[Leica] Noctilux Depth-of-field

[Larry Kopitnik <kopitnil@mra-inc.com>]

I've always read -- or inferred from what I've read, I suppose -- that the
depth-of-field from a wide open Noctilux was as about as minimal as
depth-of-field can get. I've also read that due to that minimal
depth-of-field, only a rangefinder can reliably focus such a lens
accurately enough. And I've always accepted both thoughts as likely facts.

But playing with the internet Depth-of-Field calculator at
http://ue3.ee.nus.sg/DOF.html now raises doubts.

According to the calculator, for a 50 mm lens at f/1.0 and focused at 3
feet, the depth-of-field extends from 2.969 feet to 3.031 feet, or 0.062
feet. Depth of an 85 mm lens at f/1.4 and 3 feet is 2.985 feet to 3.014
feet, or 0.029 feet.

For a 50 mm lens at f/1.0 and focused at 20 feet, depth-of-field is 18.648
to 21.563 feet, or 2.915 feet. Depth of an 85 mm lens at f/1.4 and 20 feet
is 19.325 to 20.723 feet, or 1.398 feet.

Assuming the calculator is accurate, the depth-of-field of my 85 mm f/1.4
lens wide open is less than half the depth of field of a wide open Noctilux
would be. And I can focus the 85 quite accurately on my FM2; the bigger
problem tends to be the subject moving the intended focus point slightly.

This is in no way intended as a knock at the Noctilux. Between its wider
aperture and shorter focal length, I'm guessing I could handhold it in two
stops less light than my 85, and that remains the main reason I covet one.
But one of my supplementary justifications -- to get photos with the
smallest depth-of-field known to man -- is now dashed.

And I'm now curious about my other long-held, accepted-as-fact belief. If
my SLR can focus the 85 just fine, could it perhaps focus a 50 f/1.0 --
were such a lens available for it -- with more than twice the
depth-of-field of the 85, as well as an M6 after all?

Larry