I basically ignored him. He says he was shooting at 2 meters. Even
assuming an unrealistically high resolution of 60 lines/mm the DOF at
100mm and wide open at f/2 is still approx. 13mm on either side of the
point of focus. Using a more realistic resolution number like 40
lines/mm the DOF is about 19mm on either side of the point of focus.
And if he was shooting stopped down as far as only f/4 the depth of
field is about 39mm on either side of the point of focus.
Even though the theoretical DOF calculation says the DOF is nearly
perfectly symmetrical on either side of the point of focus I do accept
that the real lens may distribute it slightly differently. However, I
don't believe his claims of extraordinary precision in focus even if he
claims he's focusing on pixels. I don't believe he can focus any more
accurately than to be somewhere within the DOF at the stated
resolutions. Humbug I says. :-)
Dr. Focus
On 9/29/2011 8:11 PM, usher99@xxxxxxx wrote:
> Before digging into full wave dof calculations vs thin lens
> approximation and correspondence with Nasse at Zeiss,
> would not have thought such a dof discrepancy
> could be real. It seems very possible now. Real FL differences also
> change the distribution of the dof, but don't know
> the actual measured FL for each of the Zuikos and doubt they differ by
> very much. First thought the differences
> might be due to focus shift----see the demonstration below with the
> Zeiss 85/1.4. Fast lenses of certain designs w/o floating elements
> often shift focus stopping down. Dr. Nasse seems to get pinged on this
> issue a bunch.
>
> http://www.the-digital-picture.com/Reviews/Zeiss-85mm-f-1.4-ZE-Planar-Lens-Review.aspx
>
> Then AG said he had focused at the shooting aperture, which
> ruled that out.
>
> Still, a similar set up with the Zuikos might easily resolve the
> question unequivocally.
> Would need to be cautious with the experimental design. Dr. Focus
> appears to be around to help.
> Anyway enjoyed the post.
>
>
> Mike
>
>
>
> When shot at exactly the same apertures, and carefully
>> selecting the same focus point, the 100/2.8 always seems to have a
>> touch
>> more DoF on the near-side. At this time, I'm calling this a human
>> error, yet
>> time and again, when focused in exactly the same manner on exactly
>> the same
>> point, the 100/2.8 is sharper closer in. I've even focused the lenses
>> stopped down and the same thing occurs. Before letting this go,
>> totally,
>> what I'm wondering is whether or not the rate of defocus either side
>> of the
>> plane of focus is assymitrical between the two lenses.
>
--
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