Re: Kodak And Hyperfocus(long)

[LDAEnt@xxxxxxx]

I finally got the initiative to try to tackle the question raised by
TimMaf@xxxxxxx about the Kodak depth of field markings on the lens being
mismarked.  Be forwarned, this discussion is long and winding, if you just
want my conclusions scroll to the end.  I will assume that the sharpness at
infinity is judged by viewing a transparency (not prints), which is the only
non scientific way to do this unless one has optical measurement instruments.
  One small item, you say to place the infinity mark opposite the aperture
being used to try to hyperfocus.  This isn't quite the definition of
hyperfocus.  When you hyperfocus you set the hyperfocal distance at the focus
arrow.  The DOF (depth of field) markings on the lens should indicate that
the far limit is infinity. This is close to what you said in practice, as the
infinity mark should be at the DOF aperture mark on the far limit side.  But
it can be off due to the inexact markings for infinity on the lens.  For
example, the hyperfocal dist. at f/8 is 10.5 ft.    If you set 10.5 at the
focus arrow you see that the near focus is about 5.5ft. and the far focus is
a tad shy of infinity.  If you set the infinity mark at f/8 you get a reading
of about 12 ft. at the focus arrow.  So it is obvious that the markings on
the lens  have poor resolution for precise setting of any distance unless it
is one of the marked distances.  So setting an accurate infinity opposite an
aperture number is inexact.  This problem should be common to many cameras.
The DOF markings on the lens are determined by a circle of confusion value.
 This value is not constant in the industry, and each camera manufacturer may
use something different.  My Wirgin stereo DOF chart is not the same as the
Kokak DOF readings on the lens, and these are different from a Realist chart.
 So I suspected that Kodak was using a different C of C.  The choice of C of
C has a direct bearing on how sharp a photo will appear throughout its' depth
of focus.  If the lens was marked with a very small C of C in the DOF
calculations then your aperture marks would be at a different place (closer
together) than a larger C of C lens marking.  Therefore the judgement of what
is in focus would be different.  So, I calculated (the best I can from
reading the lens DOF marks) the Kodak C of C.  It is approx. .048mm (.0019
inches).  Most cameras that I have used use around a .001 inch value.   The
Wirgin uses .0015 and the Realist uses .001.  With each different C of C
value you get a different hyperfocus distance at the same aperture.  So the
cameras will be marked differently even though they all have identical focal
length lenses.  The .001 C of C markings will produce a narrower DOF but
sharper points than a .0019 one when set for each hyperfocal distance.  I
will now say that the Kodak C of C is too large for good results at infinity.
 This is subjective, but it seems to make sense.  If the Kodak lens was
remarked for a .001 value in the calculations, the aperture numbers would be
closer together and  the infinity mark would move more towards the infinity
direction when lined up with the aperture number (far side).  Try this test
with the Kodak: at a .0019" C of C (Kodak) the hyperfocal at f/8 is about 10
ft. Set the focus arrow at 10 and read at f/8; its a tad shy of infinity. Now
, at a C of C of .001" the hyperfocal is 19.6 ft. (same f/8). Set this at the
focus arrow.  You can see that infinity is way beyond the f/8 marking,
showing that the lens is focused closer to infinity, and should produce a
sharper picture.  Boy, I think I have this straight.... it's late at night.
 If anyone sees error in this discussion please jump in. 
I checked the accuracy of the lens markings by comparing the calculated DOF
distances at the Kodak C of C value versus the actual lens markings.  They
compare very well, close correlation.  So the lens is marked OK, just with a
large C of C value.  I also checked the total lens travel (with a dial
indicator)  when the focus is changed from infinity to 4 ft.  It matches my
Wirgin lens travel exactly, so I feel that the lens is traveling the correct
distance for the markings on the lens.  An inexact test, but both my cameras
would have to off the same amount if there was a problem, and my Wirgin is
never off! ;-)
I feel it's the combination of inexact resolution with the lens markings and
a too large C of C value in marking the lens.  Maybe I should make a stick on
label with the tighter markings to cover up the current markings. 


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