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10-06-2008, 01:10 AM   #1
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lens scratch not affecting photos; how so?

I'm looking at buying a very expensive (for me) used lens. The owner says it has a "small scratch on the outside lens that does not affect photos at all".

I have questions:

1. how can a scratch not affect the end result (i.e., the photo)? I mean, if you look thru a pane of scratched glass I'd figure you will always "see" the scratch

2. is it feasible to send such a lens to Pentax for repair? Can you have just the scratched lens replaced or would this 'repair' really mean replacing the enitre unit?

10-06-2008, 01:46 AM   #2
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QuoteOriginally posted by arthur pappas Quote
I'm looking at buying a very expensive (for me) used lens. The owner says it has a "small scratch on the outside lens that does not affect photos at all".

I have questions:

1. how can a scratch not affect the end result (i.e., the photo)? I mean, if you look thru a pane of scratched glass I'd figure you will always "see" the scratch
I don't think you will see the scratch if you look through the lens, not focusing on the glass elements.

QuoteOriginally posted by arthur pappas Quote
2. is it feasible to send such a lens to Pentax for repair? Can you have just the scratched lens replaced or would this 'repair' really mean replacing the enitre unit?
That wouldn't make sense because scratches usually don't affect picture quality. They just affect resale value...
10-06-2008, 01:47 AM   #3
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QuoteOriginally posted by arthur pappas Quote
1. how can a scratch not affect the end result (i.e., the photo)? I mean, if you look thru a pane of scratched glass I'd figure you will always "see" the scratch
It most certainly affects the picture Ö Itís just that the effect is smeared out to all the photograph.
The extend of the degradation is another matter ...
10-06-2008, 02:34 AM   #4
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If possible, ask the owner to shoot some photos of the blue sky with white cloud from maximum aperture to minimum aperture and send you the RAW file to inspect. Visually - I mean by naked eyes, a scratch usually does not affect pictures at larger apertures, but, depending the size of the scratch, may affect pictures at smaller apertures like f/22, f/18, or even f/11.

For light scratches they may show up as black dots on the photo, just like those due to a dirty sensor. Not a big deal, but they do affect the resale value.

10-06-2008, 02:44 AM   #5
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QuoteOriginally posted by arthur pappas Quote
I'm looking at buying a very expensive (for me) used lens. The owner says it has a "small scratch on the outside lens that does not affect photos at all".

I have questions:

1. how can a scratch not affect the end result (i.e., the photo)? I mean, if you look thru a pane of scratched glass I'd figure you will always "see" the scratch

2. is it feasible to send such a lens to Pentax for repair? Can you have just the scratched lens replaced or would this 'repair' really mean replacing the enitre unit?
1. It is out of focus, therefore you will not see the scratch as a sharp feature. Try placing a hair on the front lens or the last lens of one of your current lenses. It will not give a sharp feature. Only if it is close to the censor will it give a sharp feature on the picture.
But the scratch (as well as a hair or dust) will scatter some light in strange directions that was not intended. This will reduce the sharpness, contrast of the lens. How much depends on the scratch, its position, the light conditions and the aperture. Closing the aperture to a higher f value will cut off more and more of the light beams that has gone wrong, and hence reduce the problem. If we are talking a really large surface of scratches the lens will probably go soft at all apertures. If we are talking a really deep scratch, you would probably easily get strange flare.

One of out forum members have posted an example of a lens with scratch front lens and what sort of picture it is still able to produce at our lens review data base.

2. It may be possible if it is a lens that is still in production. Go ask them.
If it is out of production, probably the only chance is to look for another broken lens with that piece of glass intact.

Personally I would not buy this lens if it is one that is easy to get hold of. If it is a rare lens of a focal length I really want I could consider it, but would then like to try it, or at least see examples of what the current owner managed to do with the lens, preferably the same motive at different apertures. Good luck!

Douglas
10-06-2008, 01:20 PM   #6
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QuoteOriginally posted by arthur pappas Quote
1. how can a scratch not affect the end result (i.e., the photo)? I mean, if you look thru a pane of scratched glass I'd figure you will always "see" the scratch
I don't want to over reassure you, but do bear in mind that many used lenses are full frame lenses for film. With the APS-C crop factor you are only using the centre of the lens. So it is possible to have a scratch on the front element that will not affect your picture if that scratch is at the edge of the lens.

I got a rather battered Carl Zeiss Jena 35/2.4 for about 1/3 the price they usually sell for because the photo in the ebay auction showed scratches at the edge of the lens. That put off the fanatical I-want-it-at-any-price bidders, and the bling bidders who buy simply for the street cred to have a "classic" lens on their camera (and don't even know what to do with it). The lens works well, and gives good images because the centre part of the front element is nice and clear.

