Re: [Leica] OT pinhole lenses

["Daniel Post" <dpost@triad.rr.com>]

I was wondering if one took a small glass sphere- and here I was thinking
specifically of a round sapphire lens about 2mm in diameter that Edmund
Scientific sells for about $10- and mounted it in a sheet of  something like
shim brass, equipped with a pinhole aperture, if the image would be any
good....?
Dan (With entirely too much time on his hands)Post

- ----- Original Message -----
From: "Malcolm McCullough" <MM4@mm-croy.mottmac.com>
To: <leica-users@mejac.palo-alto.ca.us>
Sent: Friday, August 03, 2001 12:12 PM
Subject: [Leica] OT pinhole lenses


> D Khong stirred the pot with:
>
> >If I am not mistaken, pin-hole lenses are used in architectural
> >photography. Straight lines must be strait, everything is in focus, the
> >image is distortion free.
>
> Dan,
> I wonder if you are referring to glass lenses with a very very small
> aperture rather than a plain pinhole (hence the term 'pinhole lens')?
> Despite what someone or other claims, a plain pinhole does not focus light
> rays (LUGgers please contact me off-list if you disagree with that
> statement) so the image they produce always looks rather blurred. The
> addition of a simple refractive (eg glass) element to a pinhole that is
not
> diffraction limited improves the image significantly because it brings the
> light rays to a focus. The aberrations created by the refractive element
> will be small because of the very small aperture. This is a really easy
> experiment to carry out if you have a close-up lens.
>
> So-called 'Pinhole Lenses' are frequently used in surveillance video
cameras
> and the like. These are generally a bit of a misnomer, because they are
> small refractive lenses (often glass in the better ones) of very short
focal
> length and wider aperture than a plain pinhole. Typical specs might be an
> f/2 4mm lens or an f/1 2.8 mm lens. They can have focussing mounts - which
a
> plain pinhole would not need because it doesn't focus.
>
> For distortion-free images with a pinhole the film should be held on an
arc
> with the pinhole at the origin (strictly speaking the film should be held
on
> a spherical surface - beginning to look a bit like an eye).
>
> The diffraction-limited resolution is dependent upon the 'focal length' of
> the pinhole. The formula for diffraction limiting can be easily re-written
> to give the pinhole diameter in terms of the 'focal length':
>
> d^2 = 2 * wavelength * focal length
>
> This gives the following lengths (in mm), pinhole sizes (in mm), f stops
and
> resolutions (in lppmm) using the wavelength of red light:
> 28    0.20  f/141  2.5 (eg a Leica body cap with a hole in it)
> 35    0.22  f/158  2.3
> 50    0.26  f/189  1.9
> 75    0.32  f/231  1.5
> 100   0.37  f/267  1.3
> 150   0.46  f/327  1.1
>
> Regards,
> Malcolm
>
>
>
>
>