Leigh Youdale
Well-known
It's Hardly An Exact Science!
It's Hardly An Exact Science!
I think some people go looking for a degree of precision that isn't there and was never intended to exist. Most of the lenses I've had over the years had a DOF scale on them that gives a reasonable approximation of the zone of acceptable focus. I've never used them for more than a rough guide and if the subject was important I've always gone for a reduction at least half of the next scale marking.
What is the "zone of acceptable focus"? Generally I've found with all of the lens markings that for shop prints of 6x4 the focus is most acceptable; for prints from the same negative of 10x8 some softness is frequently observed and at 16x20 it's often not acceptable. Since the fall-off of sharpness is gradual there's no "tipping point" beyond which the result is or is not acceptable. You just have to make a judgement call and, as Roger says, if in any doubt, use a tighter scale.
It's Hardly An Exact Science!
I think some people go looking for a degree of precision that isn't there and was never intended to exist. Most of the lenses I've had over the years had a DOF scale on them that gives a reasonable approximation of the zone of acceptable focus. I've never used them for more than a rough guide and if the subject was important I've always gone for a reduction at least half of the next scale marking.
What is the "zone of acceptable focus"? Generally I've found with all of the lens markings that for shop prints of 6x4 the focus is most acceptable; for prints from the same negative of 10x8 some softness is frequently observed and at 16x20 it's often not acceptable. Since the fall-off of sharpness is gradual there's no "tipping point" beyond which the result is or is not acceptable. You just have to make a judgement call and, as Roger says, if in any doubt, use a tighter scale.
