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View Full Version : What Does E.F. Stand For?



Frank R
27-Dec-2006, 21:04
I was recently looking at a couple of old magic lantern lenses that had inscriptions that read *15 inch E.F.*. I believe they were both by Bausch and Lomb.

I was trying to determine if these were petzval designs so I unscrewed one side. The two elements had a large gap between them; too much for a petzval I think. I was unable to unscrew the other side but I think it had just one element.

So what exactly does E.F. stand for?, Extreme Field?, Extra Fast?, Easy Focus?

Does anyone know if these are petzval lenses? They weren't cheap and would be expensive paperweights if they were not petzvals.

wfwhitaker
27-Dec-2006, 21:21
Probably Effective Focus, also often EFL for Effective Focal Length.

Could also mean Ebay Fodder. :)

Lazybones
27-Dec-2006, 21:58
No, no, NO! It's ELF. Everyone Loves Fudge. Just asks the Keebler Elves.

Ernest Purdum
28-Dec-2006, 09:19
It stands for equivalent focus which is synonymous with focal length.

The basic Petzval design was modified many times over the years while being produced by many companies. The separation of the rear elements might or might not be an indication of a different basic design. If, however, you can't be sure if there are two elements in front, you had better suspect that it may be a triplet. My impression is that this would be unusual, but not, of course, impossible. If you look very carefully under a single overhead light, you might be able to see whether or not there is a faint rflection from a cemented surface.

If you have bought the lenses, determining its photographic characteristics would be what matters. If you haven't yet bought them and still can't be sure, something else always comes along.

Frank R
28-Dec-2006, 20:35
Thanks for the answers; I will pass on these two.

Doug Kerr
6-Jan-2007, 09:22
"E.F." stands for "effective focus", more commonly called "effective focal length".

This is no different than the focal length we often speak of. The peculiar formal name for it comes from the following:

Initially, in optical engineering terminology, the "focal length" of a lens was measured to the focal point (front or rear) from the corresponding vertex of the lens (that is, from the center of the outer surface of the outermost element at the relevant end of the lens). Two values might be stated, the "front focal length" and the "back focal length". (We in fact today often speak of the "back focus" of a lens with that same meaning.)

But neither of these numbers are the one that figures prominantly in many optical equations. That number is measured to the focal point from the corresponding principal point of the lens. (For a lens that is immersed in the same medium fore and aft, such as air, that is in the same location as the corresponding nodal point, but the definition relates specifically to the principal point.)

To distinguish this fundamental number from the two "focal length" numbers as discussed above, the term "effective focal length" was coined.

This terminology is still found in contemporary optical textbooks.

Best regards,

Doug

BrianShaw
6-Jan-2007, 09:32
No, no, NO! It's ELF. Everyone Loves Fudge. Just asks the Keebler Elves.

Since the answer is in I don't feel like I'm hijacking the thread... but that reminds me about Keebler and their elves...

A long, long time ago in a land not too far away I lived in an area that had three businesses in essentially the same block. On the south side of the street was a large beer brewery, on the north side of the street was a chili-burger stand, and on the southwest corner was a Keebler factory. The brewery belched copious quantities of malt and hops smell almost continuously. The Chili-burger stand belched copious quantities of burning-cowflesh smells almost continuously. The Keebler factory (that previously was a cracker baking plant) would periodically belch copious quantities of the nicest smelling cookies-being-baked (by elves, no less) smells. After a while, the Keebler factory sealed their windows in response to an air pollution citation. Then they were closed down. The brewery continues to belch copious quantities of malt and hops smell almost continuously. The Chili-burger stand also still belchs copious quantities of burning-fles smells almost continuously. The Keebler factory has been replaced by a school bus storage/repair company - they belch copious quantities of probably carcinogenic smoke all day long. Why do some people hate the elves and their colcolate-coated cookies?

Doug Kerr
6-Jan-2007, 09:46
Why do some people hate the elves and their colcolate-coated cookies?

I give up. Can I have part credit for copping a plea?

Best regards,

Doug

Lazybones
6-Jan-2007, 14:38
After a while, the Keebler factory sealed their windows in response to an air pollution citation. Then they were closed down.

"Man, you never would believe where the Keebler Cookies come from. They're baked by little elves in a hollow tree. And what do you think makes these cookies so uncommon? They're baked in magic ovens, and there's no factory. Hey!"

Turner Reich
6-Jan-2007, 22:37
EF: electromotive force

naturephoto1
6-Jan-2007, 22:48
EF: electromotive force

Sorry Turner, that would be emf.

Rich

Doug Kerr
6-Jan-2007, 22:59
electromotive force = E.M.F .

Best regards,

Doug