[Leica] Noctilux DOF Film vs. M8

[freakscene at weirdness.com (Marty Deveney)]

Frank,

To be honest I was surprised.  I found these figures in some published 
materials and thought they seemed very large.  I had a colleague check them 
in the lens design program ZEMAX and it produced nearly identical figures.  
They are definitely correct, but they are absolute, not the amount greater 
than the DoF I noted earlier.  Hence the improved resolution performance of 
the Nocti at 5.6 compared to f1 as reported by Erwin.  The best anyone can 
do is make sure that their Noctilux works optimally at f1.  On the M8, 
stopped down there is always going to be some uncertainty introduced by 
focus shift and this will be worst from f1.4-f3.5 or so, where the shift is 
greater than the corresponding increase in depth of field.  It can't readily 
be fixed, because the shift varies with the aperture.  The camera, of 
course, has no aperture coupling and even if it did, there is no way to 
adjust the RF cam for different apertures.

The Konica Hexar (the fixed-lens version, not the RF) had a combined 
active/passive autofocus infra red RF system that corrected for focus shift 
automatically, but to my knowledge it is the only camera that has ever done 
that.  Interestingly, the 35/2 lens on the Hexar shifts focus less than .  
If the autofocus sensors in SLRs improve in their sensitivity, it is 
entirely possible to have them adjust after the lens stops down but before 
the shutter fires.  In a really modern camera this would slow the system a 
negligible amount and could be designed so that the user could easily switch 
it off.  I think, however, that this may be a long way off or something that 
never eventuates.

Really thin fine-grain film emulsions can be around 120-150 microns thick, 
so this optical effect can occur to an observable amount when using film 
too.  It's less likely with the kind of fast emulsions used with the 
Noctilux.

Marty

Gallery:
http://gallery.leica-users.org/main.php?g2_itemId=7617

Most people can only judge of things by the experiences of ordinary life, 
but phenomena outside the scope of this are really quite numerous.
        Shen Kuo - 'Dream Pool Essays'


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