[Leica] Re: darkroom focusing

[henningw at archiphoto.com (Henning Wulff)]

At 7:18 PM -0700 6/15/05, Herbert Kanner wrote:
>I think focus shift is a myth. If the focus were to shift when 
>stopping down, this would indicate that spherical aberration is so 
>poorly corrected that the portion (a ring) of the lens that is 
>active between say f/8 (the enlarging aperture)and say f/2,8, the 
>focusing aperture, has a marked;y different focus distance than the 
>portion (a circle) from the center to the periphery at f/2.8.  If 
>this were true, the lens would suck; it would be noticeably fuzzy at 
>f/2.8.
>
>Herb
>--
>Herbert Kanner
>kanner@acm.org
>650-326-8204

It's no myth. Check it out; most enlarging lenses, especially the 
f/2.8 ones and older 4 and 3 element ones, suffer from it to a 
noticeable degree. I use a Micromega focusser that I bought in the 
70's, and it has no trouble detecting the difference in focal plane 
if I don't have a sheet of paper in the easel. It corresponds with 
the results I get from printing, so it's accurate enough.

Anyway, print one picture after focussing on the grain stopped down, 
and then open it up and print again. Then print the picture with the 
lens focussed wide open. Do this for a 16x print. You'll probably see 
the difference, unless your lens is one of those few that really 
doesn't have any focus shift.

Some lenses I've had or used that have had focus shift problems:
Various 50mm f/2.8 Rodagons of different eras (all)
50mm f/2.8 Componons - same
40mm Focotar
240 Componon
150 Rodagons and Componons of various ages
75 Nikkor
50/2.8 Nikkor
105 Rodagon (slight)
WA Componon 60


Lenses that have negligible or no focus shift:
Nikkor 80/5.6
Nikkor 63
Focotar 50 (second)
S-Orthoplanar 60

There were others, but I can't recall, because they were all good 
lenses and I focussed them stopped down, so it doesn't matter.

On 8x10's, it rarely matters. On 16x20's, it almost always makes a 
difference, just like aligning your enlarger.

For many pictures, it doesn't matter very much if the grain is crisp; 
on others it does make a difference. YMMV :-)

-- 
    *            Henning J. Wulff
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