New toys

[P3D John Bercovitz <bercov@xxxxxxxxxxx>]

I added some new toys to the Excel file which calculates the 
maximum allowable stereo base for a given maximum allowable 
on-film deviation.  Here are the inputs and outputs as they 
stand now.

Inputs
I1) maximum allowable on-film deviation  You calculate this by 
dividing your format's lens focal length by 30 or by entering a 
value you find acceptable.
I2) lens focal length.  This is the focal length of the lens on 
the camera (which should be the same as the focal length of the 
lenses on the viewer).
I3) nearest point in scene  Enter the distance from the camera 
to the nearest point.
I4) farthest point in the scene  Enter the distance from the 
camera to the farthest point.

Outputs
O1) distance for best average focus  This figure shows where you 
should focus your lens so that the nearest and farthest points in 
the scene are equally sharp on the film.  After calculating this 
distance, I like to actually set an object at this distance 
temporarily and focus on it with my lens wide open.
O2) effective focal length of the lens  Since you move the lens 
outward towards the objects when you focus, this tells you how 
long your lens is pretending to be in this situation.  For 
instance, if you have a 35 mm lens and your acceptable on-film 
deviation is 1.7 mm and your near point is 100 mm and your far 
point is 140 mm, the lens will be out at 50 mm.  So you would want 
to use a viewer with 50 mm lenses in it to get the correct 
magnification.
O3) maximum allowable stereobase  This is the maximum amount you 
can shift your camera on your slide bar without exceeding the 
maximum allowable on-film deviation you input above.

Additional input
I5) desired resolution in minutes of arc  This is the figure you 
want to input to calculate your correct f/number setting using the 
standard geometric formula for calculating f/number.  If you don't 
stop down enough, you won't have the depth of field you need.

Additional outputs
O4) nominal f/number   This is what you set the f-stop lever or 
barrel on your lens to.
O5) effective f/number  This is the effective f/number of the lens 
considering you had to focus the lens out and that lengthened the 
operating focal length.  Use this to calculate shutter speed.
O6) diffraction-limited resolution  This is the system resolution 
due to diffraction effects.  If you stop down a lot, you'll have a 
small aperture and your resolution will go someplace in a 
handbasket.

Additional input
I6) film & lens resolution in lines per mm  This is the resolution 
of the film including any fixed errors of the lens.  (The loss of 
resolution due to diffraction was calculated separately.)  60 
lines per mm is a pretty good figure.  Maybe 40 lpmm is more 
realistic for many systems.

Additional output
O7) summation of all three resolutions in quadrature  This is a 
really a suspicious equation I just grabbed out of the air.  I 
don't stand behind it (or anywhere near it, for that matter).  It 
will give you the general idea of your total resolution, however, 
giving roughly the right weight to all the various causes of loss 
of resolution, IMHO (only).  It's at least a figure of merit.

Observations
If you're doing really close work with ordinary camera lenses, 
you're going to lose a tremendous amount of resolution by stopping 
down to increase depth of field.  Play with the inputs and you 
will see what happens.  You can balance these effects by jockeying 
the figures.  Still, when you get all done, it would be nice to 
have a reasonable resolution.  Under good, high-contrast 
conditions, 1 minute of arc would be right.  Under lower-contrast 
conditions, you might get away with 2 or even 3 minutes of arc.

The FAQ and the Excel files are in my anonymous ftp site.
ftp ftp.netcom.com and go to pub/be/bercov/photography/3D

John B

PS: I can't seem to translate the .xls to windows on this Mac so 
I'll do the translating at home.  For now, the Excel file is in my 
site as a binary file and also as a .Hqx in case the binary gives 
you any trouble.

John 


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