RE: [Leica] I eat crow

[Henning Wulff <henningw@archiphoto.com>]

At 2:50 PM -0500 1/14/03, Austin Franklin wrote:
>  > I retract my questioning of Martin Tai's calculation.  I was thinking
>>  in terms of the image space instead of the object space.  While
>>  mulling over this while eating breakfast, I realized my error.  I
>>  calculated the correction assuming that the 50mm lens is a thin lens
>>  since I don't know the distance between the image and object
>>  principal planes of a Leica 50mm Summitar, e.g.  I came up with a
>>  setting of 94.61 cm, which is pretty close to Martin Tai's 94.7.
>
>Herb,
>
>The amount of deflection is SOLELY based on where the pivot point is, and
>obviously different pivot points will vary the distance.
>
>If the pivot point is the middle of your body (such that you take the
>camera, in your hands, and turn your entire upper body), then yes, you can
>get a substantial deflection...but it's up to YOU to control that.  If you
>simply "adjust" the camera by turning it in your hands, you will get a very
>small deflection, well under 1/2".
>
>This "issue" is obviously technique dependant, but I must admit, I have
>never had this be an issue, and I shoot wide open all the time, and do
>"re-focus" for quite a few shots...focus is spot on for me.  No "calculated
>adjustments" required.
>
>I'd suggest anyone who is questioning whether this "methodology" is of any
>use for them, to simply do the experiment them selves.  Shoot some with, and
>some without.
>
>Regards,
>
>Austin

Technique has nothing to do with it.

What Martin Tai's page is about is the fact that if a lens has a 
perfectly flat field, ie, it images a flat plane in front of the 
camera onto a flat piece of film, then the parts of the image that 
are at the edges of the frame are further from the camera than the 
part of the image that is at the center of the frame.

It's like standing on a sidewalk, looking at the opposite side of the 
street exactly perpendicular to the row of buildings along the other 
side (we're assuming a straight street with all buildings starting 
the same distance from the sidewalk).

We are, say, 25m from the building (building A) directly opposite. If 
we use a lens that has a 90 degree angle of view horizontally, the 
edges of the frame will image buildings that are still the same 
distance from the sidewalk we're standing on, but from our camera 
position the distance is longer than 25m; it is (25m x the square 
root of 2), or 35.4m away. However, our camera is focussed at 25m, 
and this is correct. If we now want building A opposite us to be at 
the edge of the frame, we have to turn our camera 45 degrees and are 
now centered on a building that used to be at the edge. Now, to get 
the camera to image Building A sharply, we have to set focus at a 
distance of (25m/square root of 2) or 17.7m. As before, we are 
focussed at 17.7m (which happens to be in the middle of the street, 
but the edges (also perfectly in focus) are a lot further away; 25m 
in this case.

In large format work this discrepancy is more important due to the 
longer focal lengths involved, but less important due to the small 
apertures used. So it is generally ignored as well.

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    *            Henning J. Wulff
   /|\      Wulff Photography & Design
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