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).
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.
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