[pure-silver] Re: Large Format Tilt
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Thu, 29 Nov 2007 15:04:31 -0800
----- Original Message -----
From: "Helge Nareid" <hn.groups@xxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, November 29, 2007 2:31 PM
Subject: [pure-silver] Re: Large Format Tilt
Richard Knoppow wrote:
[... big snip ...]
As a matter of fact, for this kind of calculations, the
distances should be measured from the front and rear
nodal points of the lens, which are rarely the same as
the physical surfaces _nor_ the entrance and exit pupils
of the lens.
- Regards
Helge Nareid
The exact locations of the principle planes (same as
nodal points when the entire lens is in air) may be
important for some special circumstances such as micro or
macro work but are not for general work.
Sorry Richard
You are of course right - I was being somewhat excessively
pedantic in my previous post. One definition of a nodal
point is that the image does not move if the lens is
rotated around the nodal point, that is indeed equivalent
to the nodal point being exactly one focal length from the
focal plane (at infinity). This can actually be moderately
useful for large format photography, in that if your lens
is mounted in such a manner that the rear nodal point
coincides with the rotational axis of the front standard,
there will be minimal requirements for reframing and
refocusing with a lens tilt or swing. However, this is
rarely the case with any practical camera/lens
combination.
However, having worked with bottom tilt axis, central tilt
and swings axes and asymmetrical axes (as used by Sinar),
I will concur with the advice that you really need to
check your focus with a high magnification loupe on the
ground glass - don't trust any of that fancy stuff.
Of course, for any movements, it helps if the swing or
tilt axes are actually within the image area - bottom
tilts always require refocusing of the entire image, which
can be a right pain at times.
Personally, I tend to prefer front tilts and swings - rear
movements do affect the perspective of the image (as in
converging lines). As for shifts, except for close range
work, it doesn't really matter which standard you use.
Finally, any rear swing or tilt can be shown to be
equivalent to a front swing/tilt combined with a camera
rotation and a shift (and for that matter vice versa). The
proof of this can be left to the student ...
- Helge Nareid
I should have mentioned your last point. Some cameras
do not have front movements but the equivalent can be had by
a combination of rear movements i.e., swing and shift. The
problem is some cameras do not have full movements of either
end.
Nodal points, that is where the image does not shift
when the lens is tilted tilted around the axis of the point,
is the same as the principle planes where the entire lens is
in the same medium but can be different where more than one
medium is being used for instance, a lens with the image in
air and the front in water or some other medium Oil
immersion microscope objectives fall into this catagory.
But, of course, you are an optical designer so you know
all this stuff:-)
I don't mind a little pedanticism (is that a word?),
its fun and I am probably guilty of a great deal of it.
BTW, I agree about center vs: base tilts. I think base
tilts exist because it may be easier for the camera builder
to make the camera rigid with them. Maybe not. Ansco
redesigned their 8x10 cameras after WW-2 changing the rear
tilt to a base tilt from a center tilt.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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