Archived posting to the Leica Users Group, 1996/11/06
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I have recieved several replies concerning the 75mm/1.4; 90/2.0; and, 90/2.8
lenses. Other than the weight of and blocking of the viewfinder by the
75/1.4, the main problem with the lens was near focusing at 1.4. To better
understand the problem I calculated the DOF for the 75/1.4 at its closest
focus (.75 meter) and together with the 90/2.0 and 90/2.8 at 1 meter (their
closest focusing distance) and at 2 meters.
I used the formula set forth in Platt "The Ultimate Photo Data Guide," (1989):
dn (near) = U x F(2)/F(2) +(U x c x f)
df (far) = U x F(2)/F92) -(U x c x f)
F(2) = focal length of lens (squared)
U = Subject distance
c = circle of confusion (here for 35mm - 0.036mm)
f = f-number of lens
All demensions are meteric, length = mm
(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"
(2) 75mm - 1.0 meters (39.37"):
75/1.4 17.93mm .706"
75/2.0 25.61mm 1.00"
75/2.8 35.86mm 1.41"
(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"
(4) 75/1.4, 90/2.0, 90/2.8 - 2.0 meters (78.74"):
74/1.4 71.7mm 2.82"
90/2.0 71.13mm 2.80"
90/2.8 99.61mm 3.90"
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>.
The other thing is that the 75 at 1.4 and the 90 at 2.0 have virtually the
same DOF.
How accurate can the M6 focus??? At 3 feet the DOF is about 3/4-1" and at 6
feet it is about 3 inches. However, below 30-32" we are looking at 3/8-1/2"
DOF. No wonder focusing is this range is difficult.
Well, since I did this I thought I would post it FYI. I make no guarantees
either to the formulae or the calculations.
Dick Hemingway
Norman, OK