[Leica] Noctilux DOF Film vs M8

[kididdoc at cox.net (Steve Barbour)]

On Apr 20, 2008, at 9:05 PM, Peter Klein wrote:

> Further musings on M8 focusing:
>
> The tolerances needed for everything to be exactly right on an M8  
> are tighter than with a film M.  Film has depth.  As long as the  
> rays come to a focus somewhere within the emulsion, they will excite  
> the most silver halide molecules at this point, and you have an in- 
> focus image.  Not so with the M8. Its sensor is a 2-dimensional  
> plane as far as the image is concerned. You're either in focus or  
> you're not.
>
> Now, suppose a lens focus shifts. Then the question becomes, "what  
> exactly is correct focus, anyway?"  At f/1.4, f/2 and f/8+? Or at  
> the stops between? Or some compromise that makes it within  
> acceptable limits most of the time?
>
> Because on the M8, you might have to choose.  And to face a question  
> most Leicaphiles don't want to ask:  "How much error is  
> acceptable?"  We want perfection.  With some lenses, we ain't gonna  
> get it.
>
> My guess is that with film, Leica set up a standard that put most  
> lenses in focus with most films, regardless of aperture. As a  
> practical matter, focus shift was mostly taken care of in the  
> tolerance given by the thickness of the emulsion. Maybe with film,  
> we had a fudge factor of 3 gnat's eyebrows. Now it's got to be  
> within half a gnat's eyebrow.
>
> Leica has provided some information about all this. See the LFI  
> article that showed photos of wine bottles taken with the 35/1.4  
> ASPH at various apertures.
>
> As I've said before, on the M8 I can detect a bit of backwards focus  
> shift on *all* my lenses of f/2 and faster.  I've tested with tape  
> measures at 1 and 2 meters, and cereal boxes on the roof of my car  
> at 2-6 meters, and I always get the same basic result.  I too have  
> noticed that with the M8, at middle apertures especially, it's best  
> to assume that most to all of the DOF is behind the point of focus,  
> not distributed in the classic 1/3 front, 2/3 back configuration we  
> knew with film.  This may be why some people say that "all lenses  
> back focus on the M8."
>
> Focus on the closest thing you absolutely must have in focus, and  
> you'll be OK most of the time.
>
> The good news is that with many lenses, the error may not be  
> photographically significant at sizes that most of us print at.  I'd  
> advise people to stop pixel peeping at 100% and assuming something  
> is wrong. If the screen looks good at 50%, prints up to 11x14  
> probably will, too.
>
> The Noct is a special case.  Its focus shift was already significant  
> with film.  It is even more significant with the M8. Since Noct  
> owners bought the lens for f/1, I'd suggest using it at f/1, and if  
> it works there, chalk the middle stops up to the universe and  
> physics (or learn to the appropriate amount in front of what you  
> really want).  But if adjusting the camera to your Noct at f/1 puts  
> your other lenses' focusing off, I'd suggest putting the RF back the  
> way it was, and getting the Noct adjusted.
>
> The big unknown in all this:  Has Leica has upgraded their equipment  
> enough to adjust fast lenses to best advantage on the M8?  From some  
> leaks and rumors I've read, maybe they have in Solms.  Have they  
> done so in NJ?  I don't know.  There have been an awful lot of  
> stories about people sending their lenses in, but they come back  
> focusing no better, or worse. There have been too many of these  
> stories from too many good photographers for me to attribute it all  
> to photographer error.  This is why I trust DAG more than Leica in  
> terms of lens adjustment these days.

a wonderful review and summary Peter...

what I really don't understand is the following two points and their  
(apparant) incongruity...
re fixing "back focus"....

we either change the set screw in the body with an Allen wrench, OR we  
send our lenses somewhere, WITHOUT the body, to be adjusted...

if you can fix the problem by changing something in the body, how can  
fixing the lenses with NO regard to the settings/status of the body  
possibly work?

Steve