back tilt, coverage?

[sammy_5100]

hi,

I know that back tilt does not require as much coverage. but how much less than front tilt? for instance, if i get 5 degrees of front tilt, could i (instead) get 10 degress of back tilt? is there an equation?

[James E Galvin]

Consider yourself very small and standing inside the back of the lens, looking at the film. Tilt and swing the lens, and the lens points away from the center of the film, the opposite corner of the film is now further away from the lens axis, using up coverage. With the lens back to normal, tilt and swing the back. You see the film rectangle foreshortening. The more you tilt the back, the less coverage you need.

[Steve Hamley]

Back tilt and swing do not require ANY excess lens coverage. The dimension "seen" when the back is tilts is the heigth times Cosine of the angle, which is always 1.0 or less.

Steve

[sammy_5100]

what if i "simulate" back tilt by tilting the entire camera down, then tilting the back to be perpendicular to the ground. would this require *no* coverage? or less coverage?

[Dave Moeller]

By tilting the camera down and then repositioning the back to vertical, you're simulating front tilt (and front fall if you don't bring the lensboard back up to even with the back), not back tilt. In this case, you do need more coverage.

[sammy_5100]

yeah, but not as much as regular front tilt (without the front drop)? that is also my question.

[Steve Hamley]

The same.

Steve

[James E Galvin]

Make a sketch of the lens and film in cross-section. Draw lines from the middle of the lens to the film corners (the film diagonal is 155 mm) The middle of the lens should be its focal length away from the film. This is the coverage needed with no motions, Now sketch in the motions you are considering. Does the film go outside of the two lines? If so, you are using up coverage. Draw lines with the specified angle of coverage of the lens. You have to keep the film corners inside of these lines.

[Alan Davenport]

You can use up to 90 degrees of back tilt and the lens will still completely cover the film. Good news: with that much tilt, you can stop worrying about Cosine^4 falloff, because the falloff will be something else... ;o)

[Leonard Evens]

This whole question can be quite confusing. Let me try to clarify some points. The angle of coverage of the lens is centered about the lens axis. How the film frame interacts with a tilt, either front or back, is determined both by the angle the lens axis makes with the film plane and also by the relative position of the film plane with respect to that axis. For the sake of discussion, assume you have framed the picture so the lens axis passes through the center of the frame with both standards vertical. The coverage is defined by a conical angle centered on the lens axis, with the frame contained within, centered on the axis, and perpendicular to it. If you now tilt the back, after perhaps refocusing, depending on the kind of tilt, the frame will still be centered, but the proximity of the top and bottom of the frame to the cone of coverage will change slightly. It will be further on top and closer on bottom, so you might in extreme cases lose some coverage on the bottom. If on the other land, you tilt the lens down and keep the back vertical, the axis is tilted up and intersects the film frame at a position much further up. The bottom of the film frame may now be well below the cone of coverage.

Now of course, one can correct for the shifts in either case by moving the rear standard up, perhaps considerably in the case of tilted lens. So in purely geometric terms of relationship between lens and film, there is no effect on coverage, and no difference between front tilt and rear tilt. But, aside from issues of physical limits of the mechanism for rises, this ignores one other important issue. The subject itself also provides a conical angle with vertex at the lens. That determines what you will see in the frame. It is the relation of this subject angle to the angle of coverage that is all important. Even with the standards parallel, a rise or fall could take you out of the angle of coverage. Back tilts will ordinarily make relatively small changes in the relation of these two angles, but front tilts could make a significant difference. For example, in the situation described above, where no initial rise or fall was involved, if you tilt the lens down, you might have to move the back up a considerable amount to get adequate lens coverage, and that would substantially change the framing of the subject.

One comment about fall-off in illumination. Whenever you tilt either lens or back, you move part of the film closer to the lens and part further away. While the cos^4 law may become less relevant, the inverse square law, which is at the basis of all illumination calculations, is still very relevant. The light intensity will fall off from those parts closer to the lens to those parts further away.