Archived posting to the Leica Users Group, 1996/11/07

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Subject: RE: 75mm, 90mm -DOF
From: KEVIN BURKE <KBURKE@iterated.com>
Date: Thu, 07 Nov 96 12:30:00 EST

>From: Richard W. Hemingway

>[...]

>(1) 75mm - .75 meter (29.5"):

>        75/1.4          10.08mm         .3968" (3/8"+  or abt 25/64)
>       75/2.0          14.40mm         .57"
>        75/2.8          20.17mm         .79"

>[...]

>(3) 75/1.4, 90/2.0, 90/2.8 - 1.0 meters (39.37"):

>        75/1.4          17.93mm         .706"
>       90/2.0          17.79mm         .7004"
>        90/2.8          24.29mm         .9563"

>[...]

>Two interesting things emerge.  First the extremely small DOF of the 75/1.4
>at .75 meter, about 3/8ths of an inch.  Or, as one responder stated "about
>the thickness of two Leica Fotografie International magazines.  Actually,
>this is incorrect - according to my Starrett micrometer, two issues are 
only
>0.228" thick, three issues are closer, ie, 0.344" thick <gr>.

That would be me.  I stand corrected.  Now, will the editors of LFI please
start putting more stuff in their publication so its thickness grows?  I 
usually
read an issue in one to two days and don't like waiting for the next one.

>The other thing is that the 75 at 1.4 and the 90 at 2.0 have virtually the
>same DOF.

This is certainly the case at the same distance.  Since the 90 is limited
to 1m, this is the closest where we directly can compare the two.  Maybe
if the 90 went to 0.75m then maybe similar issues would arise with this lens
as well.  But, considering these limits, it is during actual use where I
believe complications arise.  If the user frames the subject to the same
image size on film (rather than maintain perspective) with the 75 as they
would have done with the 90, then the camera must be moved closer to
the subject.  Adding to that, I frequently ran into the near focus stop on 
the
90mm.  So, with the 75 I find I can snug up just a bit more which I most
likely would have done with the 90 if it were possible.  The bottom
line being that I have indeed been trying to use the lens in the nearest
portions of its focus range.  Danged difficult, that is...

Thanks for the numbers Dick!

 -Kevin
kburke@iterated.com