Lens separation, stopping down to avoid? Possible?

[michael_tolan]

This question arises more out of optics-ignorance than anything

else. I tried several searches and didn't find what I was looking

for. If I have missed something obvious, please email me so I dont

clutter up the page with repeat questions. Anyway, here is my

concern: (1) Suppose you stumble across a 4x5 or 8x10 lense with a

great reputation (1st-rate optics, no debate); and, (2) The lens has

either slight fungus or separation around the extreme edge of the

glass - and ONLY around the very edge(s). If you stop down enough so

that the aperature blades come in beyond the fungus/separation (as

you look straight through the lens), have you then gone, how you

say, "beyond the problem"? If I am not clear, let me have a second

try: If you look through the lense and see some sort of abnormaility

at the very edge(s), then stop down from wide open enough so that

when you look through the lens you see only perfect glass, have you

eliminated the chance of the less-than-perfect area effecting your

finished pic? Or, are optics more complicated than this? I ask

because I sometimes see famous telephoto lenses at incredibly low

prices, because, there is some abnormality - often at one edge/curve

of the back element. I would rarely if ever shoot wide open anyway,

and if all you need to do is stop down to avoid the "tarnished area

of the glass", why dont people snap up such lenses? Does the

separation spread quickly? Is there any way to gauge how fast this

would occur - assuming this is a very real problem, spreading? Just

curious if Iam missing out on some nice glass when I pass by $500-700

lenses for $50-150?

[victor_lioce]

Just might be a bad recollection, but don�t the elements bring the light to a point in the optical center of the lens? So assuming the outer edge of the lens had fungus, it would decrease sharpness of the outer edge of the image. Unless, if the image was being cropped by the format space so the fungus might be in an image area not captured by the film space. But, the fungus, or other abnormalities should also contribute to other factors such as flare, diffraction in the lens, etc. Am I getting close?

[jerry_lehrer]

Victor

 

Your answer is closer to the truth than the original questioner.

 

Those lenses CAN be repaired by John Van Stelton and E K Grimes.

They both do fabulous work at honest prices.

 

Fungus and separation are progressive deteriorations, like a cancer.

Fine lenses are worthy of these fine craftsmens efforts.

 

There are a lot of old Dagors and Tessars that have been recovered

and are turning out beautiful photos, thanks to these two companies.

 

Jerry

[duane_kucheran]

I work with optics and fiddle with photography as a hobby. Michael has the right idea - stopping down a lens can eliminate the direct abberations caused by separation or fungus at the edge. This has to be true because of the definition of the relative aperture of the lens (f/ = focal length/diameter). Stopping down the lens reduces the effective aperture or diameter of the lens. The actual amount you have to stop down though is not simply what you would see by looking thru the lens. This is because you have to consider the path of the marginal rays (the outermost boundary) of the light from the object, thru the lens elements and iris and onto the film.

 

There will be very little if any effect on the sharpness of the lens by minor separation or fungus. More obvious will be the effect on contrast, especially in unshaded conditions. It depends on the lens and on how discerning you may be.

 

Separation can be caused dropping the lens or subjecting it to thermal extremes. Like cancer it can stay dormant for years or get worse suddenly. Generally though, it tends to be stable unless something is making it get worse.

 

My experience with lenses and attendant problems has been more political. It seems as if when I'm buying one & the seller says "that's only a small blemish, you'll never notice it" he'll want close to full price. If I'm selling "it's a POS" and not worth anything. Personally, if I got a $500 lens for $100 with a bit of separation I'd snap it up. For $400 I wouldn't.

 

Cheers,

[ron_shaw]

Yes, stopping down will make it usable. One of my old LF lenses has fungus tracks around the outside of the elements, but by F8, they are diaphramed out (and I have never shot at even F8).