Do lenses sag???

[ernie_gec]

I remember a physics teacher once explaining that glass has some

properties of fluids, ie. over time it "flows." I know window pane

glass does distort after many years, and so my question to the optics

expert out there is... does optical glass share these characteristics?

One might say that the degree of movement is likely to be small,

given the thickness of the elements typically used, but then we're

talking about nanometer tolerances. Is a 30 year lens likely to be

out of whack on this account? Should we "rotate" our glass in storage

like fine wine cellars do in order to avoid problems? Has anyone ever

researched this issue?

[altaf]

Generally its the photographer sagging and the len's doing just fine.

[jerry_lehrer]

Ernie;

Granted that glass is a supercooled liquid, not a crystalline

solid, but check the windows of some 1000 year old cathedrals

in Europe. No sag. Be practical. Or turn your lenses every day.

 

Jerry

[jeff_drew4]

Actually, I have seen a sway-backed telephoto! It was really a cheap, flimsy one in a thin tube with bad optics to begin with. I doubt anything like it is on the market today. But, technically, the glass itself was not saggy. Mis-handling & poor construction probably contributes to mis-alignment, sort of like old body parts! Rotate your glass! Shoot some pix! Happy New Year!

[cwphoto]

Yup, they do!

I have a Nikkor 500 f4,and when I'm not using it on a camera, it is mounted on a gimbel axis and rotated once a day at random.

[todd frederick]

Never use foam inserts in your camera bag. Just drop everything is very loose and give it a good shake every now and then. That keeps the glass in-line. Worked on my kids!

[konrad_beck1]

Do lenses sag? No, at least not with respect to the glass part. This question has been well investigated, and summaries can be found at:

 

http://www.ualberta.ca/~bderksen/windowpane.html

 

http://madsci.wustl.edu/posts/archives/mar97/856645740.Ph.r.html

 

http://physics.hallym.ac.kr/education/faq/glass.html

 

http://www.sciencenews.org/sn_arc98/5_30_98/fob3.htm

 

Due to the higher elasticity of the metal (and especially todays plastic) barrel, you should be -if at all- much more concerned on any deformations in these parts of the lens. Just look in your (old?) physics books on the problems of deformation in one-point supported springs: if you have a long lens with a heavy front element, supported at the camera end, the front will bend down, and the optical axis isn't straight any more (as you wanted to speak on "nm" tolerances). Also any deviation in temperature from "normal" will result in several micrometer (i.e. more than thousand fold as what your tolerance level is) elongations/shortening in the metal parts. In brief: use your lenses only at 25+/-0.1 oC (or ask the manufacturer for which T the lens was designed, accept only well-supported test data), and support every inch with its own solid tripod (use 2 tripods for a 50mm lens, 20 tripods for a 500mm lens) adjusted to the correct hight by interferometry to avoid sagging of the barrel.

[ernie_gec]

Thank you Konrad for an exellent referral to the cited articles. Very interesting stuff.