Focusdifference when stop down to f16greenspun.com : LUSENET : Large format photography : One Thread
Yesterday I did a long testing in my little studio with flashlights etc. because allways when I get a new objetiv I will test it a bit. The ojectivs I`m talking about I get together with a occ. Horseman so one is a f 8 90 mm Schneider from 1952, the other is a Symmar 210 f 5.6 from 1976. On a distance of 2 meters I got a focus difference on the 90 from about 30 cm to the front when stopped down to f 16, and with the Symmar 10 cm to the front, with the newer Rodenstock 150mm I have this not! Is this the price for old lenses? Had enybody similar expiriences?
-- Armin Seeholzer (email@example.com), March 15, 2001
Is this about depth of field, or a focusing error, or focus shift on stopping down, or what?
Depth of field will not be affected by the age of the lens.
If any lens made in the last 100 years doesn't focus properly, then it's been damaged, and has nothing to do with how old it is (deliberate soft-focus lenses excepted).
-- Pete Andrews (firstname.lastname@example.org), March 16, 2001.
I`ts focus shift or difference, when I was focusing with f8 on the 90mm to a distance of 2 meters sharp, after stopping down to f16 the sarphest point drifted to about 1,70m, but I din`t anything on the bellows or camera just stopped f8 to f16! Now I know what Ansel Adams meant by focusdifference when stopped down!
-- Armin Seeholzer (email@example.com), March 16, 2001.
A lens would have to have pretty severe spherical aberration to shift focus that noticeably. Neither the 90mm SA, nor the Symmar should show spherical aberration to that degree.
There's a possibility that they haven't been assembled on the shutter properly after mounting in the lensboard. If the lensboard thickness prevents the back of the lens screwing fully home into the shutter, that's the sort of effect I'd expect.
How are you testing for this focus shift?
If it's just visually, you can't rely on it. The image gets pretty dim at f/16, and the characteristics of the ground-glass screen might change with a narrower cone of light, especially if it's a fresnel enhanced type.
-- Pete Andrews (firstname.lastname@example.org), March 19, 2001.