Distortions Exposure and the ground glass

[1erCru]

I shot my first distortion this weekend. I haven't had time to develop it yet. I noticed the amount of perceived light coming through the ground glass was reduced as I rotated the standards.

Is this a result of less light actually making it to the film back thus requiring a lower aperture to compensate or simply a perceptual issue as the ground glass has swiveled off axis resulting in an anomaly as light passes through the fresnel.

Thanks!

[Bob Salomon]

If you are doing movements and your eye does not remain centered in the optical path the view will become dim.

[Drew Bedo]

I have done some shots of that involved both swing and tilt without noticible fall-off or other change in the negative or print.

How much movement is involved in a "distortion" exposure.

[ic-racer]

I shot my first distortion this weekend. I haven't had time to develop it yet. I noticed the amount of perceived light coming through the ground glass was reduced as I rotated the standards.

Is this a result of less light actually making it to the film back thus requiring a lower aperture to compensate or simply a perceptual issue as the ground glass has swiveled off axis resulting in an anomaly as light passes through the fresnel.

Thanks!

The illuminance on the film plane falls off as the fourth power of the cosine of the off-axis angle.

[1erCru]

By distortion I meant an aggressive movement that hopefully will elongate part of the image.

[Doremus Scudder]

By distortion I meant an aggressive movement that hopefully will elongate part of the image.

Two things going on here.

1. Whichever part of the ground glass is farther from the lens will receive less light than the nearer part. Illumination decrease is due to the angular coverage decreasing. This is simply due to the increased "bellows draw" between one part of the viewing screen and the other. There will be a corresponding change of illumination at the film plane too, but this is seldom a problem.

2. The viewing angle often changes significantly with extreme movements, taking your eye out of the "hot spot." If your eye isn't lined up with the angle of light projected by the lens, the ground glass can seem very dark. This can be exacerbated with a Fresnel screen. This "darkness" for viewing, however, doesn't affect illumination on the film. You can compensate for this by searching for a better viewing angle. You may need a loupe with a skirt that you can adjust the angle on, or a loupe that you don't have to rest on the ground glass so you can simply angle it to align with the line from lens-to-ground-glass.

As for "hopefully" elongating part of the image: View cameras are WYSIWYG. You should easily be able to see the result of any movement. If your distortion is apparent on the ground glass, it'll be on the film; if not, it won't.

Best,

Doremus

[Leigh]

By distortion I meant an aggressive movement that hopefully will elongate part of the image.
For that type of distortion (e.g. turning a circle into an oval) the film must be at an angle to the subject.

You will likely need a wide-angle lens with significant coverage, perhaps 65mm or shorter on a 4x5.

- Leigh

[1erCru]

Yeah I should have used the term " slight elongation ". Nonetheless I can't believe have fascinating this camera is.

[Lou Baleur]

Yes--to accentuate the effect of perspective, the rule is the closer you are to the subject, the more perspective "distortion" is enhanced--so shorter focus lenses produce more effect than longer focus lenses.

[Bob Salomon]

Yes--to accentuate the effect of perspective, the rule is the closer you are to the subject, the more perspective "distortion" is enhanced--so shorter focus lenses produce more effect than longer focus lenses.

You are not describing distortion. Distortion, at its most basic, makes lines curve.

What you are describing is an effect due to optical physics. Foreshortening. That means things closer to the lens reproduce larger then things further from the lens. This is what creates large front shoulders, larger noses and foreheads in portraiture or that minnow that someone caught that looks like a trophy fish.

[Lou Baleur]

Correct--as far as I know, the only thing that camera movements do is allow a change in perspective or focus plane. I don't see how a camera movement would make a straight line appear to be curved.

[Doremus Scudder]

We've been over this question of whether foreshortening is a form of distortion or not and whether camera movements can really change perspective or whether only changing the viewpoint can do that. The consensus was inconclusive at best.

Still, when you turn a circle into an oval by positioning it at the edge of the frame, using a very near camera position and a short-focal-length lens, I consider the circle to be "distorted." Sure, it's not a lens defect, just a property of the projection. Seen from a purely geometric perspective (pun intended), any projection of a real-world 3-D scene onto a plane introduces distortion. I'm more than comfortable calling the elongation/foreshortening of image elements distortion, since it differs greatly from our expectation.

Same with perspective: I'm happy with calling correcting converging verticals changing perspective. It sure changes the perspective point where the lines meet, which is a classic definition of the use of perspective in art. Of course, changing the size relationship of objects requires moving the camera.

Best,

Doremus

[ic-racer]

Front standard movements do change perspective and object relationships, but I'd argue that front standard movements are "moving the camera around" because it changes the position of the lens in space. Important to note that the tripod does not necessarily "moves the camera around." You can move the tripod around and keep the lens in the same place, if you want. For example move tripod back and rack bellows out, etc.

[Bob Salomon]

Front standard movements do change perspective and object relationships, but I'd argue that front standard movements are "moving the camera around" because it changes the position of the lens in space. Important to note that the tripod does not necessarily "moves the camera around." You can move the tripod around and keep the lens in the same place, if you want. For example move tripod back and rack bellows out, etc.

Not if you have optical axis center tilts on the front. You do if you have base tilts.

[Lou Baleur]

Perhaps it can be viewed this way:

Regular camera movements seem to be pretty much like moving the camera in the x/y directions--up and down / left and right, and tilting back and forth. This changes the perspective and produces what people consider to be "distortion" in one way.

Moving the camera forward and back from the subject also changes the perspective and produces what some people consider to be "distortion" or "foreshortening".

So if your subject is the center of the sphere, regular movements allow the camera to move over the sphere surface and produce the sort of "fixed distance" distortion. But if you move in and out of the sphere, you get a "fixed latitude and longitude" but "variable distance" type of "distortion."

[Dan Fromm]

Perhaps it can be viewed this way:

Regular camera movements seem to be pretty much like moving the camera in the x/y directions--up and down / left and right, and tilting back and forth. This changes the perspective and produces what people consider to be "distortion" in one way.

Are you sure? I ask because an inch of rise disappears a lot more foreground and brings a lot more steeple into the frame than simply moving the whole camera up an inch. Neither distorts anything.

[Lou Baleur]

Are you sure? I ask because an inch of rise disappears a lot more foreground and brings a lot more steeple into the frame than simply moving the whole camera up an inch. Neither distorts anything.

I think so--you're forgetting the tilt I mentioned. Rise is the same as a camera movement plus a camera tilt--then you correct the standards back to parallel to the subject plane--can you picture this?

[Dan Fromm]

Lou, I know the difference between direct and indirect decentering movements. No matter what you say, decentering movements are not equivalent to moving the camera. They just sweep the cone of rays the lens projects across the film plane, keeping it perpendicular to the film plane.

[Lou Baleur]

That's not exactly what I said. But I understand what you're saying. It's a communication thing, so let's just say I think we are in agreement and leave it at that.

My main issue got derailed and I've now totally lost interest. I think this back and forth is mainly an issue over turf and who gets to speak with authority on certain subjects in this forum. Apparently, some people are allowed to know things (those in the old boys click) and some are not (the rest of us), and the rest must always "kiss the ring" or face the wrath of the click.

I won't kiss any rings of any kind.