Telephoto vs. long focal length?

[aad]

A recent thread about LTM Visoflex setups made a reference to lenses with a long focal length-elements at one end of a tube-versus telephotos. What is the difference?

I noticed my 90/4 Elmar has that design of the glass at the front with a big space in the lens body. When I got it I thought I may have purchased a partial lens!

The same thread mentioned a certain type of distortion that gave a 3D effect-something else I notice in the Elmar.

 

[remrf]

I'm not sure I understand the questions. The word "tele" conotes a lens that is longer than the "normal" lens for a given format. And the focal length of that lens can vary from format to format. In 35 mm format a 135mm is considered a telephoto lens but in 4x5" it is in the normal range.

The term focal length is an exact optical description : the distance between the focal point of the lens to the film plane(*). The term telephoto is not a specific optical term but rather a description of a type of lense that would be "long" for the format.


And I have never heard of a "3d effect lenses".

* With only one exception that I know of. There are long focal length view camera lenses that have have a distance to film plane that is LESS than the stated focal length of the lens. So you could have a 400 mm lens that would take less than the normal 15.something inches required. This allows the view camera to move in closer without having to double the length of the bellows for close work. Lenses of this type are called "tele" lenses and in view camera land that has the specific meaning just described.

 

[aad]

On further research, which I should have done earlier, apparently a true "telephoto" is designed so it focuses to infinity at a distance from the film plane less than the effective focal length.

The 3D effect is something I've seen in a few pics, that another poster to the Visoflex thread said was due to a type of distortion. I'll look that up, too.

 

[Finder]

Yup, telephoto lenses are shorter than their focal length suggests. In common usage, it also means "long focal length" or having a focal length longer than a "normal" lens. "Normal" means having a focal length equal to the diagonal of the film area. For 35mm, that would be 43mm. The 50mm "normal" lens is somewhat of a mystery as it is longer than "normal." "Wide angle" means a lens with an angle coverage greater than about 52 degrees - technically, a 35mm lens on a 35mm camera is not wide angle. Nor does wide angle have to give a large angle of view; a long focal length lens can be wide angle as most large-format lenses are. The common use of "wide angle" is anything shorter than "normal."

What was the name of the distortion?

 

[Ronald M]

In the old catalogs, Leitz refered to them as either telephoto or long focus.

Long focus were lenses longer than 50 mm that were not telephoto construction to reduce the physical length.

90 elmar-135 hector-90 elmarit- first 90 summicrons were all long focus.

90 tele elmarit-135 4.0 elmar-135 4.0 tele elmar were of telephoto construction.

 

[aad]

The distortion was called "curved field aberration". I wondered if this was responsible for the effect.

 

[Finder]

"Curved field abberation" or curvature of field would mean that the plane of focus is curved so things at at the same distance at the side of the frame would not be brought to the same focus as things at the center - sharp in the center, but fuzzy on the outside.

Could it be from depth of field? The subject seems sharp, but the background is fuzzy. Can you show an example?

 

[aoresteen]

The difference between the a long focus and telephoto lens is based on the location of the rear focal node.

The rear focal node will tell you if the lens is long or tele. When the rear focal node is in the center of the lens it is a "normal" lens and thus called "long focus". When the rear focal node is in front of the ceneter of the lens it is a 'telephoto' lens.

Wide angle lenses have the rear focal node behind the lens, closer to the film plane.

That is how a 90mm lens cane be a wide angle (as in 4x5 large format), a normal lens (as in 6x7 format) or a telephoto (as in 35mm format). Mount any of them of a 35mm camera you will have the same angle of view but big differences in the coverage of each lens.

 

[Finder]

Wide angle lenses have the rear focal node behind the lens, closer to the film plane.

Not on large-format lenses. :angel:

 

[varjag]

"Wide angle" means a lens with an angle coverage greater than about 52 degrees - technically, a 35mm lens on a 35mm camera is not wide angle.

Technically, 35mm lens on a 35mm frame gives you the coverage of about 62 degrees by the long side.

 

[Finder]

Technically, 35mm lens on a 35mm frame gives you the coverage of about 62 degrees by the long side.

You're right. I wonder where I got that idea? 😕