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Author Topic: perpective control computer vs lens  (Read 4001 times)

kers

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perpective control computer vs lens
« on: December 14, 2012, 10:33:44 AM »

I found this interesting article about how to do perspective control in the computer in a proper way...
thanks to the link on the cambo-site and Rodenstock for writing it.

http://www.rodenstock-photo.com/mediabase/original/Entzerren_am_Computer_A4_e_Druck_7860.pdf

I was busy for myself figuring it out but got stuck- now the search is over...
maybe it is helpful for others as well..
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Pieter Kers
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walter.sk

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Re: perpective control computer vs lens
« Reply #1 on: December 14, 2012, 12:28:35 PM »

Thanks for the link to a very helpful tutorial.  I had been aware that the height relationships in an image changed when correcting converging verticals, and usually was able to pull up the top of the image judging by eye.  This tutorial gives an easy way of arriving at a more precise restoration of perspective.
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bill t.

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Re: perpective control computer vs lens
« Reply #2 on: December 14, 2012, 01:00:42 PM »

Ah, at last!  Perspective control in the computer is a good justification for those 36mp cameras!   More pixels = more perspective control before quality is lost!  :)

Although my personal preference is to shoot high density stitches in those situations.
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RFPhotography

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Re: perpective control computer vs lens
« Reply #3 on: December 19, 2012, 09:13:37 AM »

They've made the entire process seem much more complicated than it needs to be; starting the process, then deciding that the canvas size should be changed, then having to start over with the selection, but in the end the result is the same.  There is degradation in the image from the transformations done in the computer.  Not sure there are many people who don't know that, particularly those who do this kind of work as a part of their photography business.  Not a lot of people, I don't believe, are still doing this type of work with large format cameras.  I think most of it is being done now with medium format and DSLRs.  But there are other considerations.  Cost for convenience, or affordability.  A Schneider 24mm T/S lens is over $2k, so is the Canon 24mm and the Nikon 24mm is right at $2k.  That's a lot of coin for a lot of photographers.  Some likely can't charge enough to justify that kind of cash outlay.  Time is also a factor.  Is it quicker to do it in the field or on the computer.  What are the images going to be used for?  Will the degradation from making the adjustments in Photoshop be sufficiently evident that the images will be unusable?  It's absolutely true that using a T/S lens is going to produce a better quality end picture.  But there are lots of other considerations.
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elliot_n

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Re: perpective control computer vs lens
« Reply #4 on: December 19, 2012, 05:59:10 PM »

I've heard many recommend that when using Transform>Perspective in PS, you should pull out the top corners to the same degree that you pull in the bottom corners. This article is suggesting something more complicated. Does it make sense? (I'm struggling with the English - is it a Google Translate of the German?)
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BartvanderWolf

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Re: perpective control computer vs lens
« Reply #5 on: December 21, 2012, 09:42:37 AM »

I've heard many recommend that when using Transform>Perspective in PS, you should pull out the top corners to the same degree that you pull in the bottom corners. This article is suggesting something more complicated. Does it make sense? (I'm struggling with the English - is it a Google Translate of the German?)


Hi Elliot,

They sure make it look a lot more difficult than it needs to be, if one decides to do it in software rather than at the capture stage. The most straight forward software solution is to use Pano-stitching software. Such software functions based on re-projection of image data that originates in one type of projection space (usually rectilinear) and, after changing the center of view from 'the middle of the image' to 'the horizon of the scene', projecting that on another surface (usually also rectilinear but from a different angle).

Such a flat plane (re-)projection is required to avoid straight lines from distorting, but the resulting anamorphic distortion also produces a perceived stretching of the image when viewed from the wrong perspective viewpoint (which is common because we shoot from below and view from the middle of the scene). This can be optically compensated by either shrinking the image (vertically in case of vertical keystoning), or by manipulating the vertical offset parameter in the panostitcher.

Even a free pano stitcher such as 'Hugin' will allow to do such perspective corrections (it auto-detects the required rotation(s) when vertical and/or horizontal control points are placed in a single image), and it uses a superior resampling algorithm as well.

Cheers,
Bart
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elliot_n

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Re: perpective control computer vs lens
« Reply #6 on: December 21, 2012, 02:35:53 PM »

Thanks Bart, I have PTGui, but I only ever used it for the occasional stitch. I never thought of using it for a single image. I'll give it a try.
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kers

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Re: perpective control computer vs lens
« Reply #7 on: December 22, 2012, 07:17:38 PM »

I've heard many recommend that when using Transform>Perspective in PS, you should pull out the top corners to the same degree that you pull in the bottom corners. This article is suggesting something more complicated. Does it make sense? (I'm struggling with the English - is it a Google Translate of the German?)
To Eliot: The idea is that the distortion of the proportions is also a function of the angle the camera is tilted and the viewpoint position relative to the subject. So it is not a s simple as you suggest.
to Bart.. I would not think leaving photoshop and starting up ptgui will be a much quicker solution.. i will try.
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Pieter Kers
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Oldfox

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Re: perpective control computer vs lens
« Reply #8 on: December 23, 2012, 03:29:22 AM »

From the article:
"What takes less than half a minute using distortion correction with the camera, occupies more than 15 minutes when doing it with the computer."

