Apo-90 Cron Asph. vs. Apo-75 Cron Asph.

[adan]

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Jan: As to the apparent increase in focal length, that is a real effect. If you pull a slide projector further from the screen, the image it throws gets larger. The same thing happens with a lens that focuses by increasing the distance from the lens to the sensor or film - it throws a larger image when focused close.

 

(Incidentally, when the first 28-105 zooms hit the market, which used primarily internal focusing or front element focusing, users were shocked to discover that their zoom at the 105mm end did not match their 105 prime lenses for "perspective" or magnification when used at the same distance for portraits. It looked more like an 85-90mm, and they thought they had been short-changed in zoom range. Pop Photo demonstrated that the lens really was a 105, comparing framing at infinity focus, but that there was no "zoom" effect at closer distances, because the lens itself was not moving further from the film, just a few elements in the middle. Presumably someone who has access to both the 180 APO-Elmarit (internal focusing) and one of the older 180s (full-lens focusing) could test this effect.

 

In theory, Leica could redesign their M telephotos to be fully internal focusing, and eliminate most of the magnification change with focus distance - but then the viewfinders of all existing cameras would be out of whack. One would get the sloppy infinity framing at ALL distances, including close-up).

 

And, yes, that is why the framelines of a rangefinder are only exact at one focus distance.

 

Also, yes, focusing, and the lens extension required, and thus the extra magnification due to the lens extension, are non-linear. On my 90 APO, moving the lens forward 1mm changes focus from infinity to 4 meters. Moving it another mm changes focus from 4m to 2 meters. Another mm moves focus from 2m to 1.4m. Another mm moves it from 1.4m to 1.1m. So as one focuses closer, it takes more and more lens extension for each incremental decrease in subject distance. So the magnification change from 2m to 1m with the 75 IS smaller than from 1m to 0.7m.

 

Rob: Quite right. Leica chose to use the "view" at minimum focus for all their lenses, on the assumption that we'd rather not cut off things accidentally close up, and could always crop on long-range shots.

 

And yes, the longer focus range of the 75mm, relative to its focal length, is why its framelines are the most troublesome at longer distances.

 

4 months ago this issue was discussed and I came down on Leica's side - better to crop intentionally than unintentionally. Since then, I have decided that the 75 framing variation, for me, is simply too extreme - so I switched to a 50/90 combo.

 

As another side note, both the Contax G2, and the old Leica multi-focal-length zoom viewfinders (VIOOH), used black masks to adjust the framing. In the G2, the masks slid down and sideways for parallax correction, and also tightened up noticeably at close range to account for the lens extension effect. The Leica zoom finder actually has separate index lines and click-stops for close-up and infinity framing, so that with a 90 or 135, using the close-up mark will tighten the edges of the frame (zoom in a little).

 

Unfortunately this is difficult to implement in a bright-line finder that covers multiple focal lengths. Although perhaps Leica has done it in the zooming 16mm-28mm "Frankenfinder" - I haven't tried one to see.