Doug Kerr
Well-known member
In a recent thread on the Manfrotto 410 three-axis geared tripod head, I mentioned the issue of how the camera is mounted on the head. I though I would discuss a little the theoretical issues involved.
First, a mea culpa. I have often referred to the three angular movements of a tripod head as yaw, pitch, and roll, terms borrowed from marine, aeronautical, and astronautical science.
In fact, those three terms only properly apply to angular movement about three axes that are always mutually perpendicular and are fixed within the frame of reference of the object whose motion is being described (an aircraft, for example).
So from here on, I will use more appropriate notation.
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No straightforward mount can provide us with consistent true motion around the yaw, pitch and roll axes. And for us, this is good, since such a repertoire would actually not be generally useful to our photographic needs.
Rather, in typical photographic situations, we want the following "stacking" of movements (we must be concerned with the order of stacking since the axes, unavoidably, will not be always remain mutually perpendicular).
At the lowest level, we typically want movement in the azimuth direction - around a vertical axis (or nearly such).
In the "layer" above that, we want movement in the elevation direction - around a nominally-horizontal axis, in any case perpendicular to the azimuth axis.
In the topmost "layer", we want movement in the roll direction. Here that term is used precisely; its axis is fixed in the frame of reference of the object being moved: the camera. The axis is (ideally) parallel to the lens axis, no matter where the camera is pointed by movement around the other two axes.
We can call this the "roll over elevation over azimuth" configuration.
For typical three-axis geared tripod heads, when the camera is mounted in the suggested way (the camera plate on a Manfrotto 410 has an arrow marked "lens", which is intended to point in the direction the lens aims), that will be the arrangement in effect.
In some cases, we may find that we cannot practically mount the camera in that orientation with regard to the "top layer" of the head, but must mount it at right angles to that orientation.
The result is a geometric arrangement we can describe as having these "movement layers", from top down:
• Pitch (used rigorously) - rotation about an axis running left-to-right across the camera body, parallel to its bottom.
• Rotation about a horizontal axis perpendicular to the pitch axis (roughly "fore-and-aft"). There is no precise technical name I am aware of for this axis.
• Azimuth - rotation about a vertical axis.
What are the implications of this over the "normal" (roll over elevation over azimuth) arrangement? The basic difference can be thought of in terms that arise in the following "thought experiment".
First, we consider the "normal" arrangement.
• We have the camera aimed at a substantial elevation angle (perhaps negative - imagine 60° down).
• The roll (top) movement is at "zero".
• We use the azimuth (bottom) and elevation (top) movements to aim the camera "crosshairs" at the scene point we desire to have at the center of the frame.
• There is a roughly-horizontal line in the scene we would like to run precisely horizontally across the frame.
• We turn the roll (top) movement until that line runs horizontally across the frame.
• The camera crosshairs are still exactly on the target point. (This assumes that the subject is a substantial distance form the camera, so that the displacement between the lens axis and roll axis does not result in significant "translational" image movement.)
Now, we go to the "alternate" configuration.
• As before, we have the camera aimed at a substantial elevation angle (perhaps negative - imagine 60° down).
• The "second" movement is at "zero".
• We use the azimuth (bottom) and pitch (top) movements to aim the camera "crosshairs" at the scene point we desire to have at the center of the frame.
• There is a roughly-horizontal line in the scene we would like to run precisely horizontally across the frame.
• We turn the second movement until that line runs horizontally across the frame.
• Now, the camera crosshairs are no longer exactly on the target point.
Is the latter intolerable? No, we can re-aim the camera. But is it perhaps uncultured - like non-parfocality of a lens. And in any case, it is a difference from the "normal" situation of which we should remain aware.
So if that's the only practical way to mount the camera, why not. But let's not do it otherwise.
First, a mea culpa. I have often referred to the three angular movements of a tripod head as yaw, pitch, and roll, terms borrowed from marine, aeronautical, and astronautical science.
In fact, those three terms only properly apply to angular movement about three axes that are always mutually perpendicular and are fixed within the frame of reference of the object whose motion is being described (an aircraft, for example).
So from here on, I will use more appropriate notation.
*********
No straightforward mount can provide us with consistent true motion around the yaw, pitch and roll axes. And for us, this is good, since such a repertoire would actually not be generally useful to our photographic needs.
Rather, in typical photographic situations, we want the following "stacking" of movements (we must be concerned with the order of stacking since the axes, unavoidably, will not be always remain mutually perpendicular).
At the lowest level, we typically want movement in the azimuth direction - around a vertical axis (or nearly such).
In the "layer" above that, we want movement in the elevation direction - around a nominally-horizontal axis, in any case perpendicular to the azimuth axis.
In the topmost "layer", we want movement in the roll direction. Here that term is used precisely; its axis is fixed in the frame of reference of the object being moved: the camera. The axis is (ideally) parallel to the lens axis, no matter where the camera is pointed by movement around the other two axes.
We can call this the "roll over elevation over azimuth" configuration.
For typical three-axis geared tripod heads, when the camera is mounted in the suggested way (the camera plate on a Manfrotto 410 has an arrow marked "lens", which is intended to point in the direction the lens aims), that will be the arrangement in effect.
In some cases, we may find that we cannot practically mount the camera in that orientation with regard to the "top layer" of the head, but must mount it at right angles to that orientation.
The result is a geometric arrangement we can describe as having these "movement layers", from top down:
• Pitch (used rigorously) - rotation about an axis running left-to-right across the camera body, parallel to its bottom.
• Rotation about a horizontal axis perpendicular to the pitch axis (roughly "fore-and-aft"). There is no precise technical name I am aware of for this axis.
• Azimuth - rotation about a vertical axis.
What are the implications of this over the "normal" (roll over elevation over azimuth) arrangement? The basic difference can be thought of in terms that arise in the following "thought experiment".
First, we consider the "normal" arrangement.
• We have the camera aimed at a substantial elevation angle (perhaps negative - imagine 60° down).
• The roll (top) movement is at "zero".
• We use the azimuth (bottom) and elevation (top) movements to aim the camera "crosshairs" at the scene point we desire to have at the center of the frame.
• There is a roughly-horizontal line in the scene we would like to run precisely horizontally across the frame.
• We turn the roll (top) movement until that line runs horizontally across the frame.
• The camera crosshairs are still exactly on the target point. (This assumes that the subject is a substantial distance form the camera, so that the displacement between the lens axis and roll axis does not result in significant "translational" image movement.)
Now, we go to the "alternate" configuration.
• As before, we have the camera aimed at a substantial elevation angle (perhaps negative - imagine 60° down).
• The "second" movement is at "zero".
• We use the azimuth (bottom) and pitch (top) movements to aim the camera "crosshairs" at the scene point we desire to have at the center of the frame.
• There is a roughly-horizontal line in the scene we would like to run precisely horizontally across the frame.
• We turn the second movement until that line runs horizontally across the frame.
• Now, the camera crosshairs are no longer exactly on the target point.
Is the latter intolerable? No, we can re-aim the camera. But is it perhaps uncultured - like non-parfocality of a lens. And in any case, it is a difference from the "normal" situation of which we should remain aware.
So if that's the only practical way to mount the camera, why not. But let's not do it otherwise.