[MF3D.FORUM:1377] Re: build your own medium format stereo camera, cheap and easy

["Don Lopp" <dlopp@xxxxxxxxxxxxxxx>]

I think you are on the right track generally speaking but serious testing
will show you what the best pin hole diameter is sharpest ( my experiments
indicated that .0180 inch was the best for a 6 inch pin hole best used on
the thinnest material available--Also exposure is determined by the f/ stop
of the pin hole after making adjustments for reciprocity which is a lesser
problem with T Max film than most. You are very correct in advising thin pin
hole material when building a wide angle camera. My experience indicated
that sharper pictures were made by using G or A filters which narrow the
wave length of light being forced through the pin hole.
----- Original Message -----
From: "Tom Hubin" <thubin@xxxxxxxxx>
To: "Medium Format 3D Photography" <MF3D.Forum@xxxxxxxxxxxxx>
Sent: Wednesday, November 29, 2000 1:14 AM
Subject: [MF3D.FORUM:1370] Re: build your own medium format stereo camera,
cheap and easy


> 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
>
>