicates a certain carelessness about the handling the lens
has received. Its not difficult to clean lenses without
scratching them. No magic solution is needed only pure
alcohol or ordinary lens or window cleaner and a supply of
cotton balls or Kimwipes. Dust can often be blown off with a
hand blower. Use one-time brushes made by rolling and
tearing Kodak Lens Tissue or Kimwipes (the lens tissue
feathers better). Most lenses probably get cleaned too
often. _Please_ don't use the lining of your tie as I
remember old time press photographers doing.:-)
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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