You could take the lens to optician's, they have devices that tell the
focal length (or rather diopter strength) of a lens.
Use this formula to get focal length from diopters:
fl=1/D
fl you get is in meters and this is the closest this lens should come to
the sensor/film (infinity focus)
knowing Pentax flange distance (45.5mm) you can easily subtract that to
get how far this lens should be from the mount at infinity
Krisjanis
Boris Liberman <[EMAIL PROTECTED]>
2005.11.23 13:32
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Subject
Question about optics (re single element soft lens)
Hi!
As you know I have a Helios 44K4 (formerly 58mm f2) lens modified to
be a single element soft lens. The process of making this kind of lens
has many side effects ;-)... One of those effects is that focusing
range is thrown out of whack. It is usually taken care of by attaching
an extension ring between the lens and the body.
Originally, when my monocle was made, I used Pentax #1 extension ring.
It did not solve the problem completely. That is, I could focus far
beyond infinity ;-), and the closest focus distance was rather far.
Today I received Kenko 12 mm ring and tried it... I think I achieved
the perfect infinity focus and then of course proper close focus
distance too. According to Boz's site, Pentax #1 ring is 9.5 mm in
height.
So here is the question... Is there a way to actually compute the
distance between the glass element and focal plain? Or in other words,
is there a way to compute the height of the extension ring to be
inserted? Do I have to know the curvature of the glass element in
order to make this computation?
Thanks in advance.
--
Boris
