RE: [Leica] Flat vs curved field, was Focusing the M6

["Henning J. Wulff" <henningw@archiphoto.com>]

At 1:40 PM +0800 10/26/00, Ken Lai wrote:
>Ted,
>
>Technically the problem does exist, whether it matters, however, depends on
>the photographer. In some cases, if the photographer ponders for half a
>second to re-calculate the focus distance before hitting the shutter, the
>picture will be gone and so it makes no sense to think about this problem.
>
>Also the focus discrepancy is covered by the DOF in most cases as long as
>the subject is not near the far edge of the frame. In the case of the Noct,
>if your subject is about one third the frame width from the edge, the
>subject will be covered by the DOF even at f1.0 and 0.7m and there will be
>no focus problem.
>
>Ken Lai

Once again we come across the mythical Noctilux that focusses to .7m. 
I wish I had one :-).

As anyone who has use a Noctilux regularly knows, nailing the focus 
is not that easy, and certainly not consistent. Besides DOF, and the 
fact that for most people shots an eye at the very edge of the frame 
rarely makes a decent picture, this basic fact of Noctilux picture 
taking would generally cover up this focussing discrepancy. I've been 
aware of it since I got a Summarex, and my efforts to compensate have 
not been worth it.

Another point to consider is that while a lot of modern lenses, even 
very fast ones, have fairly flat fields at infinity, most don't at 
closer distances. Fortunately for the above focussing problem, the 
type of curvature usually seen is the type that has the center 
focussed at a greater distance than the edges, neatly counteracting 
this problem.

Also, the angular displacement of teles is less, so the cosine 
problem is less with those lenses that have shallower depth of field. 
The wideangle lenses have more of a cosine focussing problem, but the 
depth of field covers any errors better.

Still, as I pointed out above, the most common focussing problem is 
getting the basic focus to be dead on, which has to do with your 
personal focussing accuracy, the camera/lens accuracy at that 
distance, film flatness, and most important, PRACTICE and EXPERIENCE.

Don't sweat it. Shoot it.

- -- 
    *            Henning J. Wulff
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