Meaning of yaw

[Leonard Evens]

I think I finally understand what yaw is about. I must admit having been confused about it in the past and even giving some misleading, incomplete advice. I will try to explain what I now think and see if anyone thinks I'm right.

Yaw, pitch and roll refer to terms used to describe the change of attitude of a rigid body such as a ship or an airplane, and in view camera photography, we use tilt, swing, and yaw to describe what happens to a standard. (Identifying which corresponds to which depends on how you set up the reference system.)

As we use the term yaw, it refers to rotation of the standard about an axis perpendicular to it, e.g., the lens axis for the front standard. Yaw may result from performing a tilt followed by a swing, or, vice versa. Exactly what happens depends on on how the axes for tilt an swing are attached to the camera frame.

Yaw could clearly be a problem for the rear standard because it would change the orientation of the film frame with respect to the horizontal and vertical. But I never saw why yaw would be a problem for the front standard. After all, the image is symmetrical about the lens axis, so you you can rotate the front standard about that axis to your heart's content without seeing any difference.

What I've finally figured out is the following. It is not yaw itself that is the problem for the front standard. It is what caused the yaw in the first place. Let me elaborate.

Suppose you apply a tilt. If you have axial tilt, this will be a rotation about a horizontal axis through the lens. But even if you have base tilt, you can adjust the position of the standard by rise/fall or movement along the rail to accomplish the same thing, rotation about a horizontal axis through the lens center. So let's assume that is the case. The question then is what happened to the swing axis. For some cameras it may remain vertical where it was. That will generally be the case for cameras with axial tilt, but it is not generally the case for cameras with base tilt. For the latter, the swing axis stays fixed in the standard and hence is tilted with it. This is usually phrased by saying that in the good case the "point of attachment" for tilts is "above" that for swings, but I've never understood exactly what that is supposed to mean, so I just try a tilt and see where the swing axis goes to be sure.

What happens next is a bit subtle. You do get yaw in the bad case where the swing axis has moved, but something else happens that is more complicated. Consider what happens to the hinge line. You hope you can move it through tilt followed by swing so it coincides with where it should be in the desired subject plane. After the tilt, the hinge line is horizontal. In the good case, as you swing, the hinge line rotates in the vertical plane parallel to the image plane about a fixed point---that is harder to see than you might think, but it is right. If you got the tilt right, which turns out to be feasible, you can rotate the hinge line to where it should be by a swing. The amount it rotates is related to both the tilt angle and the swing angle by a complicated formula, but the important thing is that for fixed tilt angle, the swing angle and the hinge line rotation angle determine one another, and you don't need to know what that formula is.

In the bad case, whatever tilt angle you choose, the hinge line will move in its entirety, no point in it remaining fixed. I don't see in that case how you can possibly predict in advance just how much to tilt in the first place, without doing elaborate calculations. You would have had to have overshot or undershot the tilt by exactly the right amount, and thaat would require knowing the formulas. So you end up having to go back, adjust the tilt, readjust the swing, etc. Of course, since in practice, you never get anything exactly right, you may have to do that anyway. But if you go about it right, fewer adjustments should be necessary.

I'm sure someone will say that the effect I'm talking about is an obvious effect of yaw, but it wasn't at all obvious to me. I had to look exactly at what happened before I saw the importance of the hinge line for this, and the geometry was not that simple. It only occurred to me when I was thinking about something else entirely. It seems more enlightening to me to say that both yaw and the effect on the hinge line result from the same basic cause. In any case, I think it is well worth understanding the importance of the hinge line in all of this. Of course, if you already have a yaw free camera, you will know from experience that something is better, in which case you may not care why, but if you don't have such a camera, which was true for me, it may take a while to figure it out. Anyway, I think I now understand the reason why many experienced view camera users extol the advantages of being yaw free.

Let me mention as an aside that usually base tilt cameras will work just fine in this regard if you just do the swing first. At least for all I've looked at, including my Toho FC-45X, the tilt axis doesn't change when you swing, so the good case becomes the bad case, and vice versa. I didn't know this because no one seemed to mention it anywhere, so it wasn't until now that I realized that I should swing first and then tilt. For the axial tilt cameras I've looked at, you run into the same problem if you reverse the roles of tilt and swing. I haven't looked closely at any cameras purported to be yaw free, but for that to be really true, the swing and tilt axes would have to be entirely independent of one another. I'm sure that can be managed by an appropriate mechanism, but as far as I can see few view camera manufacturers bother.

Any comments would be appreciated.

[Jan Pedersen]

Yaw means rotating something horizontal (Perhaps not the scientific expression) If you have your camera set up on your tripod and you rotate the camera from let's say south to west, then you are yawing CW.
Hope that makes sense.

[Leonard Evens]

P.S. I've now gone back and looked at some explanations of yaw, and now that I understand what is going on, they make sense. One explanation even highlighted the crucial point: that in order to avoid difficulties, you want to be sure the axis for the second operation is not moved by the first operation. But most of the explanations seem to be saying that the difficulty in focusing is caused by the yaw, which I still think is misleading. Also, the explanations say that in the presence of yaw you have to go back and forth continually readjusting the tilt and the swing. My mathematical intuition and knowledge told me this statement is false. If you know the formulas, then both the tilt angle and the swing angle, no matter how they are done are completely determined by the final position of the subject plane, even just by the hinge line lying in it. What is true is that you can't easily determine those angles without doing elaborate calculations. I guess this amounts to the same thing, but I misinterpreted the statement to mean it was literally true.

