0.25mm Pinhole Aperture
22mm Stainless Steel Disk.
These pinholes are the best value around. Made from .003" 301 full hard stainless steel these pinholes are thin and nearly indestructible.
These pinholes are sized and beveled to less than one thousandth of an inch at the aperture making this the tinniest pinhole possible without sacrificing the durability.
The second photo shows this pinhole under the microscope at 800x magnification. As you can see it's flawless   
 
 Most people will mount these in a camera body cap, digital or film. You could also build your own camera around this pinhole out of something as simple as a cardboard box.
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