T3D DOF spreadsheet explanation

[john bercovitz <bercov@xxxxxxxxxxx>]

I just wrote the explanation below yesterday for 
a guy who wanted to use my depth of field spread-
sheet which can be found at

http://www.angelfire.com/ca2/tech3d/index.html

under downloadables at the bottom of the page.
Seems like the documentation for the stuff I do
is always weak and this is an attempt at fixing
that problem for this spreadsheet.

John B

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-- Explanation of the spreadsheet, DOF.xls --

Depth of field is actually easier to calculate for 
the case of 3D photography than for the usual cases 
because in 3D photography, we view from the center 
of perspective, or very close to it, else the 
reconstructed scene is badly distorted.  The Excel 
spreadsheet, DOF.xls, uses Kingslake's approach to 
calculating DOF as explained in "Lenses in 
Photography" and its latest edition, "Optics in 
Photography" (ISBN 0-8194-0763-1).  A graphical 
explanation of his approach is given in kdof.gif.

You should only make entries in columns which say 
<enter> in blue in the 4th row.  The other columns 
have formulas in them.

The first column (A) is where you enter what is 
going to be acceptable fuzziness for you.  What is 
acceptable depends on scene contrast and fineness 
of detail in the scene.  For general use, 1500 is 
probably good.  That's what the better companies 
use but many use 1 in 1000 and no one complains.

The second column (B) is where you enter your 
camera's focal length as printed on the lens 
barrel.

You can enter f/numbers in the third column (C) or 
use the selection that is there already.

The fourth column (D) tells you what the linear 
aperture (not the relative aperture aka f/number)  
is.  Linear aperture sets the DOF in cases like 3D 
where you view from the center of perspective.

The hyperfocal distance is probably familiar to you 
and it is found in column E.  Set your focus to 
this distance and everything from half as far away 
out to infinity meets the sharpness criterion you 
have input in column A.  This number is a function 
of linear aperture and your fuzziness criterion and 
nothing else in the 3D case.  I calculate it first 
since it is useful further on.

Column F is where you enter the distance at which  
you have focussed.  

Column I tells you how near an object can be to the 
camera (or more precisely its lens) and still meet 
your sharpness criterion as you define sharpness in 
column A.  Column J is similar and tells you how 
far an object can be and still be sharp.

Column K gives you the distance from the near 
object to the far object.  I put this in because 
you may have some objects cast in concrete and thus 
are a fixed distance apart and you may want to fool 
around with the spreadsheet to see how far away 
from the objects you have to be to get both of them 
into acceptable focus.

Column M has to do with how much fuzziness is 
introduced by diffraction.  Column O just adds this 
additional fuzziness to the fuzziness which you 
allow in column A.  So the total actual fuzziness 
is in column O.  Column A is your input but column 
O is what you really get.  Column O will show you 
what happens if you get too clever and replace the 
aperture in your lens with a pinhole.

John B  9/02/26


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End of TECH-3D Digest 455
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