Why does tilt work?

[Jim Michael]

I'll give it a shot: Tilting the lens rotates the plane of focus. Things that are closer or farther from the lens have different planes of focus. The film ideally intersects those planes so that each item's focus lies along the film plane.

[poco]

Alan,

Believe it or not, I'm on the road to understanding -- at least if you turn the explanation around a bit. So if a corolory of tilt creating different object distances from a single vertical plane is that it brings into focus elements from *different* vertical planes through tilt induced object differences, then it starts to make sense. Kind of...

Bottom line, the lens is focused upon some plane or other -- whether angled or not. All parts of that lens are focused on that plane so it doesn't matter that light is gathered from all parts of the lens.

[aduncanson]

Absolutely, your corolary is perfectly valid. I should have expressed it explicitly myself, but I was being brief (for me.)

Tilting the lens allows focusing a tilted object plane on to a vertical image plane (But not because tilting the lens makes all object distances equal. After all, the image distances are measured along the tilted lens axis as well. If all image distances were equal, then the image plane would be parallel to the lens plane) But it does work!

All parts of the lens focus the object plane onto the image plane (Unless you consider the, hopefully negligible, lens aberrations; but let's not get into that.)

[Hiro]

Every object in front of a lens forms a focused image behind the lens--just at different distances. Focused images are there no matter where the ground glass is.

So another way of looking at it is that tilt allows the film to intersect those various vertical planes, capturing focused images at different distances. With the attached simplified illustration, tilt achieves something like: 1) taking 10 photographs with varied focus, 2) cutting each film in 10 strips, 3) putting together 10 strips (the focused part from each film). Increasing the number of shots from 10 to infinity (or to the number to match the resolution of the eye) gives you tilt. I believe some people do this sort of thing with digital cameras.

Front and rear tilts are exactly the same. Only the relative positions of the two planes in free space matter. It may help to ignore the camera (bellows, frames, focusing mechanism, etc.).

Hope I'm making sense.

[poco]

Thanks, Hiro.

I think I've nailed it down to my satisfaction.

Love the diagram -- really brightened up my day. he he

[Scott Knowles]

The best way I learned it is setting the camera up on a sidewalk, pointing the camera down the sidewalk. From the vertical (the three planes parallel) slowly tilt lens and you'll see the length of sidewalk come into focus from only the distant in focus (parallel) to the centire length in focus. It's the easiest way to see how it works, and with only a slight amount of tilt.

The same applies to a mountain slope full of flowers, starting with the distant and tilting to bring the entire slow into focus. It was fun showing hikers one day in Mt. Rainier NP the entire frame (slope, Mt. Rainier, etc.) in focus on the GG, and asking, "Can your camera to that? And produce a cool 4x5 slide? And I don't need no software to do that, only the one in my brain."

[ki6mf]

The gotcha of Scheimpflug is that foreground objects which are tall will be out of focus! This is best used when you have a broad foreground with short objects! Thanks for the explanation!

[Mark Barendt]

The gotcha of Scheimpflug is that foreground objects which are tall will be out of focus! This is best used when you have a broad foreground with short objects! Thanks for the explanation!

This gotcha can be used to great advantage. Tilting the plane of sharp focus around allows you to control isolate the subject with "vertical" focus.

Not every image needs absolutely sharp focus from lens to infinity.