Anytime you have a lens with a defect you can see, even slight ones, the obvious question is, "what effect is the defect having on image quality"? There are only two ways that I can see to answer this definitively. One is to take a perfectly good lens and create a similar defect and then compare images taken with that lens without the defect against images taken under the same conditions with that lens with the defect. The other way would be to reverse the process and take a lens with a defect that can be repaired, in the coating, say, and have it re-coated--comparing the images taken both before and after. Although this would, obviously, have an added complication in the quality of the job done in re-coating the lens.
I have not had the opportunity to do either of these comparisons. However, I think the next best thing would be to compare various lenses of the same make and model with a range of defects against lenses of the same make and model in mint condition with no observable defects. I have had the opportunity to do this and the only firm conclusion I can offer is that newer lenses are better than older due to improvements in production and technology and, perhaps, quality control and, particularly, the use of improved coatings.
When I first started using large format equipment, I kept buying old Artars in attempting to get both lenses in better condition and to duplicate several of them for two identical cameras, kept in separate locations, to avoid having to haul them back and forth. Up until then, using 35mm or medium format lenses, I had never had the opportunity to use a lens that was not in "like new" condition, at least so far as the glass was concerned. In fact, until I began using large format cameras, I had never actually compared two lenses of the same make and model, against each other, in any condition.
After some time, I ended up with a range of Goerz, Apochromat "Red Dot" Artar, lenses in various focal lengths, ranging from 8 1/4 inch to as large as 32 inch (210mm to 762mm). One of these, a 12 inch (305mm) Red Dot Artar, has some marks all over the surface of the front element that are just strange, looking like worm tracks and very noticeable when viewed through the lens while holding it up against a strong light. Others have the common cleaning marks that you describe, more or less. So, with a particular make and model lens of high quality, in a range of smaller to larger with more or less damage to either the front or rear surface or both and from different periods of production I have been able to compare them against each other and satisfy myself that I can't tell much, if any, difference between them, based on the images they produce, that I can attribute to cleaning marks. At least to relatively slight cleaning marks, that is. I do have another type of lens, a 300-500 convertible lens, that appears to have had the front surface of the front element scoured with a brillo pad and it shows and is objectionable. The various Artars, however, even the 12 in. lens, the one with the worm tracks, do not exhibit any difference, that I can see under normal use, in the quality of the images produced.
I can see a difference in comparing the older lenses against newer ones with the later production lenses seeming to have better contrast, but I believe this is due to better lens coatings on the late production lenses. Even when comparing lenses with mint condition glass and no marks or haze of any kind, I think the contrast is better in the later production lenses. Also, I have heard and read where it is possible that lenses with surface marks may have more problems with flare, but I can't say I have seen this. In general, like the various Angulon lenses I have had an opportunity to compare, I can see more variation in lenses due to the period of production, with later being better than earlier, than I can based on slight cleaning marks.
Another thing that I have observed regarding the Artars is that, in the older Apochromat Artar, not Apochromat"Red Dot" Artar, lenses, it is normal to find one or more tiny to not-so-tiny--but still very small--bubbles in the glass. I have read that these bubbles were tolerated in manufacture because the manufacturer was unable to prevent them when casting blanks of the particular glass that was being used and unable to obtain lenses of comparable performance without using that particular glass. Again, except for the difference in contrast, that I ascribe to improvements in the lens coating technology, I can't really tell any difference with or without the bubbles.
I think that the condition ratings that are used by KEH in describing the used lenses they offer for sale are a good indication of what the problems are with marks on lenses:
"Bargain" 70-79% of original condition. Shows more than average wear. May have dents, dings and a goodly amount of brassing and finish loss. Glass may have marks that should not affect picture quality."
""Ugly" Very rough looking. Multiple impressions in metal, excessive finish loss and brassing are likely. Glass will have marks, fungus or haze that will probably affect picture quality.
emphasis added.
In other words, the real difference between lenses with very slight to slight defects, what we might call "cosmetic" defects, is economic. You do not want to buy a lens with a defect without deeply discounting the price. This is because, should you want to re-sell the lens, you can be sure that any future potential buyer will want to apply a similar discount.
The best examples of this, in my experience, comes from two other lenses I purchased. The first was a 300mm Apo Symmar that was pictured with the elements reversed in the shutter, the front element having been threaded into the rear of the shutter and the rear element placed in the front. I was the high bidder for this lens for an amount that was less than I would have expected to pay for a mint condition lens like it. When I got the lens, sure enough, reversing the elements produced a mint condition lens with absolutely nothing wrong with it.
The second example was a 240mm f5.6, Rodenstock Sironar S, that the seller disclosed had a very small mark in the coating at the outer edge of the lens, just inside the lens retaining ring. Again, I was high bidder at a very low price for such a high quality lens. When I got it, inspection showed it was the only defect in what would have otherwise been a lens in "like new" condition. The defect itself, hardly more than a slight pin prick at the extreme edge of the lens, has, in my opinion, absolutely no observable effect on the performance of what is a very high quality, current production, lens.
I have found differences between lenses that were of the same make and model and in mint condition where I have been unable to find any identifiable reason to explain the difference. This, to me, is the real risk in buying a used lens. When I first began buying used lenses, I was not aware of the fact, as I learned later, that glass never becomes a solid and, as I understand it, can be disturbed by being subjected to a blow or excessive vibration, even though it appears to be undamaged. Since learning this, I have become considerably more careful in how I store and carry lenses and more careful in buying them. On the other hand, except for really ugly surface defects, cleaning marks, etc., I have not been able to see any difference in the performance of a lens that I could determine to be caused by the defect. If you do get a lens that does not perform well, for whatever reason, all you can do is replace it with one that does. If, however, you have a lens that seems to perform well, but has some small, but observable defect--don't worry about it. Just be sure you discount the price you pay for such a lens and enjoy the use of it.
