P3D Re: DOF Table on Realist lens cover varies among samples!

["Greg Wageman" <gjw@xxxxxxxxxx>]

From: Paul Talbot <ptww@xxxxxxxxx>


>In which case aren't we down to semantics?  Observations by Mike K.
>and me (jointly)--and Gabriel singlehandedly--were that the DOF tables
>vary.  We want to know why.  A different CoC parameter would give
>different DOF tables, but what does that tell about the cameras?

Nothing.  Changing one's choice of CoC changes nothing about the camera; it only
says how fuzzy an image one is willing to call "in-focus".  The camera will
produce the same images regardless. :-)

>We're looking for why a different DOF table is specified, or, asking
>the same question another way, why is a different CoC specified, as
>the cameras and lenses (as far as we know) are identical?

I thought I explained that-- it's a matter of choice.  Although if the marketing
department is known to be doing the chosing, I might look twice at the numbers.
:-)

>P.S. As a matter of practicality, I suspect that a dial made for
>the DOF scale on Mike K's Realist would have the same scale, but
>would have different markings (for ease of use).  My dial has marks
>at 3, 4, 5, 6, 8, 10, 15, 25, and Inf; a dial for his camera would
>likely have somewhat different marks to make it easy to set focus
>to the "Set to" value of the DOF table for his camera.


Paul, I'm sitting here as I type this with two Realist cameras.  One is serial
number A3265, an Ilex Paragon Realist that has not been retrofitted with a DoF
scale or double-exposure prevention feature ( in other words, a very early
Realist).  It has, however, the DoF chart under the lens cover, which reads in
part:
f-#      set   in focus from-to
f/11 - 11' - 5.5'-inf.;
f/16  - 8' - 4'-inf.;
f/22 - 5.5' - 2.25'-inf.

 I also have a late f2.8 Realist with oversized rewind knob, serial #025358.
The focus knob on both is marked identically: 2.5, 3, 4, 5, 6, 8, 10, 15, 25,
Inf.

Using the DoF scale on the later model gives, approximately (because of the need
to estimate the readings between markings):

f/11 - 12.5' - 6-Inf.
f/16 - 10' - 5-Inf.
(There is no indication for f/22 on this camera.)

As Mike said, these indications leave lots of room for interpretation (e.g.
should infinity be right at the f-number on the far scale, or slightly ahead of
it; is the distance scale linear between markings, or logarithmic as it appears
throughout its range?).

Given the uncertainties, the DoF scale readings vs. the chart don't seem THAT
far out-of-line with each other.  I think we're making a mountain out of a mote.
:-)

     -Greg W. (gjw@xxxxxxxxxx)



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