T3D Re: DOF spreadsheet explanation

[john bercovitz <bercov@xxxxxxxxxxx>]

> But wait!  If you are in the same boat as I am, I found a free
> little program at:
>
> http://www.dl-c.com/dl.html
>
> which ran just fine on my Windoze NT box; so chances are good it
> will run OK on a more mainstream version of Windoze as well.  :-)

All right!  This is easy to use.  For 3D use, all you have to do
is figure the film plane resolution so you you can enter it into
this program.  So let's say you are into medium format and so you
shoot and view with an 80 mm lens.  You've decided to declare a
fuzziness of 1 part in 2000 acceptable (the eye can see 1 in 3400
under very good circumstances and 1 in 2000 in more ordinary
situations).  80/2000 = .04 mm from line to line and 1/.04 = 25
lines per mm.  Of course you could just as well do 2000/80 = 25
in the first place.  So you enter 25 in the correct box in the
program and also 80 mm for the focal length.  This is so simple
that I shouldn't have feared damaging my dainty brain trying to 
figure it out yesterday!  8-)

OK, so I've entered 80 for focal length, 16 for the f/stop, 25 lpmm
for the resolution, and 1.5 m for the distance to focus and the
program off the web spits out near focus of 1.313 m, far focus of 
1.748 m, and hyperfocal distance of 10.080 m.

So I try the same thing on my spreadsheet.  I enter 1 part in 2000,
80 mm, f/16, and distance to object of 1.5 m.  It spits out 1.000 m 
for the hyperfocal distance, 1.304 for the near focus, and 1.765 for 
the far distance.

Why is there a difference?  Because the program off the web measures 
from the film plane (which is standard) and I measure from the lens
which is more accurate because the unknown hiatus doesn't come into
play.  The web program assumes the hiatus is = 0 which is fine if you
don't do macro work.

John B


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