I can, of course, be wrong and your rocking method of focusing (which I
also use) *may* work. But I just find it extremely doubtful in this
particular case. My assumptions (since I don't have a 100/2): That,
like the 85/2, the 100/2 has a 180 degree focusing motion from its
closest focus of 0.7 meters to infinity and that the lens's motion is
linear with respect to focusing ring turning angle.
A 100mm lens has to move 16.67mm to cover that range. That means that
each degree of rotation moves the lens a distance of 0.0926mm. However,
if DOF is taken as 13mm on either side of of the plane of focus the
total range of lens motion to move the lens back and forth within that
26mm span is only 0.07mm which amounts to total focusing ring rotation
of 3/4 of one degree or 3/8 of one degree one either side of the plane
of focus.
As I said, I can (and even have been) wrong. I just find it doubtful
that you can move the focusing ring that accurately. I think that, by
the time you see a difference in focus you're already outside the range
and can't accurately reposition within the middle of that range. To
emphasize the point: I assume the diameter of the focusing scale on the
100/2 is about 65mm. If there were 1 degree markings on that scale they
would be 0.57mm apart. The total linear range of motion for 3/4 degree
would be 0.425mm and, of course, only half of that on either side of the
plane of focus. Maybe you can do it. I don't think I can. Of course,
if I relax the CoC to a resolution of 40 lines/mm vs 60 lines/mm the DOF
extends from 13mm on either side of the plane of focus to 19mm. That
extends the focusing ring rotation from 3/4 degree to about 1.2 degrees
or about 2/3mm on a linear focusing scale and 1/3mm on either side of
the plane of focus. Easier but still mighty hard in my book.
Chuck Norcutt
On 9/30/2011 3:41 PM, Ken Norton wrote:
>> I basically ignored him.
>
> Thanks. I love you too.
>
>
>> ...I don't believe he can focus any more
>> accurately than to be somewhere within the DOF at the stated
>> resolutions.
>
> And that's what I said! When you can't find the exact plane of focus,
> then you rock the focus back and forth to identify where it becomes
> less sharp. Then you adjust to somewhere between those points. It's
> simple and works quite well most of the time.
>
> If all I did was adjust focus from one direction and stopped when
> things appeared sharp enough, then it would matter which side of the
> plane of focus I started from. In this case, if one lens was focused
> from infinity, I could stop on the near side of the DoF. But if the
> other lens was started from close-up, then I could end up stopping on
> the far side of the DoF. THAT is why I always focus to either side and
> bump to the middle. If I follow exactly the same procedure with both
> lenses, I don't see where the problem is. If you have a better method,
> I'm all ears. But if you claim that the best you can do is achieve
> focus to somewhere within the DoF without any ability to narrow it
> down further, I'm going to call foul. This particular method works and
> works quite well. Oh, and it's been a published methodology since
> time-immortal.
>
> I'm going to AGAIN do the tape-measure test. Not to prove anything to
> you, but to answer this lurking question in my own mind as to why the
> two lenses aren't focusing the same even though they appear to be
> focusing at the same point. The Zeiss article definitely points to
> this as being either a nodal-point difference or a focus shift due to
> aperture adjustment. The Zeiss article also addresses the cone
> differences at identical apertures from lenses of different maximum
> apertures.
>
> In the world of assymetrical lens elements, all bets are off.
>
> AG
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