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

[jacob@xxxxxxxxxxxxxx (Gabriel Jacob)]

The limited time offer has expired, so.....
ok, ok, enough foolishness. ;-) 

Awhile back Paul Talbot wrote,

>>Greg Wageman wrote:
>>Perhaps they changed the size of the Circle of Confusion.
>>This is a key factor in DOF scales, and you're comparing
>>apples to oranges unless you know this parameter.

>Paul replies,
>Greg, do you mean "changed the parameter for purposes of the
>calculations" or that the CoC of the lenses actually changed?
>
>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? 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'm not ready to accept the idea that this whole group is
>stumped for the explanation of the DOF table/CoC difference.
>Who's keeping secrets out there?

It certainly isn't me! Recently while browsing an old copy of
"Handbook of Photography, 1939 ed", I came across the formula
for determining the depth-of-field and hyperfocal distances.
Paul (and others, including myself) wondered why there were
differing tables and the conclusion was because the CoC
parameter was probably changed. Different CoC are used to
reflect if it's a slide, small print, large print, etc. and
consequently yields different tables. But this begs the
question, why did they change the table (since slides were
used in each case)? Was it to fudge the performance or was
it a reflection of better lenses, film, etc.?


On an Realist with Ilex lenses and another Realist with David
White lenses I found these two differing tables.

David White f3.5 lenses
Hyperfocal Table (serial number A55495)
---------------------------------------------
Diaphragm    Set Distance  Camera is in focus  
 Setting        Dial On         From   To 
   f2.8           44'            22'-inf.
   f3.5           34'            17'-inf.
   f4             30'            15'-inf.
   f5.6           22'            11'-inf.
   f8             15'           7.5'-inf.     
   f11            11'           5.5'-inf.
   f16             8'             4'-inf.
   f22           5.5'          2.75'-inf.

Ilex f3.5 lenses
Hyperfocal Table (serial number A8314)
---------------------------------------------
Diaphragm    Set Distance  Camera is in focus  
 Setting        Dial On         From   To 
   f3.5           45'            22'-inf.
   f4             40'            20'-inf.
   f5.6           28'            14'-inf.
   f8             20'            10'-inf.
   f11            14'             7'-inf.
   f16            10'             5'-inf.
   f22             7'           3.5'-inf.

>From the two tables above, it looks like the CoC was increased
for the DW lenses but I wasn't sure by how much.

Refering to the formulas given in the handbook, the hyperfocal 
formula is given by,

        F x F
H = -------------
     (f x C x 12)

where H, hyperfocal distance (feet)
      F, focal length of lens (inch), (35.2mm=1.3858inch for a Realist)
      f, aperture (f/number)
      C, circle of confusion (inch)

 
Solving for C, the Ilex lenses results in a CoC (C) of 1/1000
(.001 inch) of an inch. Later with the DW lenses they increased
the CoC to 1/750 (.00133 inch) of an inch.

Now one would think that they would decrease the CoC for the DW
lenses, since they are supposedly not as sharp as Ilex lenses but
we see the opposite. From all this it would seem to indicate that
it was done for purely marketing purposes.

Any other theories, ideas, thoughts, opinions, etc...

Oh yea, before I forget, here is the DOF formula.

         H x a                      H x a
Dn = --------------        Df = --------------
     H + (a - f/12)             H - (a - f/12)

where Dn, near distance (feet)
      Df, far distance (feet)
       H, hyperfocal distance (feet)
       a, camera focussed to distance X (feet)    
       F, focal length of lens (inch), (35.2mm=1.3858inch for a Realist)

So Paul, with the above, you can generate your own hyperfocal/DOF tables!
Enjoy! ;-)

Gabriel


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