long focus vs. telephoto

It has only been recently since I heard these term?

what do they mean? What is the difference between and a long focus

and a telephoto lens of the same focal length?

The tele lens will have shorter back focus. Back focus is the distance from film to the lens rear element when the lens is focused at infinity. "Normal construction" long lenses are typically sharper and produce images with less distortion that telephoto lenses do. The big gain from teles is that they can be used on cameras with less extension.

 

Why did you ask?

Thanks for the info. I didn't know there was a distinction until a few months ago. I have a 90mm LTM Elmar that I guess is a long focus lens. Is there any secret handshake way of knowing what kind a particular lens is?

Generally, if the rear element or group is a negative lens, the design is a telephoto -- these are effectively a shorter focus lens with a built-in tele-extender, but one that's optimized for the lens so it doesn't degrade the image quality the way generic extenders commonly do. The result can be (for instance) a 300 mm lens the length of a 135 mm -- there was a Tele-Xenar for the M42 that was just about exactly this, in fact.

Long-focus lenses generally will have little or no distortion endemic to the early telephotos, but they can become unwieldly at focal lengths greater than about 150mm. By its very nature, a long-focus lens is as long, or longer than the stated focal length, minus the distance from the mount to the focal plane. The less complex construction gave significantly less light loss for uncoated lenses.

To throw a little triva into the ring, if one uses the Ihagee Exa cameras with the sector shutter, long-focus lens over 100mm focal length will vignette. I've heard though that more modern telephoto designs of equivalent focal length avoid this problem.

"Is there any secret handshake way of knowing what kind a particular lens is?"

 

In general, if you find a lens with a barrel longer than your arm (like the older Nikon teles, Leitz Telyts) then you know they are long focus lenses.

 

In the medium format digest site, you will find examples of these. They could be adapted for MF as well. Coverage also is more than sufficient for 6x6.

There is an easy way to tell a long-focus lens from a telephoto, generally: The long focus lens will have glass only at the front end of the barrel, if you look in from the rear it will look quite empty. An equivalent telephoto lens will have a negative lens group at the rear, typically pretty far back toward the mount.

 

Someone mentioned vignetting in the Exa: That is an Exa problem due to its shutter design, but it is also a differentiating feature of the lens type. A long focus lens will vignette more than a telephoto for any given focal length and camera mount. Generally this is not an issue if the lens is designed for the camera, but you would want to watch for it if you're experimenting with interchangeable-mount lenses, bellows, or other optics such as telescopes.

My 200mm f/4.5 Telyt is a true telephoto, as is the 400mm f/5 Telyt.

I believe that my 500/5.6 TeleAthenar II is a long focus lens, not a tele.