Optics Involved in Forward Tilt and Front Risegreenspun.com : LUSENET : Large format photography : One Thread
I've read in several places that when you perform a front forward tilt (say to increase depth of field), you should then also use a front rise. Why? As a compositional device, I have observed the obvious, that the forward tilt in effect lowers the composition. I can correct this in two ways: (1) tilt the entire camera upward slightly, or (2) use front rise. I understand that tilting it upward will not only change what subject is included in the composition but will affect the perspective. In my experience, however, this effect on perspective, given the small amounts of forward tilt I use, is negligible. Does using front rise (as opposed to tilting the entire camera upward slightly) also recenter the lens optically so that you are using the "sweetest" part of the lens? Are there other optical effects at issue here that I'm missing?
-- Howard Slavitt (firstname.lastname@example.org), July 12, 2000
Using forward tilt would require using fall if you want to keep the optical axis of the lens centered on the film plane (the sweet spot).
-- Ron Shaw (email@example.com), July 12, 2000.
Doesn't this depend on where one's particular camera locates its front tilt axis relative to the lens' nodal point?
-- Sal Santamaura (firstname.lastname@example.org), July 12, 2000.
Tilting the lens, or the back, doesn't change the depth of field as such. Instead, it tilts the plane of sharpest focus. Where did you read that you should also use front rise? Seems very strange to me.
-- Alan Gibson (Alan@snibgo.com), July 12, 2000.
Sal, I recall if the pivot of tilt is the same as the rear nodal point of the lens, then the composition is not affected from my past reading of Basic Photographic Materials and Processes (now new edition!). Usually, composition is done at the rear and swing and tilt are done at front if you do not want to distort the image and if the camera meets this requirement. Tilt front downward and raise the rear to get back to where you were and to the sweetest part of the lens. Maybe I'm missing something??
-- Masayoshi Hayashi (email@example.com), July 12, 2000.
For LF camera operation a website that is outstanding, highly recommended and must be visited is Merklinger's. This can be found at http://fox.nstn.ca/~hmmerk/HMArtls.html Merklinger also has several publications available online which deal with depth of field, camera movements, etc. The video illustrations are specially useful. Even if you do not remember all that Merklinger teaches you will have gained a great deal of understanding about how it all works.
-- Julio Fernandez (firstname.lastname@example.org), July 13, 2000.
Masayoshi, I don't think you're missing anything. Key in your post was "..if the camera meets this requirement." I assume Howard is discussing his Horseman VH, and was considering checking out this situation on my own VH, but realized there are too many variables to reach a useful conclusion. I don't know where Howard's lenses' nodal points are. Also, I recall Howard has at least one lens on a recessed board, which would negate any attempts Horseman might have made to precisely locate the VH's front standard pivot point.
-- Sal Santamaura (email@example.com), July 13, 2000.
Sounds like what you read is for cameras with the tilt axis at the bottom of the standard rather that through the lens. If that is the case, then tilting the lens forward is going to lower it a bit, too, though you'd have to tilt an awful lot to make a lot of difference.
I say just look at your composition on the ground glass. If you need to use rise to get want you want, then use rise.
-- John H. Henderson (firstname.lastname@example.org), July 13, 2000.
If the amount you are tilting the lens forward is negligable, why not just tilt the back backward? This will solve a couple of problems. 1. you won't need to concern yourself with vignetting that sometimes occurs with lenses that have limited coverage and 2. assuming you have an axis tilt at the rear of the camera, you won't impact the composition significantly. If you have a base tilt, you can compensate with a little front rise. If you are really close to a foreground element, the rear tilt will exagerate it's apparent size (making it larger), which may or may not be a desired effect. You have to be the judge. Also, by using the rear controls, you'll be closer to what's going on and have an easier time making that critical adjustment of the focus plane. Depth of field, by the way is determined by focal length, aperture and subject to lens distance. The adjustment of tilt simply changes the plane of focus. Correctly making that adjustment can reduce the amount of DOF needed to bring all elements into acceptable focus without using an aperture so small as to make diffraction artifacts annoying.
-- Robert A. Zeichner (email@example.com), July 13, 2000.
Why are you worried about what a book tells you instead of going out, setting up your camera, and focussing the damn thing. If you have a camera that has the horizontal axis across the middle of the lens, then the answer is no. If your front standard has the tilt axis anywhere along the vertical struts, like at the base(base tilt) then yes you need to employ front rise or fall depending on how you are moving the front standard. That is why I have always found axis tilt to be easier to use than base tilts. With axis tilts there is no need for additional movements such as rise or fall. Just look at the ground glass and let it tell you what you need to do. James
-- james (firstname.lastname@example.org), July 13, 2000.
It helps to understand what's going on when you apply camera movements by imagining a line running through the middle of the lens, at right angles to the lens board. If you tilt the lens down, on any axis of rotation, then where that line intersects the film plane will move up from the centre of the film. To keep the lens axis centred on the film, either the lens board must be dropped, or the film back raised.
Tilting the rear standard backwards will achieve the same effect only if the whole camera is tilted forward to keep the back vertical. Then, if you think about it, you've ended up with the back and the lens in the same relative position in both cases. There're more ways than one to skin a cat, or to get a given lens/film plane geometry.
-- Pete Andrews (email@example.com), July 14, 2000.