Do we need to worry about lens perfection?

Yes, it can be done, and newer tools have improved over the Viewpoint 1 I used. Interesting, I need to update my test with some newer tools. This version will need to be cropped far more on the RH side for some reason.

I wonder if Viewpoint 3 has improved the scaling, compared to VP1. Time to update.

Isn't simpler and more precise, just to use the right lens for the job? Key stone correction is a good work around when using an ordinary lens. I use this method too. But with a shift lens I have precise control over the composition.

You are absolutely correct. A S/W tools only address correction on one plane and in reality that should be the focal plane. However, a simple consideration of the projection of parallel planes in front of and behind the focal plane when these planes are not perpendicular to the lens axis on a fixed axis camera, shows that the keystoning is not equivalent and the correction can only be precise for the focal plane. This is what you are seeing on the diagonals.

Bottom line a camera is mathematically a projective transform of a volume in three space onto a plane. Projective transforms do not preserve either distance or angles. The only way to correct all the planes in front and behind the focal plane accurately is to insure the lens axis is perpendicular to these planes. That is only obtainable by lens shift. Once one points the camera so that the lens axis is not perpendicular to a building, the perspective of the projection of planes in front and behind the focal plane is no longer equivalent to that on the focal plane.

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I agree that you need a lot of time and patience to set up a scene with a shift lens on a tripod, but I also enjoy the process of getting what I want in the viewfinder.

But with mirrorless, things are a bit different, almost game changing in my opinion. When I cannot use a tripod I use the in camera level to compose with my shift lenses. I can enlarge in the viewfinder to get critical focus and exposure is easy to control with the WSWG viewfinder. With IBIS I can close down a bit and the Z7 is still quite noise free at 1600

This was hand held. It just needed some very minor adjustment in post.

Update.

I must have done my original combinations using Capture 1 keystone tools. Using DXO Viewpoint Ver 1 I can get a good match with 2 direction keystone correction.

Looks like not all keystone correction tools are equal. Viewpoint seems to do a very good job.

This conversation has prompted me to take another look at keystone correction in post.

I made 2 quick conversions using the same files. The pictures were taken with the same 24TS form the same position

DXO Viewpoint does a good job

Capture One is quite disastrous

I do not quite know what is going on here, and I might do some more experiments.

However, that's what professional profilers do.

Maybe my title did not have surgical precision. Lens quality today, seems to be tied up with MFT resolving power, test charts and blurred corners. I forgot Bokeh of course. This is what my post was all about. Nobody, talks much about contrast, micro contrast and other factors which make up the lens image quality package. The Samyang 24TS which I owned briefly was slated for various distortion issues in the reviews I read, but none talked about the horrible low contrast this lens had.

Your observations regarding Leica and Fuji images, may be due to some sensor micro lens magic, or software processing. Who knows.

I bought a Schneider Kreuznach PC Curtagon 4.0 35mm shift lens, old new stock for €240, out of curiosity. It has a very original shift mechanism. The few tests I found were not encouraging. But I like the rendering this lens gives me. Like the two Nikon PC lenses, it seems a bit more "creamy" than my sharper modern lenses.

I might dig out my old Nikon 180 2.8 to see if it has anything to offer, when I photograph architectural details.

The diversion of this thread turned out to be interesting. It seems diagonal shift, can be reproduced in post if you use the right software. Which begs the question about what transformations are being done when we use the key stone correctors in our software.

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Lens shift tilt on projectors is not the same as "keystone" correction which is the same concept behind keystone correction in imaging S/W.

www.lifewire.com/tip-lens-shift-vs-keystone-correction-1847335

Keystone correction is a projective transform known as a homography and such corrections belongs to the prevue of geometry over projective space - in the case of images over the two dimensional projective space over the real numbers.

en.wikipedia.org/wiki/Homography

Projective geometry does not preserve distance and the concept of angles does not exist. That is the distance between two points one on the focal plane and another on a different plane will not be preserved in a keystone correction algorithm. Since there is no concept of parallel lines in the projective plane (all lines intersect in projective geometry) keystone correction off the focal plane cannot accurately address points that come from reflectors on planes other than the focal plane. In fact if S/W keystone correction could accurately remove all perspective distortion for the image of the from using S/W. That would be equivalent to being able to recreate the Euclidian geometry (which preserves both distance and angles) from a single image. It is pretty well know you need two - each taken at different points of reference - a.k.a. stereo imaging to accomplish.

en.wikipedia.org/wiki/Projective_geometry

Projective space rather than affine space is the appropriate way to describe images which is why it is the foundation of computer vision.

www.hhi.fraunhofer.de/fileadmin/Departments/VIT/IMC/Team/schreer/doc_pub/1998_Lecture_ProjGeometryIntro_Birchfield.pdf

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When you use a shift lens, you are basically cropping in camera. A 24PC is actually a 21mm I believe. You are using just a part of the image circle. There will be some "stretching" effects due to the rectilinear wide angle conformation. The glass will have no effect

Tilting the lens upwards and correcting in post will have keystoning, but vertically the detail of how you see such things as window sills remains the same as a line drawn from the sensor to the detail remains at almost the same angle compared to the horizontal. We just need horizontal stretching in post. It is all a question of trigonometry.

You can sketch things out with a pencil and paper.

I agree, this true.

This is not true. A 24mm lens is a 24mm lens.

A 24PC or TS lens has bigger angle of view and so bigger image circle, but not shorter focal length. All that speech of crop factors and equivalent focal lengths is confusing people.The 24mm TS-E has ~103° AOV at 48x36mm sensor which is equivalent with 17mm lens at 24x36mm sensor. It's focal length is still 24mm.

JACS' suggestion does not involve geometric corrections in software. Use a lens wide enough to cover the top of the edifice, held parallel to it, and simply crop out the bottom and sides. That's what we did before Nikon introduced the world's first shift lens and we didn't have a view camera.
Edit: I haven't read beyond tprevatt's post, so if this is redundant, please disregard.

True, I should of written equivalent to a 21mm lens.

I think he did answer to his own question when asking:

When using PC or TS lenses we are using UWAs and cropping a part of image to use. The same with view cameras. We used to use wide angles with view cameras to keep vertical lines parallel. No need to correct in PS/LR. AOV of my widest view camera lens is 110°.

Or shorter...😎