Rub marks, but will not affect images !

[srtiwari]

I see this all the time in ads for lens, and never know quite what to make of it. Does this mean that the description of "Perfect glass" in an ad is relevant only with regards to its value as a collectible ? If so, what was the point of "Multi-coating" a lens ? (This, I guess is what gets removed in a rub mark)
Or does this suggest that the effect will not be detectable by cursory examination of a certain sized image ?
Personally, this description has caused me some hesitation in buying many a lens, but maybe I am being unnecessarily fussy. I am curious to see how others deal with this little 'Fly in the ointment'. 😱

 

[Al Kaplan]

Multi-coating is mostly to cut down on INTERNAL reflections INSIDE the lens. It's the light bouncing back and forth between surfaces that reduces contrast. What bounces off the front surface of the front element gets lost, true, reducing the amount of light getting to the film (or sensor) but otherwise causes no harm. As for sharpness it should have no effect at all. It's mostly just a good excuse to bargain down the price on the lens.

 

[JohnTF]

My friend had a Nikon Mount Zoom I had given her, 28-200 or so, and it had some condensation so she took off the UV filter and the camera "fell" on a rock, putting at least a one inch or more very deep gouge in the outer element.

I took the lens in for repair, and before we sent it in, we shot a roll of film, various openings, in to the sun, in to reflections, and were totally surprised at the lack of any sign in the images. Still had the lens repaired.

I have also heard if you get a "ding" in the outer element it can be filled with black ink.

Finally, the lenses with bubbles in the glass, generally considered to be of good quality.

I do not recommend mistreatment of lenses, but am inclined to agree with what Al says re: the rub mark, it is ugly, and knocks down value, but my old boss at the shop said "a lot of photos have been take through a lens with a scratch".

I have a FSU 85 F2 with a rub mark of some sort in the coating, so it is a user, I just shot with it last week and can find no evidence of image degradation, hardly a scientific test.

I do not clean lenses in the field if I can avoid it, seen too many with "cleaning" marks, I have heard the ones on the back have a greater affect on image? The old soft coatings were famously delicate, and I used to stop in to the shop to get any serious cleaning done.

Same with filters, tests I have seen, seem to indicate it has to be a pretty poor filter to negatively affect image quality.

Just stuff I have seen and heard. It is more painful to look at the damage you have done yourself, rather than a slight mark on a user lens you got at a terrific price.

Almost any Leica body with scuff marks from the meter, or a slight impression or ding, or marks from the straps drops a lot in value, but if you get it cheap, you get an excellent user you do not have to worry about keeping in the case or on a shelf. I picked up a spare M3 single stroke I am figuring out what to do with, and it has a tiny bit of vulcanite missing as well as the scuffs from the meter, so it may get left in the car trunk in a beat up case and a lens with a rub mark. I may look for some "experienced" glass and always have something to shoot with when something comes up on a day I am not expecting it.

Regards, John

 

[Roger Hicks]

Dear Subhash,

Scratches and 'rub marks' or 'cleaning marks' are rather different. It's a 'bald man' or 'slippery slope' argument. (Take a man with a full head of hair. Pull out one hair at a time. When you have finished, he will be bald. Where between the start and the finish did he go from hairy to bald?)

A single scratch can indeed be filled with dead black paint and will have no detectable effect on the image: the 80/2.8 on my Graflex XL has a spot of black paint on it for this very reason.

A very scratched lens will lose a lot of contrast, and this will reduce its ultimate resolution. After all, resolution is a matter of contrast between black and white line pairs, and if contrast is reduced, so is resolution.

In between...

'Cleaning marks' or 'rub marks' WILL affect lens performance, but a lot will depend on the light, whether or not a lens hood is used, the subject matter, the way the lens is used (hand held or on a tripod), the user's expectation, and above all, on how extensive they are.

My 150/4.5 Apo Lanthar has 'cleaning marks' and they make very little difference, but then, I normally use it on 4x5 inch where the limiting factor for resolution is normally film location, and besides, even 50 lp/mm will still give a staggeringly sharp 20x16 inch/40x50cm print.

In 35mm, I once tested a badly scratched and rubbed 24/2.8 Nikkor for a magazine article, and was surprised at how good it was -- but it was a contrast grade softer than a pristine example. Personally, I'm less worried about a scratch on otherwise pristine glass than I am by overall rub marks.

Cheers,

Roger

 

[JohnTF]

Roger, well stated.

How much loss would you expect in general in hazy lenses, as a WAG I would think internal haze could easily cost you more contrast?

I do not have the tools and experience to break down lenses, but have friends and repair folks who do and can clean some lenses as needed and know which not to try. A few years back I used to be able to get a repair shop in Prague to cla easy stuff quickly and at a reasonable price. Olympus and Nikon factory repair are both in Prague, and labor was quite reasonable, so you could drop a lens off, get quick, complete, service, and Czech law requires a 6-12 month warranty.

I was thinking really of a small and perhap single rub mark, but did not clearly state that. Am also speculating that an off center rub mark would not create as much of a problem?

When swapping items at a camera show, the guy on the other end will argue either way depending on ownership. ;-) As your bald argument is a good one, I brush my hair a bit more gingerly these days.

When in the field, I will tend to try and blow stuff off, avoiding any vigorous scrubbing. At home I have the micro fiber cloths and ROR, etc. but rarely use much. I try to be very cautious on old glass or let someone with experience help out.

The dust was so fine in the Canary Islands, I kept my Contaxes in the bag.

I did have some lens cleaner remove most of the coating on a 60's Omega enlarger lens.

John