Richard
10-06-2008, 03:07 PM   #7
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As others have said, with just a scratch you'll probably see very little or no effect on most photos. Somewhere on the web there's a page showing the results of a CRACKED lens with all kinds of other damage, and it still takes good pictures -- but I can't find it right now.

But you might see (a) occasional weird flare, or (b) reduced contrast, both of which would be the result of the scratch causing light to bounce around the lens in directions it ordinarily wouldn't. So if it's a problem, use a hood to avoid flare (and don't shoot directly into bright lights). In my (admittedly, and fortunately, slim) experience, reduced contrast is unlikely to be all that visible from just a single moderate scratch.
10-06-2008, 05:53 PM   #8
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QuoteOriginally posted by arthur pappas Quote
I'm looking at buying a very expensive (for me) used lens. The owner says it has a "small scratch on the outside lens that does not affect photos at all".

I have questions:

1. how can a scratch not affect the end result (i.e., the photo)? I mean, if you look thru a pane of scratched glass I'd figure you will always "see" the scratch

2. is it feasible to send such a lens to Pentax for repair? Can you have just the scratched lens replaced or would this 'repair' really mean replacing the enitre unit?
If the scratch is small, and the glass is fairly large, it should very well go unnoticed in most situations, though there is no way it can have no effect.
If the degree of effect is below the threshold of notice ability, then it is, in essence not affecting the image.
Resolution wise, a scratch can only lower the resolution by a factor of it's % of area to the area of the entire element., so a small nick on a larger front element really is negligible.
Where you are most likely to see scratched front glass affecting things is when a point source light is shining on the front of the lens. Scratches can cause really cool flaring if the light catches things right. This is a fairly rare occurrence in my experience, and a lens hood can go a long way towards ameliorating things.

I wouldn't let these facts deter me from approaching the lens with a small amount of suspicion, certainly marred glass should reduce the selling price by a reasonable amount, unless the lens is extremely rare.

Scratched glass can often be "repaired" by filling the scratch with India Ink This renders the scratch unable to transmit light, which reduces the flare problem.

10-06-2008, 07:09 PM   #9
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Thanks for all the helpful replies. The lens in question is a FA*85/1.4 with an asking price of $US550 due to 'the scratch'. I'm going to look at it on Wednesday.
10-06-2008, 07:43 PM   #10
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I have a small scratch on my Tamron 18-200 DiII and it hasn't affected the quality of the images at all.

That said, hard to tell if the scratch is only to the coating on the glass or the actual glass itself.
10-06-2008, 08:04 PM   #11
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What if the owner means the scratch is on the outside of the lens not on any of the glass elements?

Ask the seller to photograph the scratch so you can see where it is on the lens.
10-07-2008, 06:11 AM   #12
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QuoteOriginally posted by arthur pappas Quote
Thanks for all the helpful replies. The lens in question is a FA*85/1.4 with an asking price of $US550 due to 'the scratch'. I'm going to look at it on Wednesday.
For that kind of money, I'd probably buy the thing and count my blessings.
10-07-2008, 09:26 AM   #13
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QuoteOriginally posted by Wheatfield Quote
Scratched glass can often be "repaired" by filling the scratch with India Ink This renders the scratch unable to transmit light, which reduces the flare problem.
That's an interesting observation. so presumably filling-in the scratch using a black felt tip pen would do the same thing? Although the front lens would look a little weird with a black line on it...

I've heard that scratches on a CRT glass screen can be repaired - there are companies that will fill-in the scratch and make it "disappear", I wonder if that would work too? (I learned about this from a thread elsewhere where the toddler daughter of the poster had repeatedly pushed a toy car back and forth over the front of a brand new, big screen, plasma TV and scratched it. Unfortunately, the screen was plastic, but there were lots of suggestions for fixing scratches on glass-fronted TVs!)

Richard
10-07-2008, 09:40 AM   #14
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one of my earlier stick waving posts...

https://www.pentaxforums.com/forums/everything-else/27933-scratched-unaffect-image-quality.html
10-07-2008, 12:15 PM   #15
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QuoteOriginally posted by richard64 Quote
That's an interesting observation. so presumably filling-in the scratch using a black felt tip pen would do the same thing? Although the front lens would look a little weird with a black line on it...

I've heard that scratches on a CRT glass screen can be repaired - there are companies that will fill-in the scratch and make it "disappear", I wonder if that would work too? (I learned about this from a thread elsewhere where the toddler daughter of the poster had repeatedly pushed a toy car back and forth over the front of a brand new, big screen, plasma TV and scratched it. Unfortunately, the screen was plastic, but there were lots of suggestions for fixing scratches on glass-fronted TVs!)

Richard
I think the felt tip pen is a bad idea.

You would block some of the light (not as much as india ink), but if the ink is soluble in alcohol (or any other ingredient in whatever lens cleaning fluid you use) you could end up with a big mess the next time you clean your lens.
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