In addition, some of the implications implied that being yaw free meant it didn't matter which order the operations were done, and then went on to describe mechanisms whereby tilt followed by swing was yaw free, but not vice versa. I agree that a truly yaw free camera would work that way, but not the arrangements they described.

I'm sure there is an explanation somewhere which explains it correctly, probably in a much better way than I did, but if so it is hard to find among the many misleading or incomplete explanations.

[steve simmons]

If you level the camera left to right and front to rear you will never have yaw.

steve simmons

[Gordon Moat]

I am reminded of some Arca Swiss and Linhof cameras, which claim yaw free design. What I see is a large section of a circle, about which the lensboard/standard can move at angles. However, if the effective focal distance is not the same as the flange to film plain distance, wouldn't that mean there is always a focus shift, even with these cameras?

Also, in Swing or Tilt, using the front standard, I have seen where the focus moves to either the left or right, or higher or lower, on the ground glass. Is this an example of focus shift, or just an aspect of lenses when using movements?

At least in the Arca Swiss brochures, they mention focusing once, then doing movements without needing to refocus. Would that essentially be yaw free movements, because of that capability?

Ciao!

Gordon Moat Photography

[Sheldon N]

For me, the simplest way to understand what a Yaw free camera does is this:

If you use indirect rise/fall (meaning point the camera up/down then relevel both standards to vertical to create effective rise/fall) and THEN attempt use swing on either the front or rear standard....

1) A "Yaw Free" camera will have the swing work normally, i.e. on a flat horizontal plane.

2) A camera that is not "Yaw Free" will do strange things with the image/focus plane because the swing axis of rotation is not horizontal, but rather arcing on the plane aligned with where you originally pointed the camera (up/down).


If your camera is not "Yaw Free" you have to do what Steve said - keep the camera oriented on a perfectly horizontal plane in order for swing to function normally.

[Bob Salomon]

I am reminded of some Arca Swiss and Linhof cameras, which claim yaw free design. What I see is a large section of a circle, about which the lensboard/standard can move at angles. However, if the effective focal distance is not the same as the flange to film plain distance, wouldn't that mean there is always a focus shift, even with these cameras?"

A yaw free camera is one that has a tilt point below the swing point. On Linhof and Sinar yaw free cameras that means that there are two tilt points. The bottom one used for moving the standards to the proper point for leveling and the second for doing the camera movements.

"Also, in Swing or Tilt, using the front standard, I have seen where the focus moves to either the left or right, or higher or lower, on the ground glass. Is this an example of focus shift, or just an aspect of lenses when using movements?"

Those are the lines that you focus far and near on when the camera has assymetrical axis movements. That is an additional feature to being yaw free and are not part of making a camera yaw free.


Any camera that is yaw prone when used upright become yaw free when swung 90 degrees and used on its side. That puts the swing below the tilt point by making the tilt point into the swing point and the swing point into the tilt point.

You only get yaw when the tilt point is below the swing point and the camera has only the one tilt point.

Gordon Moat Photography

[Gordon Moat]

So if I understand what Bob is stating, if a camera has the swing and tilt rotation axis intersecting at the same plain, then the design is yaw free? Please feel free to correct my terminology, and I hope that question makes sense to someone. Also, it seems that some of the Silvestri designs are yaw free, but I don't recall them making that claim/statement.

Ciao!

Gordon Moat Photography

[Sheldon N]

I'm not sure if that's the best way to put it...

It's more like if you can use tilt (usually base tilt) to relevel the swing axis back to a horizontal plane, then the camera is yaw free.

As Bob pointed out, you really need two tilt points to make a camera yaw free and fully functional. One tilt point to relevel the swing axis, and one tilt point for any additional tilts that may be needed.

For example, my Arca 4x5 w/orbix is yaw free. I can use base tilt to bring the front/rear standard back to vertical. When the standard is vertical, the swing movement pans around on a purely horizontal plane. I can then use the orbix tilt movement (axial tilt above the base tilt and swing movements) to dial in additional front tilt to go with the swing.

If I didn't have orbix, I could not use any front base tilt movements that deviated from vertical and still keep the camera "yaw free" for swing movements.

I don't think that the problem of yaw comes up very often in real shooting situations, maybe more for product shooters.

[Bob Salomon]

If I didn't have orbix, I could not use any front base tilt movements that deviated from vertical and still keep the camera "yaw free" for swing movements.

Yes you could. Just rotate the camera on the rail 90 degrees so that the swing is now the tilt point and you are yaw free.

All yaw prone cameras become yaw free when mounted on the side and all yaw free cameras become yaw prone when mounted on their sides.

All you need is a tripod and head strong enough to hold and support the camera when it is rotated 90 degrees.

This is also why the Linhof TK systems have an extra spirit level for when the camera is rotated 90 degrees to be yaw free. Hardly anyone ever does that though.