Quote Originally Posted by Toffle View Post
Hi Ralph,
Where did you get your figures? (I don't doubt them) My calculations, from the designer at pinhole.cz came to .282 mm, but I figured .3 was about as precise as I could get without specialized tools. ...
Two optimum pinhole diameter equations exist. One is optimized for resolution d = sqrt(3.66*lambda*f), the other for sharpness d = sqrt(2.44*lambda*f), lambda being the wavelength of light and f being the focal length. In your case, the former returns d = 0.29 mm for resolution and the latter d = 0.23 mm for sharpness, assuming a wavelength of 555 nm and a focal length of 40 mm. Most people, subjectively, prefer sharpness over resolution.

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It's not a big deal, but you may want to experiment with both and see which you like best.