Buying a lens

Ali Walker · November 18, 2014

I've seen a anamorphic 20-120 in perfect working order except has "slight/small amount of dust between elements that doesn't effect performance." There is no fungus etc etc and its £1200. Has anyone brought a lens with a small amount of dust like this? Thoughts?

Dom Jaeger · November 19, 2014

A small amount of dust is fine, you'll never notice it. Even little scratches in the front element (only coating scratches mind) or "light cleaning marks" generally won't be a problem. Fungus is something to avoid (although often it can be cleaned), as are deep scratches, chips or badly damaged coatings.

Harder to check but arguably more important is the mechanical condition - back focus, possible play in the zoom cams, worn focus threads or decentred elements. Does it hold focus through the zoom range? Is it softer on one side than the other? Anamorphic zooms are almost always spherical zooms with a rear anamorphic adapter fitted, it's important that this is correctly set up. Sample footage is helpful, but generally you'd want a lens tech to give it a check over if you're planning to shoot anything important.

Dennis Couzin · November 19, 2014

Your tolerance for dust, like tolerance for scratches, should depend on your tolerance for flare (veiling glare). Dust scatters light so the darkest, or most color saturated, parts of the image are veiled with the rest of the image. The classical test for lens flare is to photograph an extremely black spot (black velvet or better a light trap) against a white background. Modern multicoated lenses have less than 1% "flare factor" meaning that the utra black spot exposes the film less than 1% as much as the background does.

Dust adds to the flare factor according to how much of the surface is occupied by the dust. It is common to over-estimate that area, but if dust really covers 1% of the surface I think it will add about 0.5% to flare factor, which should be intolerable to users of modern multicoated lenses.

So-called "light cleaning marks" are the most offensive flare sources. They too operate as scatter sources and they can easily occupy 1% of the surface. Clean-clean-clean. Single deep scratches are much less offensive because they occupy much less of the surface. An isolated scratch on a lens can be painted-in black and then have no optical effect at all.

The cinematography establishment in the US was slow to appreciate low-flare optics. Even in 1980 it had proponents of uncoated glass filters vs. coated ones, the former being more durable!

Dom Jaeger · November 19, 2014

Single deep scratches are much less offensive because they occupy much less of the surface. An isolated scratch on a lens can be painted-in black and then have no optical effect at all.

I agree with most of what you say about veiling glare etc but the black painted scratch solution only works for relatively long focal lengths. Once you get below even 50mm, at deep stops a black line on the front element surface can start to become visible when you focus close. So the trick works much better for larger photographic formats than cine ones.

Alternatively, leaving a scratch unpainted will only usually become apparent when there is strong front light that can refract inside the scratch, so it's one of those "depends on the circumstance" flaws.

Dennis Couzin · November 19, 2014

Dom is certainly correct that the black painted scratch poses a danger of being in focus! It's a depth-of-field problem to decide if the plane of the front surface of the lens is sufficiently out of focus.

It's not focal length per se that determines this. For example, if the 50 mm lens has its entrance pupil at the lens front surface the black line on the front is totally out of focus and has no optical effect at all. So the seldom mentioned lens dimension "entrance pupil depth", E, needs to be reckoned in the danger equation.

When a lens is focused far, the blur circle from an object at the front surface has diameter f*f/N/E. In the numerator twice, f is the focal length; decreasing it tightens the blur circle. In the denominator, N is the f-number; decreasing it enlarges the blur circle. Also in the denominator, E is the entrance pupil depth; decreasing it enlarges the blur circle. For the black painted scratch to have no optical effect at all, f*f/N/E must be as large as the film format. For film format 12 mm, f = 50 mm, N=16, E is then required to be less than 13 mm. You don't find many 50 mm lenses with such shallow entrance pupils, confirming Dom's point. A very narrow black line requires less blur to make it effectively invisible. This is too much trouble to calculate. Better to test empirically by wiggling a black thread on the lens front surface with the lens at its smallest aperture.

However when the wiggled black thread is found to be visible, a wiggled white thread will also be visible. The unpainted scratch is like a light grey thread. It is a light scatterer, not a refractor, because it is rough. About half the lightfall at the front surface, from all directions is scattered forward by the scratch. If this were completely out of focus, as in the earlier discussion of "cleaning marks", it would contribute veiling glare, but it can be somewhat in focus according to the f*f/N/E blur circle formula.

A scratch that would make trouble blacked-in probably makes trouble not blacked-in.

Edited November 19, 2014 by Dennis Couzin

christophernigel · November 19, 2014

A small amount of dust is fine, you'll never notice it. Even little scratches in the front element (only coating scratches mind) or "light cleaning marks" generally won't be a problem. Fungus is something to avoid (although often it can be cleaned), as are deep scratches, chips or badly damaged coatings.

Harder to check but arguably more important is the mechanical condition - back focus, possible play in the zoom cams, worn focus threads or decentred elements. Does it hold focus through the zoom range? Is it softer on one side than the other? Anamorphic zooms are almost always spherical zooms with a rear anamorphic adapter fitted, it's important that this is correctly set up. Sample footage is helpful, but generally you'd want a lens tech to give it a check over if you're planning to shoot anything important.

It's more the rear of the lens that needs to be checked , also how is it to hold and feel and focus < that for me is what counts and also how it's feels on the body of camera you are going to use , or will be using <, still need to do tests as Dom says to check set up ! ie test film .

Ali Walker · November 19, 2014

Thank for the replies- I'll test and get it check out first- first impressions are great though. Cautious that the dust will attract moisture in long run?