[Leica] Newbie question about frame line accuracy

[henningw at archiphoto.com (Henning Wulff)]

At 1:21 PM -0400 4/14/05, Scott McLoughlin wrote:
>Why aren't the framelines in the M's (I have a TTL) very
>accurate for framing?  Is there some special technical challenge
>involved?
>
>Scott

Rangefinders have two issues which make accurate framing close to 
impossible. One is parallax, 'cause you're viewing the subject from a 
different point than the lens sees it. Leicas, and most other RFs 
deal with this by moving the frame lines towards the lens as the lens 
is focussed closer. This is purely a function of the distance, so all 
frame lines can move the same amount.

The other is the fact that a lens, as it's moved away from the body 
sees a narrower angle of view. A 50mm lens moves some 4mm or so away 
from the body to focus at .7m. So instead of 51mm or so, it's now 
acting like a 55mm. I didn't do the math re: the extension, or look 
up the actual focal length of Leica lenses, but the ballpark is right.

Leica has designed the framelines so that at closest focussing 
distance, nothing gets cut off if you're shooting slides - the worst 
condition. So that defines the narrowest angle of view. If you shoot 
negatives at infinity that means that your viewfinder showed a lot 
more than you have on your negatives.

Since you have to move a longer focal length lens a lot more to focus 
at the same distance, the longer lenses have the greater error when 
shooting at infinity. 21mm lenses will be quite close; 135s have a 
_lot_ more on the neg at infinity.

The above means that medium format cameras, or any other larger 
format cameras have a lot more problems, as the 'standard' lens might 
be an 80 or 90, and therefore must extend a lot more for closer 
distances. That's why they sometimes have the feature that the 
framelines narrow and widen as you focus, to more accurately frame 
the picture. Cameras like the Koni-Omega had that. It does add 
complexity and sometimes caused problems as well as initial expense, 
but there were less framelines in those cameras compared with the 
Leicas as well. There were some 35's that had that feature as well.

Leicas of all vintages used the same criteria for frame lines. M3 
frame lines appear to be more accurate, but that was only because it 
was designed to focus to 1m (for lenses without eyes), and that meant 
that the extension of the lenses, and therefore the constriction of 
the angle of view was less.

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