in "identifying leica lenses" by ghester sartorius he mentions rectilinear focussing for some lens. what is that?

-- stefan randlkofer (geesbert@yahoo.com), January 14, 2002

Answers

Rectilinear refers to non-distortion across a rectangular field. Wide angle lenses like the Voightlander 15 are advertised as rectilinear vs fisheye (which is spherical distortion across a rectangular field). This might not be totally accurate as well because the Zeiss 30 C mm fisheye (for Blads) is a fisheye with minimal non-linear distortion.

My interpretation of rectilinear focus is that the plane of focus is flat (with respect to the film plane) anywhere in the image forming rectangle. A simple task to accomplish with normal-teles... but difficult with wide-angle/fisheyes.

Boy, really got a lot of free time today!

-- John Chan (ouroboros_2001@yahoo.com), January 14, 2002.

Stephan: I think the term you are thinking of is "retro-focus", not "rectilinear focus"...

A wide angle lens can be rectilinear in design, and John gave a good definition of what that means, but I do not believe the term rectilinear-focus is one generally used.

On the other hand, a retro-focus design lens is one that is designed to project the plane of focus behind the physical center of the lens - - the opposite of a telephoto design if you will. This term also refers to wide angle lenses and is used to allow very wide lenses to clear restrictions within the camera body. Restrictions like the swinging mirror in an SLR, or the metering cell in the M6. Some older wide angle M lenses like the 21 and the 28 were not retro-focus in design, and hence project too far back into the camera body to allow use of the meter in the M6. The new versions of the 21, 24 and 28 are all retrofocus designs to overcome this pitfall.

Hope this helps!

-- Jack Flesher (jbflesher@msn.com), January 14, 2002.

it is definitely rectilinear. for example the elmar 90 collapsible (which is pretty far off a fisheye): "...and the focusing movement was rectilinear."

-- stefan randlkofer (geesbert@yahoo.com), January 14, 2002.

Well, that makes absolutely no sense to me whatsoever! Perhaps you can quote the full text so we can better understand the complete context of his statement.

-- Jack Flesher (jbflesher@msn.com), January 14, 2002.

"Rectilinear" in this case means a non-rotating mount (I think).

-- (bmitch@home.com), January 14, 2002.

I think I would agree, what he's referring to is that the focusing ring moves the optical cell in and out without the whole barrel including the aperture ring rotating. Was Mr. Sartorius a non-native- speaker of English, or the book a translation?

-- Jay (infinitydt@aol.com), January 14, 2002.

Jay:

Isn't what you are saying what we all lovingly refer to as IF (internal-focus)?

-- Jack Flesher (jbflesher@msn.com), January 14, 2002.

Oops, never mind. Got it a little late.

;) Cheers,

-- Jack Flesher (jbflesher@msn.com), January 14, 2002.

It sounds like it could be a reference to what has been called "parallel focusing", in which a portion of the outwardly visible lens barrel moves in and out, but, as has already been stated, doesn't rotate. That much can be accomplished by the use of a helical focusing mount. With internal focusing (IF), there is also no rotation, but in this case the external lens barrel may not move at all during focusing.

I agree that the choice of terms seems to be confusing the issue. I know of no application in photography of the word "rectilinear" other than the way John defined it.

-- Bob Fleischman (RFXMAIL@prodigy.net), January 14, 2002.