View Camera Focusing and the Scheimpflug Principle

[rdenney]

That's technically correct but it's much easier to explain it as focused space and unfocused space.

Ian's question reveals the troubles of attempting a precise explanation without being careful about all the terms. Much pain and suffering on this forum has resulted from the lack of clarity (so to speak) between "in focus" and "acceptably sharp". (You could have said "apparent focus", which addresses a lot of issues.)

But then it's sort of a sport on this forum to out-precise the next guy.

Rick "guilty of that as anyone" Denney

[Cletus]

Lon - nice article! Nice illustrations! That's going in my bookmarks and my reading list. I'm just now checking back on this discussion (that I started, I'm afraid) and yours is a comprehensive and accessible explanation. Not that others explanations have been lacking, either.

I guess what prompted the question in the first place - not to get this all started again! - was the fact that to me, ANY TIME you have ~some front forward tilt and ~some back rear tilt, you have met the conditions of the Sheimpflug Principle I that lens plane, film plane and ostensibly subject plane will converge to a point. Period. That does NOT determine exactly WHERE the plane of best focus will land, but it does meet the requirements of the SP.

That was really my only point in the first place. If you happen to be focussing on a "flat" plane, whatever the orientation, it should be in sharp focus in its entirety and need not stop to f128 to get there.

[Cletus]

Oh boy. I should've kept my mouth, or fingers, shut! I get it, I was just reiterating with that last. Please disregard my comments!

[Leonard Evens]

The Scheimpflug Principle states that the subject plane, the lens plane, and the image plane intersect in a common line. So if the lens plane and plane of the gg are tilted with respect to one another, thus intersecting in a line, the subject plane must pass through that line. But the Scheimplug principle doesn't tell which of the infinitely many planes also passing through that line is the subject plane. It just must be one of them. Of course, you can see the subject plane simply by looking at your gg. It is what is in exact focus.

If what you see in focus on your gg is not your desired subject pane, you may try remedy that by focussing, i.e., by moving either the front standar or or the rear standard parallel to itself along the rail. That will of course move the line of intersection of the lens plane and image plane as well as moving the subject plane. If you chose the tilt properly, then at a certain point you desired subject plane will come into exact focus. If you did not choose the tilt properly, that won't ever happen. Thus you must change the tilt.

Here is what I do. I find it better to focus by moving the rear standard. I start by making a guess and tilting the front standard about 5 degrees. I choose two points, a near point and a far point, which I want to be in my desired subject plane. I focus by moving the rear standard until the far point is in focus. I then refocus with the rear standard to bring the near point into focus. If that requires me to move the standards further apart, I increase the tilt. If that requires me to move the standards closer together, I decrease the tilt. After two or three iterations of this process, I find both near and far point in focus, and of course my desired subject plane is in focus.

In doing this, you can be confused by depth of field since that my confuse you about where the exact focus is. You should always focus wide open and use a loupe to try to reduce the amount of depth of field so you can identify where the exact focus is.


It is also important to know what the depth of field region looks like after a tilt. It forms a "wedge" shaped region extending from a base line, below the lens, called the hinge line. the plane of exact focus cuts that region in two in that any vertical plane is cut by the region into two equal planar regions, one above the subject plane and one below. The position of the hinge line is determine by the tilt. As you increase the tilt of the lens plane, it moves closer to the lens but never gets closer to the lens than one focal length. With the tilt fixed, if you focus by moving the rear standard, the whole wedge swings about the hinge line, with the subject plane moving but always centered in the wedge as described above.

[Leigh]

...lens plane, film plane and ostensibly subject plane will converge to a point. Period. That does NOT determine exactly WHERE the plane of best focus will land, but it does meet the requirements of the SP.

You can only control two planes: the lens plane (normal to the lens optical axis), and the film plane.

Those two planes intersect in a line (provided they're not parallel to each other).
A plane drawn through that line and intersecting the subject at the point of focus defines the plane of focus. You cannot change it.

- Leigh

[Cletus]

Leigh -

Yes, I understand. I should have limited any further commentary and "reiterations" to avoid dragging this into redundancy. I read (and re-read) many of yours and others comments and it's very clear to me now. I even some test setups on a table to demonstrate to myself some of these principles. IOW - I practiced a bit!

Thank you for sharing your views and considerable knowledge regarding this rather complicated (I guess it can be as complicated as we want, yes?) subject.

Cletus

[Leigh]

Hi Cletus,

One of the problems with this and many other concepts is that explaining them in writing is much more cumbersome
than demonstrating the principle in person, or with a video or some similar visual medium.

Good luck, and good shooting.

- Leigh