[pure-silver] Re: Large Format Tilt
----- 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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