8x10 Bellows Extension and Angle of View

[Dan Fromm]

Sheldon, in the plane of best focus it is invariant. What's behind the plane of best focus doesn't count. Well, of course it does, but the angle subtended by rays going from the corners of the film in use to the corners of the subject in the plane of best focus is the same for all magnifications.

At this point we're quibbling about semantics. You've been very clear about what you see and what you mean. Thanks very much for that.

[mdarnton]

Dan, you're insisting on thinking of this as a lens issue, not a film thing. The farther a lens is from the film, the smaller the angle of what the film sees, that's all. As you move the lens away, the film sees a narrower angle of the outside world. You're thinking too hard here.

[Dan Fromm]

I'm thinking in terms of subject plane and image plane.

[Sheldon N]

Sheldon, in the plane of best focus it is invariant. What's behind the plane of best focus doesn't count. Well, of course it does, but the angle subtended by rays going from the corners of the film in use to the corners of the subject in the plane of best focus is the same for all magnifications.

Ah, but that's not the case. There can only be one angle of view of the world that the ground glass sees, whether that's near or far or in focus or out of focus, the angle is the angle. Ignore optics and just imagine straight lines coming from the corners of the ground glass, through the center of your aperture, out to your subject. Move the ground glass away from the lens, and the angle of those imaginary lines toward the corners of your subject is going to tighten up.

The implication is that as you increase magnification/bellows extension you are going to have to reposition the camera farther away from your subject than you would expect in order to properly frame the image. It's just as if your lens becomes a longer focal lens as your focus closer.

[Dan Fromm]

Sheldon, when you focus closer than infinity both conjugates move.

[Sheldon N]

Sheldon, when you focus closer than infinity both conjugates move.

I'm not particularly good at math, so excuse me if I'm not following... Can you clarify?


I guess my whole premise of this thread was me beginning to understand a couple things...

1) With 8x10, I can't just shoot like I did with 35mm and assume that my lens is going to give me a constant field of view. I took this portrait of my daughter (http://tinyurl.com/n7pkvg2) positioning her and the camera picturing a "normal" focal length field of view. However when I focused the field of view was a lot narrower than I previsualized. That interaction of focus distance and effective focal length was entirely new to me.

2) The whole idea that you need to use a longer lens to shoot a tighter portrait to get a pleasing perspective isn't necessarily true with big formats. I've seen this common wisdom espoused many, many times (ie Bob Salomon's off the cuff response early in this thread). But with bigger formats by the time you've focused close enough to shoot a tight portrait your short focal length lens is now giving you the angle of view and resulting subject distance of a much longer lens. That's not to say that longer lenses won't give a greater subject distance and improved perspective, but it explains why not many people are shooting 8x10 portraits with the theoretical equivalent of an 85mm lens (600mm).

[jb7]

... It's essentially like having a variable focal length lens which is dependent on how tight you frame the shot. ...

That's it. I call it Focus Zoom. A distinctly large format phenomenon which might be difficult to get a grip on if you're only used to smaller formats...