Lens scratches

[APSmith]

I've recently used two different glasses which had extensive scratches on the oculars. The scratches appeared to mostly be from improper cleaning. The surprising thing is that I really couldn't detect any effect while viewing. In one case, one side is considerably worse than the other, yet the view appears the same. This seems to defy logic, especially when so much is said about coatings, etc.

What is the explanation of this? Is there a way to "test" the effect of lens scratches on the view?

Thanks, APS

[mooreorless]

APS, I have some fine scratches on the oculars of my Swarovski 7x30SLC. I had to send this old binocular back to Swarovski to repair and told them if any lens needed replacement to replace. I would be charged for these lens, they used my old lens into a new unit. I am still using this binocular and have had a person that posts on here use this binocular and he liked it very much, he is a very picky person when it comes to optics.
I did find a source of a study on this in UK but you would have to pay for it.
Regards,Steve

[Tero]

For comparison, I can see even pretty light scratches on my eye glasses.

[Kevin Purcell]

Scratches add additional scatter (they're not at the focus) so they reduce the contrast of the image.

They don't affect easier to perceive qualities of the optics like sharpness and brightness so their overall effect is more difficult to discern. Perhaps if you AB the two barrels with different scratches you might see a difference.

It's rather like the reason for cleaning a lens: most of the time you shouldn't. Only if it really affects the lens should you clean the lens (and risk a scratch). Even when you do scratch the lens most of the time the quality is affected in only a very small way.

The time you'll see the biggest effect is when you have bright off axis light on the lens (that can scatter on axis from the scratches).

This is a gradual issue. One scratch makes a infinitesimal difference. More sctraches make more difference. Steel wool cleaning makes a big difference. But the effect is less than the visible impact of viewing the scratches on the objective.

If you can't tell the difference: be happy and ignore them.

[Surveyor]

........The surprising thing is that I really couldn't detect any effect while viewing. In one case, one side is considerably worse than the other, yet the view appears the same.

Thanks, APS

Good morning APS,

Last March, while checking a friends Swaro's, I found a similar problem. The view was still good, but resolution and transmission had suffered. I agree with Kevin but think I would add that if scratches are bad enough, they probably reflect light back into the eyepiece. See the photos in the post below.

http://www.birdforum.net/showpost.php?p=1149004&postcount=6

Best,
Ron

[henry link]

A friend recently gave me a pair of Minolta 8x23 Weathermatic binoculars with scratched eyelenses (see photo). He has no interest in optics at all, but he recognized that they had been ruined by one "cleaning", probably with his shirt tail, after using them on a sandy beach. Looking through them is like looking through a layer of gauze. Too bad, they were pretty nice binoculars, similar to the Nikon Mountaineer.

[Surveyor]

Henry,
At the bottom portion of your picture, is the milky looking section a light flair or is it actually on the lens?

Thanks,
Ron

[henry link]

Ron,

That's just a reflection of the sky which actually comes from an internal surface and looks milky because it's out of focus. The scratches are the only visible flaws on the exterior lens surface.

Henry

[Kevin Purcell]

I agree with Kevin but think I would add that if scratches are bad enough, they probably reflect light back into the eyepiece.

I was thinking about objective scratches with those comments.

I never really though about this but emotionally I dislike objective scratches more than eyepeice scratches (big piece of glass with a scratch on it). But logically I agree with Surveyor that eyepiece scratches are potentially a lot worse either localized scatter from the image or scattering light to the side back into the eye or "starburst" flares.

I wrecked a pair of CR39 eyeglasses cleaning them with a T shirt in desert-like Central Oregon. I didn't realize how much dust and grit had got into the T shirt over the day. Just a couple of seconds and it looked like I'd rubbed them with sandpaper. Dry dust is not a problem in the damp PNW. I'm a lot more careful about cleaning since then!

[APSmith]

Thanks to everyone for the input.

It sounds like the effects are not exactly apparent in the way a layman would expect. The bino in question in my case is a Leica Trinovid 8x32BN. After some more field testing with my example (and in light of the commentary here), there does seem to be a bit of performance falloff, noticeable in challenging lighting situations. It's a subtle thing, but the view sometimes becomes just slightly finicky compared to what I recall of a new pair I examined in the past. But, it's still not obvious like I would expect. (If I didn't know there were scratches, I wouldn't suspect it in the least.) I hope to do some careful AB testing tomorrow to see what shows.

But as it is, I just can't quit wondering: would an unscatched specimen have a noticeably superior view.

Thanks again, APS

[FIDDLER]

A friend recently gave me a pair of Minolta 8x23 Weathermatic binoculars with scratched eyelenses (see photo). He has no interest in optics at all, but he recognized that they had been ruined by one "cleaning", probably with his shirt tail, after using them on a sandy beach. Looking through them is like looking through a layer of gauze. Too bad, they were pretty nice binoculars, similar to the Nikon Mountaineer.

Did you horsewhipp him, Henry?