Think a scratched lens is no good? Think again

[koh303]

This article explains in wonderful detail how most scratches and even extreme blemishes in a lens glass mean absolutely NOTHING:
http://kurtmunger.com/dirty_lens_articleid35.html

As a university professor my students eventually accept this as the truth, but will the "1mm barely noticeable" scratch mentioning crowd be convinced? probably not.

[jbenedict]

I've been told that, if there is a big divot out of the lens element and you are afraid of weird reflections and refractions, take some india ink and put it in the scratch/divot to suppress reflections.

[BetterSense]

Shhh...don't tell anyone. I enjoy getting deeply discounted lenses due to presence of scratches and "cleaning marks". Really grungy lenses can really still be quite sharp. The extra scratches will decrease contrast, and haze will decrease contrast and speed.

[8x10 user]

Well lets do some math...

A lens element of 70mm has an area of 3848.45mm^2

A rather large scratch of 3 mm x .333mm scratch would occupy 1mm^2 of the element or 1/3848 of this hypothetical lens

10 stops (color neg)= A 1:1000 subject brightness contrast ratio
6 stops (color slide)= A 1:64 subject brightness contrast ratio

Most parts of the lens contribute light to most parts of the image. A scratch on the lens is not in focus and does not produce a hard artifact, or mark in the image. Yes the scratch would block some light but the amount is negligible compared to the total amount passing through. There is a question of flare resistance which could cause a noticeable effect. This can of course be mitigated by the use of a compendium, lens hood, avoiding certain lighting scenarios, and by blacking out the area effected on the lens.

Scratches do seems to greatly effect the resale value of a lens, even the little ones in the coating (but not the glass). In early days many of the nicer glass types could only be melted for a short time and were thicker air bubbles would often remain in the glass and presence of bubbles in the glass became know as a sign of quality.

[lenser]

I just sent an 8x10 Brass lens off to Carol at Flutot's for a CLA and new iris blades. What I'm excited about is that this $10.00 auction wonder has some serious Balsam separations going on in both front and back groups that should give wonderful soft effects and possible out of focus planes within the images. Can't wait to use it BECAUSE of it's faults!

[Fotoguy20d]

Im with BetterSense on this one - don't tell anyone.

Every time one of these threads comes up, I like to show these images. I dropped the front cell of this dialyte while cleaning it and it cracked (not neatly) in half. Still gets the job done.

Dan

[Dan Fromm]

It really depends on the lens.

Scratches. On the one hand, I have a 210/5.6 Zircon with a horribly speckled front surface. It shoots well. On the other, I have an 80/2.8 Xenotar whose front and rear surfaces have the classic "cleaned with coarse sandpaper" look. It is very soft and horribly flary. Stopping down doesn't help at all.

Separations. On the one hand, I have a 58/5.6 Grandagon with horrible separations. Rings of fire in both cells, silvery spots in the middle of the rear cell. It shoots well. On the other, I have an EKCO 25-15 wide angle attachment for the 25/1.4 Cine Ektar II that shows strong Newton's rings (really Newton's stripes) from one edge to the other. They're very visible in footage shot with it.

It depends on the lens.

[lenser]

Dan, you got me thinking outside of large format to my 500mm mirror Nikkor. The rear cells have a ridiculous amount of separation that shows as a looping rainbow when just looking at the group, but unless shooting at a light source, it still makes a remarkably sharp image.

[goamules]

Yeah, I get a laugh out of people wanting only "Excellent, like new" lenses made 50 - 150 years ago. I don't sweat a few scratches or chips, but don't want any overall haze.

[Heroique]

That's a fun article – but if the author would also describe, in simple terms, how lenses form images, his readers would better understand why scratches aren't the huge issue a lot of them think.