PC-E Lens Design Question

In looking at tilt shift lenses by Nikon and Canon, I notice that long focal length lenses have the center of rotation out in front of the lens, and shorter focal length lenses all have the center of rotation at or around the sensor/focal plane. Canon does the same thing.

It makes some sense to me that having the tilt pivot about the focal plane would mean you don't get any shift with your tilt adjustment (no cross-talk). This also eliminates any effect on focus due to tilt adjustment.

But why for a longer lens make it so you get a huge shift upward with your tilt downward, and so on?

Richard

This might just be the physics of the matter kicking in.

A few data points that when taken together support this contention, as opposed to some conscious design decision as you seem to suggest:

• The nodal points of my longer primes are generally all further forward than my wider primes.
• The nodal points of my zooms all move forward as you select longer focal lengths.
• Per your point above, if the same behavior is observed for both the N and C TS lenses, then certainly physics would seem to be at play.

It would be nice if all lenses had their nodal point right where the focal plane is - dang it would be convenient. I have all the nodal points for my lenses marked up on a laminated piece of paper, and use a RRS rail to get to the nodal point for any given lens when needed

FWIW - I own the 24mm and 45mm PC-Es.

Hope this helps.

David

BlindSquirrel said:

It makes some sense to me that having the tilt pivot about the focal plane would mean you don't get any shift with your tilt adjustment (no cross-talk). This also eliminates any effect on focus due to tilt adjustment.

Click to expand...

None of the tilt-shift lenses have the tilt pivot at the focal plane. The shorter lenses (24mm and 45mm) have the mechanical tilt center at (or very close to) the lens mounting flange. The 85mm has its tilt center near the front elements.

BlindSquirrel said:

But why for a longer lens make it so you get a huge shift upward with your tilt downward, and so on?

Click to expand...

I believe you'll find that they are all designed such that tilting produces only a minimal amount of shift. The shorter lenses are retrofocus, with the lens nodal points well separated. The 85mm is a telephoto design, which means the front nodal point is actually closer to the sensor plane than the rear nodal point is. These two types of lens designs need to be tilted about very different points, to avoid shifting.

Thank you for both of your responses. Though I have a science and engineering background, I'm very fuzzy on the physics of optics. Even so, I think your responses have pointed me in the right direction to better understand what is going on.

I also built a tilted and shifted f-mount for a Mamiya 645 MF lens, and I've found a little bit of shift doesn't really have a significant impact on the quality of the tilted photos I've taken anyway. And if it is does, it isn't hard to adjust in post process. As a result, I've decided that my adjustable tilt, fixed register distance mount won't have adjustable shift, and I wont worry too much about even picking the perfect center of rotation.

As it turns out, the part of all this that is really an issue is that the medium format lens I started with, which is sharp as can be and a joy to manually focus, isn't very contrasty, and gives everything a very odd color cast (green, but not uniformly so like a color filter would).

It's easier with "normal" (not telephoto, reverse or otherwise) lenses -- but then you have the bellows to deal with. And arguments over axis tilts, base tilts, and asymmetric tilts. Have fun!

I think a further design point is the following:

With a 17 mm lens it is hard to get more usabel image circle, because of that a tilt axis very near to the sensor is good.

For a 90 mm lens the image circle could be very big - and you could move the tilt axis away to the lens - to have less problems with the mirror housing shading.
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