Tilt-Shift DOF

I have a Canon 24 3.5 TSE lens. I've really enjoyed using it with my 20D. So far, my main uses have been to stitch panos (vert. & landscape) and tilt to control DOF. Even with the angle finder, I mainly 'focus far' and 'tilt near' for max DOF. Recently I upgraded to a 1D Mark III. Now that I have got used to most of the differences, I plan on using the 24 TSE (and maybe the 90 TSE) with the Live Preview. The 10X magnification ought to be a great asset.

My main question is calculating DOF wth tilt. I'm new to using the view camera functions, so I want to get the principles correct in my head. Normally, if I focus 1 meter from the subject at 3.5 the DOF near/far is 0.87m/1.17m with a DOF of 0.3m. Focusing at 10m gives a near/far of 3.97m/infinity. I fully understand this when normally shooting into a perpendicular plane (normal photo shot). If this same set of principles apply when tilting the plane of focus, it definitely matters where I apply the main point of focus. At the same tilt angle, focusing near would produce a shallow depth of field, focus far would produce a very large depth of field. Am I correct in this assumption?

More or less, yes. It is a little more complicated than that, but as an approximation it works. However, I find it fairly easy to see what is happening through the viewfinder. You'll find that the plane is not exactly a plane anymore. Nearby, the width of focus is less than far away, it is a kind of half ellipsoid, plus, far away the thickness of the focal plane is greater than nearby. That is how I try to visualize it. I hope this helps you .

BTW, don't forget that you are tilting the focus plane at a faster rate than you tilt the optical axis of the lens. Unless you want the focus to be very near (relatively thin), you only have to tilt a couple of degrees for maximum effect.

You may want to have an in-depth look at Lester Wareham's site:
http://www.zen20934.ze​n.co.uk/photography/ti​ltshift.htm
He worked everything out for us TS enthousiasts .

Kind regards, Wim

Gerry,
Your fundamental understanding of DOF for tilted plane is similar to DOF for perpendicular plane is correct...the only thing that changed is the angle of the plane! So near focus point is still results in shallower DOF than farther focus point.

Think of the plane of focus as a book in space. The thickness of the book is defined by how far it is from you...farther away is a thicker book. If you angle the book in space, the thickness is not changed, but merely the things in space which intersect the book's position in space.

But Wim is right that it truly is not a book, but a bit more like a wedge. And the point of that wedge is located at the Schleimpflug intersection of all the planes.

Wilt wrote in post #3883701
Gerry,
Your fundamental understanding of DOF for tilted plane is similar to DOF for perpendicular plane is correct...the only thing that changed is the angle of the plane! So near focus point is still results in shallower DOF than farther focus point.

Think of the plane of focus as a book in space. The thickness of the book is defined by how far it is from you...farther away is a thicker book. If you angle the book in space, the thickness is not changed, but merely the things in space which intersect the book's position in space.

But Wim is right that it truly is not a book, but a bit more like a wedge. And the point of that wedge is located at the Schleimpflug intersection of all the planes.

Thanks, Wilt. I was looking for the right word. It si actually a wedge with a fairly vague, i.e., non-sharp intersection. DoF fades in and out similarly to normal DoF, but now does it 3-dimensionally, and nearby it fades out faster than far away. Plus, very nearby you can actually see the plane of focus bend too, basically along the lines of Lester's plots. You can make this very visible by focusing nearby with, let's say 4 degrees of tilt at 90 degrees, IOW, tilt perpendicular to the horizon.

Ok, to define it more precisely, the 3-dimensional focal area is a wedge shaped short pyramid, so flattened rather than with sides of the same width, with a bent top, and not very clearly defined outlines . And the top is directed towards the lens and bends more towards the lens then the rest of the shape.

Kind regards, Wim

wimg wrote in post #3883811
Ok, to define it more precisely, the 3-dimensional focal area is a wedge shaped short pyramid, so flattened rather than with sides of the same width, with a bent top, and not very clearly defined outlines . And the top is directed towards the lens and bends more towards the lens then the rest of the shape.

To quote a very old TV show...

"You have entered the Twilight Zone!"

Wilt wrote in post #3883904
To quote a very old TV show...

"You have entered the Twilight Zone!"

And isn't it fun...

Kind regards, Wim

Thanks, gentlemen.... I'm going to find a quiet spot and tweak all the settings. There is definitely a lot to absorb in those charts.