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Re: [OM] Follow up on Lens specific DOF tables--Zeiss replies

Subject: Re: [OM] Follow up on Lens specific DOF tables--Zeiss replies
From: Ken Norton <ken@xxxxxxxxxxx>
Date: Mon, 29 Nov 2010 10:29:06 -0600
> I think you're chasing a ghost.  I had a look at the tables you linked
> below and did some sample calculations for 50mm at f/8 using a
> conventional DoF calculator.  Those calculations don't agree with the
> Zeiss cine lens table for 50mm but they're not far off.

Well, it depends on what focus distance. At close focus, the
differences can be significant, but at infinity or hyperfocal, the
differences are essentially almost within the margin of error.


> My conclusion is that the differences between the Zeiss table and a thin
> lens calculation is likely mostly due to the difference between a T-stop
> and an F-stop.  While the real geometry of the lens may lead to
> different values than the thin lens equation, the differences due solely
> to lens geometry are probably very small and not necessary to worry about.

I believe your conclusion is correct, although your math is wrong.  Or
your math is correct and your conclusion is wrong. :)


> Finally, the only time I really pay much attention to DoF is when doing
> hyperfocal shooting of landscapes.  Usually the near focusing distance
> has to be estimated.  If your estimate is off such that you are focusing
> too close you risk not getting infinity in focus.  Therefore I always
> focus slightly past my estimated distance or else use an aperture
> slightly smaller than my table calls for.

You'd think in this digital age that the camera companies could allow
us to program a function button on the camera to instantly place the
lens in Hyperfocal mode...

As to your method of focusing slightly past the estimated distance is
good if you want to maintain maximum sharpness at infinity, but at the
expense of the potential of loss of resolution in close details. As a
general rule, I take the estimated DoF calculations or lens markings
and I go one stop down--unless I'm butting up against diffraction
limits. The DoF calculations typically assume the viewing of an 8x10
print or something like that. The extra stop gives me about the same
DoF on an 11x14 print as the calculated DoF on an 8x10 print.

These days I've been going the opposite direction with DoF, but that's
a story for another day.

AG
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