My experience is the opposite.

(The article is written by a company that makes T/S lenses).
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kers

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Re: perpective control computer vs lens
« Reply #9 on: December 23, 2012, 11:59:58 AM »

From the article:
"What takes less than half a minute using distortion correction with the camera, occupies more than 15 minutes when doing it with the computer."

My experience is the opposite.

(The article is written by a company that makes T/S lenses).
I use both and i think it is 1 minute on site and about the same in the computer.
The good thing of doing it on site is that you know exactly what you get and in my case i need less time for presenting a picture to my clients...
and kudos for Rodenstock for taking so much time telling us how to get the perspective right in the computer; better than i have seen anywhere before...
The only thing they forget to say is that the borders of a shifted lens also has its loss in quality...
« Last Edit: December 23, 2012, 12:02:04 PM by kers »
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Pieter Kers
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langier

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Re: perpective control computer vs lens
« Reply #10 on: December 23, 2012, 11:09:55 PM »

I had weight anxieties during my last trip to Europe and had fears with the 8 KG limit on a couple of my flights and the 24 PCE stayed home, so the D800 and post processing came to the rescue.

Though the process is fun to work in the field and larger files can be stitched with the PCE, it was liberating during my trip to pack less and just shoot larger files for later (and easy to do) post processing.

This past weekend, I was back at it and enjoying the process again and working with the PCE lenses to finish up shooting for my next show. With the file bloat of the D800, it now gives you choice of when to handle the correction-- the luxury of the PCE lenses and enjoyment of the process, or cut-run with fairly good image quality and post production.

Either way, if you are in control of your craft, it takes little time to go either way. It's a nice choice to have today!
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Rajan Parrikar

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Re: perpective control computer vs lens
« Reply #11 on: December 29, 2012, 07:08:37 PM »

I bought DxO Viewpoint and found it to be quite good for perspective distortion correction. It is (or was) on sale for only $39.

A short review is found here -

http://pindelski.org/Photography/2012/12/26/dxo-viewpoint/

Ellis Vener

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Re: perpective control computer vs lens
« Reply #12 on: December 29, 2012, 09:26:06 PM »

[quoteNot a lot of people, I don't believe, are still doing this type of work with large format cameras.[/quote]

There are some but mostly if they are shootign professionally, are using medium foramt digital backs, either phase One or Hasselblad, instead of film.

I used to shoot a lot of 4x5 for clients but that went away mainly because clients got tired of paying the time costs ( waiting for processing and scanning for sepratations) and additional  costs ( Polaroid proofing, extra film for bracketed exposures, and scanning).
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Ellis Vener
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RFPhotography

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Re: perpective control computer vs lens
« Reply #13 on: December 30, 2012, 08:32:49 AM »

Yes, that was basically my point, Ellis.  That the bulk, of this work has migrated to digital and, with good slr-format perspective control lenses and the ability to do it after the fact, a lot of it is being done with DSLR cameras now, I'd venture.
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EsbenHR

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Re: perpective control computer vs lens
« Reply #14 on: January 03, 2013, 04:46:31 AM »

I found this interesting article about how to do perspective control in the computer in a proper way...
thanks to the link on the cambo-site and Rodenstock for writing it.

http://www.rodenstock-photo.com/mediabase/original/Entzerren_am_Computer_A4_e_Druck_7860.pdf

I was busy for myself figuring it out but got stuck- now the search is over...
maybe it is helpful for others as well..

Good catch.

Of course, you can do it correctly in software as well. I am biased, but Capture One will do it correctly for RAW files with known focal length (you need to know the sensor and lens to do these calculations). If it does not work correctly, file a bug.

Full disclosure: if you find a bug, it will land on my desk :-)

There are a few caveats:
1) You really can throw away a lot of pixel and resolution, so an ultra high resolution camera is preferable.
2) Shallow DOF does not work, because you don't align the plane where you (probably) want it to be. This makes it much less amenable for product photography than it is for architecture.

You can do tricky stuff, like putting the focus plane where you desire with a DSLR and use keystone to "undo" the movement. Expect to throw away *a lot* of pixels.

Or you can use it to get more image by tilting it so you crop away some sky, which can easily be filled with "context aware fill". In fact you can use this trick to increase resolution, but only if you don't need a shallow DOF.

Regards,

Esben H-R Myosotis
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SunnyUK

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Re: perpective control computer vs lens
« Reply #15 on: January 23, 2013, 08:31:52 AM »

Thank you very much for that link. Bookmarked!
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