[MF3D.FORUM:1370] Re: build your own medium format stereo camera, cheap and easy

[Tom Hubin <thubin@xxxxxxxxx>]

Hello Paul,

I did some theoretical number crunching and came up with some formulae
and tables for best results. The larger the pinhole the larger are the
image spots NEAR the pinhole. The smaller the pinhole the larger the
image spots are FAR from the pinhole due to diffraction. So the best
size pinhole depends on exactly what you plan to photograph. Field of
view and resolution depend on pinhole size, light wavelength, and film
size.

If the film is too close to the pinhole then the pinhole itself is about
all that gets imaged. If the film is too far away from the pinhole the
diverging light caused by diffraction will produce a large image spot.
So the distance from the pinhole to the film is also important. For best
results, the distance from pinhole to film depends on pinhole size and
wavelength of light. 

> How sharp?  

D (mm) is diameter of pinhole 
lambda (mm) is wavelength of light
W (mm) is width of film
N is number of resolvable spots across width of film
AFOV (deg) is angular field of view
L (mm) is distance from pinhole to film

L = D^2 / ( 2.44*lambda)
N = 0.7071 * W / D
AFOV = 2 * Arctan(W/(2*L))

If I assume that lambda is 0.000550mm for green light and that the film
width is 50mm these become

L = 745 * D^2 
N = 35.355 / D
AFOV = 2 * Arctan(25/L)    

The table below might shed some light.;)

D(mm)	L(mm)	N	AFOV(deg)
1.00	745	35.4	3.84
0.5	186.3	70.7	15.3
0.25	46.6	141	56.4
0.125	11.64	283	130
0.1	7.45	353	147

It would appear that the pinhole excels for wide angle shots. Keep in
mind that for wide angle shots the pinhole must be thin or it will act
more like a tunnel and cause vignetting and loss of resolution for very
oblique rays.

> Also, what about exposure times? 

Assume you want the same shot with a regular camera using the same film
speed and format. Choose the lens focal length that produces the same
angular field of view. That is, the focal length should be the same as L
in the table above. Based on the lighting conditions, what fStop and
exposure time would you choose for this lens? 

Now to determine the exposure time for the pinhole follow the steps
below. 

Divide the lens focal length by the fStop to get the lens aperture
diameter. Divide this result by the pinhole diameter. Square this result
to get the ratio of the lens area to the pinhole area. Multiply this by
the lens exposure time to get the pinhole exposure time.

I'm kinda shooting from the hip here. Let me know if this does or does
not compare with real world experience.

Tom Hubin
thubin@xxxxxxxxx