T3D Re: apertures

[john bercovitz <bercov@xxxxxxxxxxx>]

> I want to modify my teco-nimslo's inner 2 apertures so that they will
> shoot only f/22 so that I can use a flash with it for close ups.
> 
> Anyone have any idea how I could order or make f/22 apertures for this
> camera?

For an SLR lens, I merely made a disk of thin sheet metal and put a hole 
in its center and taped it over the iris blades.  Actually, I made several 
disks with different apertures to find out when the resolution would fail 
due to physical optics problems.  For the 50 mm lens, the resolution was
abysmal at f/64 but tolerable at f/32.  So you always want to be careful 
just how small you make your apertures.

Definitions:
linear aperture:   diameter (e.g. 2 mm) of the aperture
relative aperture: focal length divided by linear aperture 
                   (e.g. 50/1.6 = f/32)

You can calculate plate (film) resolution from relative aperture.
In lines per mm, it's about 1800 divided by the f/number.  However,
in stereo photography, we're more interested in angular resolution
since nominally we view the image from its center of perspective.
Angular resolution in seconds of arc is 115 divided by the linear 
aperture in mm.

The human eye will do about 1 minute of arc.  What is the minimum linear 
aperture which will deliver 1 minute (=60 seconds) of arc?  115/60 ~= 2 mm.
Now let's apply that to lenses we might encounter:

lens        minimum 
focal       relative
length      aperture

 25           f/12
 30           f/15
 35           f/17
 50           f/25
 80           f/40
135           f/68
150           f/75
210           f/100

All of these lenses, when set at the relative aperture shown, will 
deliver the requisite 1 minute of arc if there are no other limitations 
in addition to physical optics limitations.

In conclusion, be a little careful in stopping down short lenses.
You may not like what you get.

John B


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