Does a scratch on front element affect IQ?

I'm looking at a used 70-200 lens and there is a small scratch on the front element near the edge of the glass. From the pics it looks to be 1/2" long and not a deep gouge or anything, just a surface scratch. But it is there, and it is noticable.

The guy says it doesn't affect IQ AT ALL and I have heard others selling lenses with internal dust on the front element say the same thing.

It's a decent deal but the scratch has me worried. I admit I don't fully understand all the technical in's and out's of how lenses work though, just the basics, so maybe it's not a big deal.

Please help me understand how this would or would not affect IQ.

Thanks
Chris

NWShooter wrote in post #2748640
I'm looking at a used 70-200 lens and there is a small scratch on the front element near the edge of the glass. From the pics it looks to be 1/2" long and not a deep gouge or anything, just a surface scratch. But it is there, and it is noticable.

The guy says it doesn't affect IQ AT ALL and I have heard others selling lenses with internal dust on the front element say the same thing.

It's a decent deal but the scratch has me worried. I admit I don't fully understand all the technical in's and out's of how lenses work though, just the basics, so maybe it's not a big deal.

Please help me understand how this would or would not affect IQ.

Thanks
Chris

it could affect IQ in some cases but probably won't most of the time. the discount would have to be pretty substantial to interest me.

i figure if the lens were in otherwise mint condition i'd find the very best price on the internet and deduct 30% for depreciation and also deduct the cost of a canon repair plus postage etc.

edit: oh yeah i'd probably knock off another 10% because the guy didn't use a UV filter .

ed rader

These threads seem to indicate that it's very likely that it does not.

https://photography-on-the.net/forum/showthre​ad.php?t=226698

https://photography-on-the.net/forum/showthre​ad.php?t=204266

as said, it probably wont affect IQ, but unless its going to be an amazing deal, i wouldnt risk it.

If you plan on beating the snot out of this lens or keeping it for a long time, then buy it knowing you'll get great images from it.

If you are willing to pander to the gear ****s and worry about resale, move on.

A half inch scratch is pretty substantial though.

Just checked my 80-200/2.8 L that I bought used on eBay at a fairly good price (around USD 500 instead of the USD 800 a 'mint' version would have cost) and it has a few scratches on the front element ... maybe not that size, but still. Image quality is great, though.

I think that Ed's suggestion of a 'cheapest internet price - 30% for depreciation - cost of canon repair' would be too low ... I haven't seen a Canon L lens in mint condition offered for cheapest price - 30% depreciation yet

A scratch on the front element normally is not a big deal. Do a test and hold one of your hairs just in front of the front element and check if you can see it in the images taken ... you won't see it. It will have to be fairly thick to show up in your images. A scratch on the rear element, however, is a totally different thing...

Best regards,
Andy

I just spoke with a local canon dealer and they are saying front element replacement would run $400-500!! Wow. He didn't have an exact estimate in his book, but that was his best guess.

I tried calling Canon and, well, that was a joke. This is how it went (just so you all can have a good laugh)

Canon Repair: Hi this is Maria, how can I help you today?
Me: (explain what lens I have and whats wrong with it (scratch)).
CR: What camera do you have?
Me: 30D
CR: and what seems to be the problem with the 30D that you think needs repair?
Me: I just told you it was a lens with a scratch.
CR: Oh, what lens do you have?
Me: I just told you it was a 70-200 2.8
CR: Oh, and what seems to be the problem that you think needs repaired?
Me: You know what, never mind...goodbye.

Nice work Canon.

Why would a UV filter that is clean and not scratched affect image quality, but a lens with a 1/2 inch long scratch on the surface not affect IQ?

I'm puzzled.

Glenn NK wrote in post #2749094
Why would a UV filter that is clean and not scratched affect image quality, but a lens with a 1/2 inch long scratch on the surface not affect IQ?

I'm puzzled.

Your text should be saved by every pro-filter person for future reference.

Could not have said it better myself.

Glenn NK wrote in post #2749094
Why would a UV filter that is clean and not scratched affect image quality, but a lens with a 1/2 inch long scratch on the surface not affect IQ?

I'm puzzled.

Anything on the front element will be far too out of focus to show up on images. The clean UV will refract the light; hence more internal reflections and a softer image - as opposed to a minute refraction on a small part of the image due to the scratch.

Glenn NK wrote in post #2749094
Why would a UV filter that is clean and not scratched affect image quality, but a lens with a 1/2 inch long scratch on the surface not affect IQ?

I'm puzzled.

Because at each air/glass interface light is diffracted (bent). If the glass isn't of good optical quality, i.e. optically flat with no internal imperfections, then the light will be diffracted enough to cause a degradation (flare) in the lens. Good quality (read expensive) filters are not only optically clear, but use mulitcoatings (multiple thin coatings of materials) to minimize the diffraction inherent at the air/glass interface.

That's just physics at work.

Mark

illy wrote in post #2749135
Anything on the front element will be far too out of focus to show up on images. The clean UV will refract the light; hence more internal reflections and a softer image - as opposed to a minute refraction on a small part of the image due to the scratch.

That is not how lenses work. The image from each point is formed by light rays incident at all points on the aperture. That is why you don’t notice it.

A scratch will diffract and disperse light. A single scratch will probably disperse so little light it will not add much flare, a lot of scratches will have an effect.

A good quality filter is made of flat optical glass and will have high performance multicoatings probably as good as those in the lens. A lens has many elements and the filter is just another, this minimal effect.

I think you're both right, just different ways of looking at it. if you put a small scrap of soft paper on the front element of your lens and shoot a pic, you can test this. or you can shoot thru a screen window.

but a scratch is a special type of obstacle to the path of light. not only does it potentially block some of the light, it also can refract passing light, giving rise to new unexpected light paths.

I doubt that a scratch will affect the sharpness of the lens, but it almost certainly can produce internal reflections, and general loss of contrast. I'd ask the seller to take a photo at 70 and at 200mm, both wide open, with the sun in one corner of the image, and see if the flare is as expected.

and I'd ask for a discount.

In most cases, you can put a cracked UV filter on and not see any IQ issues until f/11, and even then it will be very subtle (slight shadow on the image in highlight areas.) There will be virtually no image quality loss whatsoever with a lens with a small scratch, and any IQ problems that you experience will be related to slightly increased flare, not resolution loss.