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-   -   Correcting perspective (https://e-group.uk.net/forum/showthread.php?t=49685)

Bikie John 31st January 2019 02:30 PM

Correcting perspective
Prompted by Paul's (pdk42) series of photos of the Flatiron Building (thread here: http://e-group.uk.net/forum/showthread.php?t=49588) and the discussion about comparison of the human eye to a camera (http://e-group.uk.net/forum/showthread.php?t=49640) I have been thinking about correcting perspective. When we shoot big buildings we usually tilt the camera upwards, which makes the building look as though it is falling over backwards, and this is quite disturbing.

Editors give us tools to resolve the converging verticals, and we used to do it in the darkroom by tilting the printing paper, but the results never look quite natural. Paul's photos show this, the building tends to come out looking top-heavy.

I have just done a little experiment to see what the editing tools are doing, and found it quite illuminating. I took a photo of a mount-cutting mat with a square grid printed on it, and shot from an angle as if shooting upwards at a building. I left plenty of space around it to allow use of the vertical correction tools without having to crop outside the edges. Here is the full shot with no edits, just resized for the thread:


Then in Lightroom I mashed the tonal values to simplify playing with it:


Here is a crop of just the mat so we can see what is happening:


As expected, the lines going away from us (which would be the verticals on a building) are converging to a vanishing point. The transverse lines stay parallel but get closer together as we get further away.

This is the result of doing a vertical perspective adjustment in Lightroom, I really don't know why it has chopped some off as I did it from the full uncropped file:


Photoshop is a little more sensible and doesn't chop any bits off:


We can see that the transformation has restored the original shape of the subject - the boxes are all nice and square. This is neat, and probably working as it should.

I think this is at the heart of why adjusted photos look wrong. When we look at a photo, we know that the top of the building is further away so the top floors should look smaller. With these transforms they are the same height which makes them look unnaturally large and hence the building looks top-heavy.

I reckon that what we need to do is add a sort of "spongy squash" from the top down so that the top is compressed more than the bottom, with a nice even gradient. If we could do that the result would look more natural.

I have been playing quite a bit this morning and not yet found anything that will do this. Do any of our post-processing wizards have any bright ideas?


PS You are right, I do need to get out more :)

Graham from Wirral 31st January 2019 03:52 PM

Re: Correcting perspective
This is interesting as I came back from Armenia and Georgia last Autumn with a lot of building shots and almost every one has needed the perspective correcting. The EM1 II is the best camera I have used in low light (ie gloomy buildings) hence the high number of decent shots needing correction!

I've found Lightroom works best on those which are not too far out, but Photoshop Perspective Warp seems to work pretty well in just about every case.

The problem for me is making sure that images are not 'squashed' and the final result bears some resemblance to how things really look without chopping off the top or bottom - I don't know if there is any easy solution to that except going with what looks right.

Sorry - this isn't a very technical response, but I'll certainly be interested in what others have to say about this.

Walti 31st January 2019 03:58 PM

Re: Correcting perspective
Just correcting some in Lightroom....

I find myself applying the perspective correction, then dialling it back a bit so that there's still some narrowing so it "looks natural" as I find completely square buildings can look unnatural, but it is a case by case this as some look far better square.

I don't think there's a scientific reasoning behind it other than what looks right!

Most of the photos of buildings I took this morning were with the 7.5mm lens so I used the in built levels wherever possible, as I find this gives a square building and a more natural viewpoint..

Walti 31st January 2019 04:07 PM

Re: Correcting perspective
Though trying to get the perspective spot on when the building isn't square to start off with...

Is this the ideal house, including old Land Rover?

OM USer 31st January 2019 06:39 PM

Re: Correcting perspective

Originally Posted by Bikie John (Post 470400)
Editors give us tools to resolve the converging verticals, and we used to do it in the darkroom by tilting the printing paper, but the results never look quite natural. Paul's photos show this, the building tends to come out looking top-heavy.

You're right. When you make the buildings square it does look unnatural. I also think that the keystone correction and the trapezoidal correction in OV3 produce distinctly different looks. Sometimes after repeated convergence correction I find that I have to stretch the image in the horizaontal as people look very tall and thin - possibly because they were not at the centre of the picture but towards the bottom. If I have the wide angle with me I sometimes put that on to make use of its rectilinear properties, rotate the camera to portrait, and take a shot with the camera pointing straight forward. Crop off the bottom half (your legs) and you have a nice looking well adjusted picture.

DerekW 1st February 2019 09:55 AM

Re: Correcting perspective
I use the perspective control in Lightroom only for minor correction.
If I want to correct say a very tall building picture that requires a significant correction I use Photoshop, first of all I double the Canvas Size in the Image - Canvas size option.
This allows the image to be stretched where needed without the subject being pulled outside the original canvas size. I then crop the image. I often then stretch the image vertically to restore the proportions of the building.

The perspective control recently added to LR has reduced the number of transfers to PS and then back on quite a few images.

Bikie John 1st February 2019 11:31 AM

Re: Correcting perspective
Thanks for the comments. Derek, that's a useful comment about increasing the canvas size in PS. It always surprises me how much you seem to need.

The problem as I see it is that all the perspective corrections, stretches and so on is that they are applied uniformly so in the example the squares at the top are the same size as the squares at the bottom. This shows that it is a really neat projection, but I think this is what makes buildings look unnatural - we expect the top storeys to be smaller because they are further way.

Of course, because we also expect to see vertical verticals, this leads to a conflict - if all the bits stay the same shape the ones at the top are too tall. But a large part of lens design is about resolving conflicts like this and giving us a result that looks natural. The comes about because of deep-seated differences in the way we understand the world with our eyes and brains, and how a camera records it.

What I would like to try is a transform that straightens the verticals while letting the transverse lines get closer together with distance, as they do in the original. It might be horrible, we don't know until we see it, but I would still like to give it a try. I haven't figured out how to yet, though.


steverh 1st February 2019 02:39 PM

Re: Correcting perspective
I downloaded the original image and processed it with DxO ViewPoint 3. This is with its automated processing, I could probably improve it a bit:


It is extremely powerful and flexible. You can dial in as much correction as you like. I think if you have the EXIF information from an original image it uses lens information as well.


Bikie John 2nd February 2019 12:24 PM

Re: Correcting perspective
Thanks for trying it Steve. It has done the same thing as PS and LR - adjusted the horizontal lines as well as realigning the verticals so we have the same problem - it it were a photo of a building it would look top-heavy.


steverh 2nd February 2019 03:01 PM

Re: Correcting perspective
True but you have very fine control over the amount of corrections you can apply so making it appear more natural.

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