From: donl@glass.esd.sgi.com (donl mathis)
Newsgroups: rec.photo
Subject: Re: Questions -- I have 4x5 cameras now.
Date: 12 Jan 93 01:58:01 GMT
Reply-To: donl@glass.esd.sgi.com (donl mathis)
Distribution: na
Organization: Silicon Graphics, Inc., Mountain View, CA, USA


In article <C0pAy2.20H@acsu.buffalo.edu>, v101gwud@ubvmsb.cc.buffalo.edu
(a man of colors) writes:

|> 		"Graflex D" [Graflex' "Speed Graphic" plus a viewing hood;
|> ~1950; no questions]
|> 
|> 		"Premo Pony" [made between 1910 and 1920]
|> 	The piston lens allows f/8 to f/64 [and accurately 1 second to 100th and
|> maybe 200th second].
|> 
|> 		I have questions [to those reading this in January 1993] :
|> 	1] For what bellows length [or it equivalent, object distance] are the
|> Premo f-stops valid?  For close-ups with extended bellows I intend to open
|> further but from what standard -- perhaps the focus scale below the bellows?

The f-stops are valid in the usual way -- at the lens' focal length.  That
is to say, the distance from the rear nodal point to the film plane when
the lens is focused at infinity.  That is to say, probably from somewhere
near the middle of the lens to the film plane.

If you want to be more precise about finding the focal length, try this:
focus the camera carefully at infinity.  Measure from the lens to some
fixed point at the film plane.  Focus the camera carefully so that the
magnification is 1:1, and measure again.  Subtract the two distances, and
that is the focal length.  If you can't focus at 1:1 because the bellows is
too short, you can use a few formuli to figure it out.  The problem is
that you have to take front measurements from the front nodal point, and
rear measurements from the rear nodal point, and you don't know where
they are.  So you have to work the equations so that you don't need to
know where they really are.

	u = distance from subject to front nodal point of lens
	v = distance from rear nodal point of lens to film plane
	F = lens focal length
	m = magnification of image

	1/F = 1/u + 1/v
	m = v/u

I have a feeling that with enough algebra, you can find the focal length
from two different focus points, if you know the magnification at each.

|> 	2] Will a piston lens snap any faster than its top setting?  Past 100th
|> second the tension knob continues a bit before reaching the Bulb and Timer
|> settings; I figured that it might reach 200th second.

You may find that you think differently about shutter speeds with a
large format camera.  Most of my exposures are between 1/15 and 2
minutes.  Sometimes I need to go as fast as 1/30, but not often.  I
can't imagine actually making an exposure beyond 1/60.

|> 	3] In covering three visible bellows holes [on corners] should I avoid
|> duct tape?  I plan to use black, fibrous duct tape.  *Your method* is
welcome. 
|> Local shops would only replace my bellows.

I found some black silicone seal at the hardware store.  I think I would
try that, on the inside.  And I would check it regularly to make sure
it's still sealed.

|> 	Will Kennedy, State University of New York at Buffalo 11JAN1993
|> 	[my opinions]

- donl mathis at Silicon Graphics Computer Systems, Mountain View, CA
donl@sgi.com