Parallelism of film and subject planesgreenspun.com : LUSENET : Large format photography : One Thread
As I contact print my 5x7s and 8x10s I occasionally discover something that was not apparent when I was shooting: that the film back was not perfectly parallel with the flat subject that I was photographing head-on. Take, for example, a person seated in a chair facing the camera. The chair and subject appear slightly twisted towards one side. Swinging the back of course would move the subject off-center. What I'm talking about here is placing the camera in front of the true center of the subject with back perfectly parallel. My current solution is to use a carpenter's tape to measure the distances between the lens and the two sides of the subject, but I'm hoping that there is a simpler way, esp. where the subject is beyond the reach of my 12' tape. Any suggestions? Thanks in advance for replies. Nick.
-- Nick Jones (email@example.com), December 05, 2001
"Swinging the back of course would move the subject off-center." - Huh?
-- Pete Andrews (firstname.lastname@example.org), December 05, 2001.
Let me correct myself and put my understanding of the movements more fully. My camera does not have rear shift, and even if it did I wouldn't want to move the subject away from the center of the image circle, unless I had to. So leaving the camera off-center, bringing the film into the correct parallel position, then shifting until the subject is centered is not an option. As for rear swing, what I should have said was that using that movement to bring the film into a parallel position with the subject, while leaving the subject centered as before, would alter its geometry, possibly for the better, but at this point I don't have enough faith in my abilities to use movements to approximate a simple zeroed-out dead-on shot. Besides, the angle of view would remain off-center. And why do so anyway, if the camera can be moved? So what's at issue here is how to place the tripod and camera in precisely the correct position, dead center in front of the subject with film plane parallel to the (flat) subject plane. Nick.
-- Nick Jones (email@example.com), December 05, 2001.
Try contacting the people at Zig Align. They make alignment products for enlargers, view cameras, copystands, etc. I use their enlarger system--it's improved the sharpness of my prints so much I won't print without it anymore.
-- Peter Latner (firstname.lastname@example.org), December 05, 2001.
Maybe I'm misunderstanding your question, but it sounds like you want to take straight on shots with no perspective correction. Rather than measure your film plane to subject distance on each side, couldn't you just ensure your lens and film planes are parallell and centered, and then just move the camera to the desired location for framing?
If one leg of the chair is closer to the camera it will naturally appear larger, using a longer lens with a greater working distance would also help diminish this effect.
I know I'm probably missing something.
-- Andrew Cole (email@example.com), December 05, 2001.
You may want to look at a laser rangefinder. They are not that expensive anymore and will have an accuracy of perhaps 1/4 inch. It would certainly solve the problem of your 12 foot tape measure not being long enough or falling off when measuring. Look at the better hardware stores that cater to the contractors.
-- Dave Schneider (firstname.lastname@example.org), December 05, 2001.
OK, I re-read your post, you seem to be asking how to get the camera precisely centered in front of a seated (or similar) subject. This can be done more simply without measuring to the edges of your camera back. From a geometric point of view you want to make your camera the top of an isosoles triangle (I was a math major a couple years ago, but I'm sure I'm misspelling that) with the base being the plane of your subject that you want to be parallell to, and then center the subject in the frame. From a measurement point of view, you might get more accuracy if you expand the base of the triangle for the purpose of your measurement (e.g. place a yardstick or longer straightedge centered on the center of your subject, parallell with your subject plane and take your measurements from the ends).
Doesn't help with you needing a longer tape measure though. I've never needed this much accuracy, I just eyeball it and compose based on what I see on the glass.
-- Andrew Cole (email@example.com), December 05, 2001.
During set up, if you are able to place a straight edge in the subject position parallel to what will be an axis through the width of the subject, then you can use a backpacking style compass. I'm thinking of the kind made by Silva that allows you to rotate the dial to get the engraved arrow exactly under the floating magnetic needle. You take the bearing from the straight edge and then make sure that a measurement taken at the film plane is the same. You should easily be able to be accurate within one degree. njb
-- Nacio Jan Brown (firstname.lastname@example.org), December 05, 2001.
Perhaps I don't understand exactly what you are asking. If you are simply interested in making sure the camera back is horizontally parallel to a wall or other sort of background (vertical you should be able to take care of with a level), then just make sure that the horizontal lines in the background are perfectly horizontal and parallel on the ground glass. If this is the case, you must be parallel to the background (within visual tolerances, at least). If your subject is then off-center, you need to move the camera and bring the back parallel to the background again. If there are no horizontal lines in you background, either make some temporary ones (hang a meter stick or draw some lines) or ignore it, since it makes no visual difference anyway. In the latter case, just make your best composition on the gg. Hope this helps a bit. ;^D)
-- Doremus Scudder (ScudderLandreth@compuserve.com), December 06, 2001.
Do you have grid lines on your groundglass? If you have a plain groundglass, you can pencil them in on the matte side. If you have a glass with a fresnel on the matte side, you can draw them in on the smooth side with a felt-tip pen that uses water soluble ink.
-- David Goldfarb (email@example.com), December 06, 2001.
Put a mirror at the subject location. With all of your rises, shifts and tilts zeroed out, move your camera around, and when you can see the reflection of your camera lens centered on your groundglass, then your camera's axis will be perpendicular to the mirror. Does that make sense?
-- Don Wong (firstname.lastname@example.org), December 07, 2001.