[pure-silver] Re: correction for bellows extension
- From: `Richard Knoppow <dickburk@xxxxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Wed, 04 Mar 2015 11:30:37 -0800
On 3/4/2015 10:07 AM, bonner wrote:
Michael,
For bellows length, are you measuring from front standard to ground glass?
-----Original Message-----
From: pure-silver-bounce@xxxxxxxxxxxxx
[mailto:pure-silver-bounce@xxxxxxxxxxxxx] On Behalf Of Michael A. Smith and
Paula Chamlee
Sent: Wednesday, March 4, 2015 11:59 AM
To: pure-silver@xxxxxxxxxxxxx
Subject: [pure-silver] Re: correction for bellows extension
It is real simple: Total length squared divided by the focal length squared
gives you the factor to multiply the exposure by.
Example: If you have a 12" lens and the bellows is out 20" you have 400 divided
by 144 for a factor of 3. Being exact with this is not critical.
While the actual measurement is from the rear nodal point of of each lens to
the other rear nodal point (and the rear nodal points are not always in the
center of the lens, just measure, as Richard Knoppow wrote, from center of lens
to center of lens. (This holds with view camera lenses.)
Michael A. Smith
Actually not the nodal point but the exit pupil. For
approximate correction you can use the formula I posted earlier. For
very precise correction its necessary to know the true focal length of
the lens and the location of both entrance and exit pupils. Since focal
length is defined in terms of the principle planes measuring the focal
length requires knowing those. They are not difficult to measure and I
have posted the procedure for measuring true focal length and finding
the locations of the principle planes and pupils several times. They
should be in the archives of this list. The nodal points are identical
to the principle planes or points when the lens is in air. For
approximate corrections the focal length can be assumed to be what is
marked on the lens and the pupils assumed to be at the center of the
barrel. This will work for most standard lenses. For telephoto or
reversed telephoto lenses the principle planes and pupils are displaced
considerably from the body of the lens so the simplifying assumption
can't be made.
When a lens is set up for unity magnification its distance from
the image plane must be extended exactly one focal length from the
infinity focus position. In fact, this is one way of determining the
focal length. At this extension the distance is double that at infinity
and by the square law the light intensity is 1/2^2 or one forth the
intensity at infinity, i.e. two stops. The effective diameter of the
stop or pupil is half that at infinity. This ratio holds for all
distances. Its possible to use the ratio of magnification of the image
as well as focal distance to calculate the effective exposure but,
except for larger view cameras, it can be difficult to measure.
Exact infinity focus is easy to find on a larger camera by
auto-collimation. This requires a flat mirror that will fit over the
front of the lens and a small flashlight. The mirror is placed over the
lens and the flashlight with perhaps a cardboard pin-hole mask, is place
on the ground glass, near, but not exactly at the center. The light from
the ground glass is reflected back through the lens by the mirror and
will fall near the source. When it is focused the lens is exactly at
infinity focus. This can be marked on the camera bed to measure
extension but is NOT a measure of focal length since you do not know the
location of the rear or second principle plane. All other measurements
can be determined from this simple set up.
--
Richard Knoppow
dickburk@xxxxxxxxxxxxx
WB6KBL
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