Archived posting to the Leica Users Group, 2001/07/08

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Subject: Re: [Leica] .58VF & 90mm
From: shino@ubspainewebber.com
Date: Sun, 8 Jul 2001 11:59:24 -0400 (EDT)

> From: "Michael Reichmann" <m_reichmann@hotmail.com>
> 
> With concerns about the rangefinder accuracy of the Summicron and the .58 VF 
> I called the product manager at Leica's Canadian distributor the other day 
> with my concerns. He replied that his personal experience and information 
> from the company indicated that there was no reason not to consider the f/2 
> 90mm, .58 VF combo.
> 
> Of course this doesn't mean that there won't be an issue but at least it 
> isn't vebotten.


on page 226 of leica lens compendium, puts gives a detailed exposition
of rangefinder accuracy and on page 228 gives a table showing the
"effective rangefinder bases" needed for the various M lenses.
according to this table, even at the more lenient 0.03 circle of confusion
tolerance, the 90 summicron needs en effective base of 48.5, which is barely
covered by the 0.72 (effective base 49.86) and not by the 0.58 (e.b. 40.17)
however, the "effective base" calculation does incorporate a number human
factors, so i suppose it's possible some will do better and some worse.

regarding relative depth-of-field, using formulae i took from 
http://members.home.net/gillettm/DOF.html
i put together a little spreadsheet of depth-of-field
of some current fast m lenses.  the 90/2 has the second
smallest depth of field after the 135/3.4 *, about 1.86 inches
at 6 feet, which is much smaller than the noctilux at 6 feet
(3.08 inches) and practically the same as the 75/1.4 at 6 feet
(1.89 inches).   i was surprised that the noctilux had over one
and a half times greater depth of field that the 90 summicron.


Dist (ft)->  3     4    5    6     7    8      9    10    11    12  
 Lens                   Depth of Field (inches)
 35/1.4    2.17 3.90 6.14 8.89 12.15 15.93 20.23 25.06 30.42 36.33 
 50/1      0.75 1.35 2.12 3.08  4.20  5.51  6.99  8.65 10.48 12.49  
 75/1.4    0.45 0.82 1.30 1.89  2.58  3.39  4.30  5.33  6.47  7.71 
 90/2      0.44 0.80 1.27 1.86  2.54  3.34  4.24  5.26  6.38  7.62  
135/3.4    0.31 0.58 0.93 1.37  1.88  2.48  3.15  3.91  4.76  5.68  

(Circle of confusion =  0.03)

- -rei

* the old 135/4 also had a smaller DOF (1.61 inches at 6 feet),
  as of course the 135/2.8, although the latter had the advantage
  of magnifying "goggles."