"Telephoto" lenses in the Age of Internet Stupidity...

[Corran]

Well maybe you should edit it to say "the physical length of the lens when on the camera, measured from the lens to the image plane."

Personally I find it obvious that the definition is written from the viewpoint of the SLR/RF. The bellows is an integral part of the equation, when looking from the viewpoint of a LF shooter. But of course there are much less of us than the aforementioned group. So it was written without regard for this subset of photographers.

[Mark Sawyer]

If you sold a Petzval lens described as having "a physical length of 12 inches" and included the focal length behind it as part of the physical length, I suspect the buyer might call shenanigans. The lens is only a part of the whole imaging system.

[Dan Fromm]

What makes a telephoto lens is rear node in front of front node.


1/ some of my Tessars (and some are also zooms ) as well as my 100 mm Hasselblad Planar are telephoto designs!

2/ that my faithful 360 mm Tele-Arton is NOT a telephoto! (H=N = at 137 mm in front of the first lens vertex; H'=N' at 63 mm in front of the first lens vertex; effective focal length = 353)

Interesting. In fact some OPIC type 6/4 slightly asymmetrical double Gauss type lenses are in fact lightly telephoto. This is why relatively early modern era OPIC type "normal" lenses for 35 mm SLRs (57 and 58 mm lenses from Konica, Minolta and Topcon come to mind) are longer than the standard generally accepted normal focal length for 35 mm still.

As for your beloved and of course faithful 360 Tele-Arton, well, I wonder about it. The latest Schneider documentation I can find for it (see https://www.schneideroptics.com/info.../5.5-360mm.htm) says that the principal points are 74 mm apart. Distances for the other f/5.5 Tele-Artons at that site also give unsigned presumably positive separations. But slightly older documentation (see http://web.archive.org/web/201110021...le_xenar_2.pdf) gives -32.5 mm separation for the 250 mm f/5.6 (sic) and 74.3 mm (no sign, presumably positive) for the 360 f/5.5. It gives nearly the same flange-focal distance for the two lenses.

Many are the possible explanations. Typographical error. Different prescriptions and untrustworthy trade names. ... I don't know what to believe.

Sorry, Emmanuel, the only thing that's clear at the moment is that I still am and always will be an ignorant barbarian.

[koh303]

Well maybe you should edit it to say "the physical length of the lens when on the camera, measured from the lens to the image plane."

Personally I find it obvious that the definition is written from the viewpoint of the SLR/RF. The bellows is an integral part of the equation, when looking from the viewpoint of a LF shooter. But of course there are much less of us than the aforementioned group. So it was written without regard for this subset of photographers.

took the words out of my mouth.

[Dan Fromm]

I cheated and went to the library. Per S. F. Ray, Applied Photographic Optics, 3d edition, p. 302

The use of a long focus lens, where the BFD of simple two- or three-element designs is similar to the EFL gives a large image size, but this configuration also requires a long lens mount and focusing barrel or a long extension bellows. The telephoto configuration of a front positive lens followed by a rear diverging lens gives a more compact design, since the rear nodal point (N2) is now located in front of the front of the lens (Figure 30.1). The BFD is shortened but the lens barrel may be quite long.

A telephoto lens is defined as one where the ratio EFL:BFD is greater than unity and is at least of value 2.0 (Booth, 1926).

The definition above is an old one, now replaced by telephoto power, the distance from the lens' front vertex to the film plane (S, in his notation) divided by focal length. P = S/f. He mentions in passing that the 400/5.6 and 800/12 Apo-Tele-Xenar's telephoto powers are 1.0 and 0.92 respectively.

Ray adds (pp. 303-4):

Some classic designs such as the Tessar and double Gauss configurations have been adapted into telephotos, but bear little resemblance to earlier designs.

This is a polite way of saying that the lenses Zeiss sold as Tele-Tessars aren't what ignorant barbarians like me would recognize a Tessars. Trade names have nothing to do with design type. The same is true of Schneider's Tele-Xenars and jes' plain Xenars.

And to get back to this discussion's start, he writes:

Erroneously, most long focus lenses are simply called 'telephotos', although they may not be.

[koh303]

If you sold a Petzval lens described as having "a physical length of 12 inches" and included the focal length behind it as part of the physical length, I suspect the buyer might call shenanigans. The lens is only a part of the whole imaging system.

From the wiki page (4th paragraph):
...But such simple lenses are not telephoto lenses, no matter how extreme the focal length – they are known as long-focus lenses.[1] While the optical centre of a simple ("non-telephoto") lens is within the construction, the telephoto lens moves the optical centre in front of the construction.

[Bill Burk]

I don't have the nerve to edit the article, but added some discussion... The Wikipedia "Angénieux retrofocus" article clearly explains retrofocus (inverted telephoto) design. Should be easy to make the definition in the telephoto article as clear.

[Jim Andrada]

Not sure what 300mm have to do with a foot but I'de rather not get into the silliness of the American measurement system, which aside from Burma is the only other country in the world not to use the metric system (and burma has recently begun a conversion scheme...).
So i will just leave you with this:
Attachment 115545

It might help with other things like using the inter web down the line...

Well, it's a little known factoid that I-19 from Tucson to Nogales is marked in Kilometers, not miles. At least it was last time I drove it. They keep talking about changing it to miles, but most of the folks locally kind of like it as it is.

Picked up a rental car in CA last week and the speedometer read in km/hr.