I had one and tested another. On my M6, both focussed perfectly at f1 which impressed me. There was noticeable focus shift as I stopped down. I wanted f1, but would have preferred to use it in the range f1 to around f4, but the focus shift was more than the increase in depth of field.
Below I've pasted in some text of a response I made on photo.net a few years ago in response to a person called Jack who said the effect could be ignored:
Jack, with respect, I'd like to challenge your statement about the Noctilux focus shift, where you say "We are talking microns of focus shift, which translate to a few milimeters of plane-of-focus shift in the 3d image."
Erwin gives figures for the focus shift of 74micron at f2 and 120micron at f5.6, but seems uncharacteristically vague when he describes the effect of this degree of focus shift. After all, a micron seems pretty small! (One thousandth of a millimeter).
My experience is that it really makes a difference at around f2 to f5.6. My use of the Noctilux was to use it in the range f1 to f2.8. The ability to get more than f1.4 in available darkness situations was a real luxury. It is a tribute to Leica that the M6 focusses the Noctilux practically perfectly at f1. But where possible, I'd like to use a smaller aperture than f1 if I could: that was my problem.
At the following focussed distances the actual distance focussed, using Erwin's figures above, are as follows:
At f2:
2 metre: 2.12, discrepancy = 12cm = 4.7inch
3 metre: 3.28, discrepancy = 28cm = 11inch
4 metre: 4.52, discrepancy = 52cm = 20inch
8 metre: 10.45, discrepancy = 2.45metre = 8feet
This was calculated using the formula we know and love, the lens equation:
1/u + 1/v = 1/f
It seems amazing doesn't it, but when you think about it, a lens doesn't move much when you change focussed distance from 3metre to 3.28metre, does it! Now although these figures seem amazing, you can verify them by photographing a 3-dimensional target with your Noctilux. Photograph a scale with it and measure it.
Now, we need some depth of field for the following reasons:
1. Our subjects are 3-dimensional
2. Our cameras are not always perfect
3. Even the photographer is not perfect at judging the exact focus.
4. Our subjects are not immobile.
If you view the above focus shifts to be accomodated satisfactorily by depth of field, you are:
1. Accepting a circle of confusion of over 35 micron, which was recommended in the 1930s when films and lenses were very inferior to today. (I know this opens up room for lots of debate, but we didn't buy Leica gear to get that level of fuzziness)
2. Even if that level of fuzziness is ok with you, you have zero margin of error on the near side of the focussed distance. (And heaps on the other side) This was unsatisfactory for me.
