Do we need to worry about lens perfection?

I have plenty of good things to say about the old Nikon 28PC, including sharpness and distortion. The rendering this lens produces is pleasing. Test charts and measurements do not say everything about the image a lens produces. The Samyang 24TS produced sharp but low contrast images that I personally hated.

I quote from James Ewing from his textbook Follow the Sun. A book aimed at professionals and students and considered a standard text.

"You might ask yourself "Do I really need an expensive tilt shift lens" Can't I just correct the perspective later in Photoshop?" The answer is yes you could correct it later, but the tilt shift lens allows you to see and feel the perspective of the images you are shooting. The final crop and ultimately the entire composition will be totally different in a shot that is corrected in post. If you cannot see the image while you are shooting you cannot control the composition and therefore you cannot effectively interpret the building. Correcting the perspective during post production causes a significant loss of sharpness and detail. The Tilt shift lens gives you accurate, sharp controlled images."

I think this says it all.

I would add that one never quite leaves enough room around the image when one shoots an image with the camera tilted upwards, with the intention of correcting in post. It is all very hit and miss.

I actually did some rather crude but useful experiments concerning the correction of verticals in camera optically and in post. For simple vertical shift the results are quite close, but for diagonal shift the results are quite different

Here is the link

The second part of this post shows that with a little diagonal shift things are very different

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If we consider lens shifts alone, the change in projection from shifting the lens can be exactly corrected by applying a mathematical transformation in software. There is no difference between a shift lens image and a software shifted image except that lens aberrations will be different in the two cases.

A photograph created with a shift lens will be seen with exactly the original perspective when viewed from the "centre of perspective", just as a photograph created with an ordinary lens will be seen with the original perspective when viewed from the centre of perspective. When viewed in this way both photos look identical. There is no difference in perspective, only a difference in the position of the centre of perspective relative to the image. It is a relatively simple mathematical transformation to go from one image to the other.

Getting the same result in post is easy if you just use simple vertical shift. Any commercial software like DXO Viewpoint does a good job. But start trying to recreate diagonal shift and you will find it almost impossible to make a matching picture with commercial software.

This is a copy and paste from my Blog

Last year, I took my 24mm TS lens to a local castle to see if there are any visual differences between using a shift lens, and correcting keystoning in post. The lens was on a tripod and in a fixed position. In one case the lens was shifted, in the other, shift was set to zero and the lens tilted to simulate a normal lens. I have never seen any sort of comparison in my textbooks or on the web, so I decided to do my own comparison to see if things change geometrically.

I looked at two cases: simple vertical shift and diagonal shift. I then made an approximate overlay of the images to get an idea of the differences using a pretty primitive hit or miss tool on the Web.

Simple vertical shift is pretty close, with very minor differences probably due to my approximations. Diagonal shift does have some more noticeable differences.

I just wanted to do a geometrical comparison. The other merits of using a shift lens compared to correcting in post is an another interesting conversation.

1- 24mm TS levelled and shifted upward. Camera centred perpendicular to the tower

2- 24mm TS levelled with zero shift and pointed upwards. Camera centred perpendicular to the tower . No correction in post

3- 24mm TS levelled with zero shift and pointed upwards. Camera centred perpendicular to the tower . Correction in post

4- Overlay - The match is almost perfect

5- 24mm TS levelled and shifted diagonally upwards. Camera centred to get the whole building into the frame

6- 24mm TS levelled with zero shift and pointed upwards. Camera centred to include the whole building . No correction in post

7- 24mm TS levelled with zero shift and pointed upwards. Camera centred to include the whole building . Correction in post in 2 directions

8 Overlay. This is pretty approximate as getting a good match was almost impossible.

9- 24mm TS levelled and shifted upwards. Camera centred to get the whole building into the frame

10 - 24mm TS levelled with zero shift and pointed upwards. Camera centred to include the whole building . No correction in post

11- 24mm TS levelled with zero shift and pointed upwards. Camera centred to include the whole building . 2 way correction in post

12 Overlay

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I did the comparisons, because I was just to curious to see if there was any differences, between "doing it in post" and using a shift lens, as i could find no real life comparisons anywhere.

I have had these sort of arguments before on DPR, in fact some people got quite nasty about it. There seems to be a strange aversion on photo forums to these lenses, because they are considered expensive specialist tools, that all conquering software can replace easily. In fact you can find an old Nikon or Canon PC lens for €200 - €300 and they still do a decent job. My more modern lenses cost me €1000 for hardly used copies. Hardly earth shattering sums compared to current lens prices. But nobody ever questions the wisdom of the bird photography brigade who often spend mouth watering sums on lenses.

My tests show that indeed doing it in post can get the same results as using a shift lens, in the most simple cases. But start doing diagonal shifting and as my comparisons show, the results are very different, and difficult to achieve in post.

But most of all photographing architecture with a shift lens on a tripod is a more satisfying experience. It is nice as well as easier to compose the near final framing in camera, without having to make guesses about the inevitable cropping that occurs with PP keystone correction.

For sure, resolution (and noise) are not the only measures of IQ, and other elements of IQ can easily matter [substantially] more so long as "deficiencies" in lack of resolution (and/or excessive noise) are not distracting. Everybody's "quality threshold" of what is "good enough" and what is not with regards to the "success" of the photo will vary, not merely from person to person, but from scene to scene, not to mention display media (e.g. smartphone screen vs 20 inch diagonal canvas print vs 85" 8K TV) and viewing distance. And it almost went without saying that no matter how much better the equipment gets, we're still ultimately limited by our vision, and hence improvements in equipment are not only subjected to diminishing returns, but often well past the "good enough" point as to be irrelevant for most.

As for what reviewers hype and don't hype, that's irrelevant to me -- I want to see [competent] samples and decide by my own weighted criteria if better is worth it to me. For most people, a smartphone is easily "good enough". It's we photography enthusiasts that keep chasing better and better because we're passionate about it.

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I took the same shot from the same spot with the same lens, shifted or with the lens tilted and then made overlays. I would say I have provided pretty good proof that in simple situations key stone correction and lens shifting provide much the same results with commercial software. I also show in more complex situations that optical correction and keystone correction give very different results.

I will also add using shift lenses lets me control the perspective effects much easier. I see the perspective effects in camera, and can change focal length if needs be.

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Frame 8 and Frame 12. Enlarge the overlays and the difference is quite marked.

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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.