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correcting perspective

Sep 16, 2011, 01:09
Post: #1
correcting perspective
My second thread here and I will need some info as well on this......

I have photographed framed art and I was using my 40 mm lens. My rectangular photographs have no discernible barrel or pin-cushion distortion....rather it seems to have a bit of 'converging parallels'... that is to say that the top right and bottom right of the piece of art are not parallel.

Is there a correction for this, other than to go to my client and tell him that I should have stayed with my previous line of work?

Thanks a lot

Ray
Miami Beach
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Sep 16, 2011, 06:34
Post: #2
RE: correcting perspective
(Sep 16, 2011 01:09)miamisunray Wrote:  My second thread here and I will need some info as well on this......

I have photographed framed art and I was using my 40 mm lens. My rectangular photographs have no discernible barrel or pin-cushion distortion....rather it seems to have a bit of 'converging parallels'... that is to say that the top right and bottom right of the piece of art are not parallel.

Is there a correction for this, other than to go to my client and tell him that I should have stayed with my previous line of work?

Thanks a lot

Ray
Miami Beach
Use the perpective tool (Perspective) in "corrective" mode. In this mode you drag the corner handles to make a 4-sides frame that Gimp transforms into a rectangle the size of the current image. This means there are two things to worry about:

1) if the correction frame is much smaller than the image, there will be some upscaling, so it's better to crop your image first to be just wide enough for the correction frame.

2) the final aspect ratio is that of the image, so you may have to re-scale the image along one of its dimensions.

3) you don't get full perspective correction, objects that are smaller due to perspective remain smaller. Fir instance, if you use the tool to fix the picture of a building, you can make all the window frames rectangular, but those of the 6th floor will be smaller than those on the 1st. In your case it may not be noticeable, however.

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Sep 16, 2011, 15:47
Post: #3
RE: correcting perspective
Well, thanks so much. That all sounds reasonable. In my case the actual size of the objects within the frame is probably not too big of a deal.... we are only talking inches 'out' on a 6 foot top-to-bottom frame....

Many thanks. As soon as I can get Peyton Manning to go all white (see my only other post here on cropping issues) then I will tackle the other photos which have mainly this perspective distortion issue.

On a side-note. My current lenses are a 17-40mm and a 70-200--all Canon. I am told that a 50 mm would be the natural choice for head-on shots of rectangular pieces of art, so as to avoid this distortion (assuming I am dead-center on the art...)

Any thoughts on that from any experienced architectural or art photographer here?

Great forum and great help, thanks again....

Ray
Miami Beach
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Sep 16, 2011, 16:10 (This post was last modified: Sep 16, 2011 16:11 by ofnuts.)
Post: #4
RE: correcting perspective
(Sep 16, 2011 15:47)miamisunray Wrote:  Well, thanks so much. That all sounds reasonable. In my case the actual size of the objects within the frame is probably not too big of a deal.... we are only talking inches 'out' on a 6 foot top-to-bottom frame....

Many thanks. As soon as I can get Peyton Manning to go all white (see my only other post here on cropping issues) then I will tackle the other photos which have mainly this perspective distortion issue.

On a side-note. My current lenses are a 17-40mm and a 70-200--all Canon. I am told that a 50 mm would be the natural choice for head-on shots of rectangular pieces of art, so as to avoid this distortion (assuming I am dead-center on the art...)

Any thoughts on that from any experienced architectural or art photographer here?

Great forum and great help, thanks again....

Ray
Miami Beach
The longer the lens the less distortion you get (actually perspective effects on the corners, that are slightly farther from the lens than the center) (but of course on an APS-C camera you can go with shorter lenses). Personally I would use my Canon 100mm macro lens, long enough to avoid distortion, and superbly sharp. The 50mm 1.4 is good and cheap, but you could also use the 60mm macro if you want something shorter than the 100mm.

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Sep 16, 2011, 23:08
Post: #5
RE: correcting perspective
[/quote]The longer the lens the less distortion you get (actually perspective effects on the corners, that are slightly farther from the lens than the center) (but of course on an APS-C camera you can go with shorter lenses). Personally I would use my Canon 100mm macro lens, long enough to avoid distortion, and superbly sharp. The 50mm 1.4 is good and cheap, but you could also use the 60mm macro if you want something shorter than the 100mm.
[/quote]

Thanks for the reply..

I am not in a position to buy any more gear just yet, but I suppose if I get substantially any more 'art' gigs I'll have to consider picking up a lens that possibly 'fills the gap' between my wide and longer zoom lenses. thanks again for your advice... much appreciated

Ray
Miami Beach
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Sep 17, 2011, 12:39
Post: #6
RE: correcting perspective
(Sep 16, 2011 16:10)ofnuts Wrote:  
(Sep 16, 2011 15:47)miamisunray Wrote:  Well, thanks so much. That all sounds reasonable. In my case the actual size of the objects within the frame is probably not too big of a deal.... we are only talking inches 'out' on a 6 foot top-to-bottom frame....

Many thanks. As soon as I can get Peyton Manning to go all white (see my only other post here on cropping issues) then I will tackle the other photos which have mainly this perspective distortion issue.

On a side-note. My current lenses are a 17-40mm and a 70-200--all Canon. I am told that a 50 mm would be the natural choice for head-on shots of rectangular pieces of art, so as to avoid this distortion (assuming I am dead-center on the art...)

Any thoughts on that from any experienced architectural or art photographer here?

Great forum and great help, thanks again....

Ray
Miami Beach
The longer the lens the less distortion you get (actually perspective effects on the corners, that are slightly farther from the lens than the center) (but of course on an APS-C camera you can go with shorter lenses). Personally I would use my Canon 100mm macro lens, long enough to avoid distortion, and superbly sharp. The 50mm 1.4 is good and cheap, but you could also use the 60mm macro if you want something shorter than the 100mm.
Wait a minute... That OTHER guy was YOU!!!!!!

Sorry about that. I am about to try corecting the perspective in one now... here is the one I will try now.....

Hope you see it.....

thanks again....

Ray


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Sep 17, 2011, 12:40
Post: #7
RE: correcting perspective
Notice the very professional white backdrop I created...... Jeez, I'm glad I am not asking for any money for this.....
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Sep 17, 2011, 12:50
Post: #8
RE: correcting perspective
OK

So everything is not as obvious as it could be.

I clicked on the perspective tool... chose 'corrective' and tried to click-hold-and-drag the far corners of the GIMP window... the corners of my artwork....and the center 'cross-hairs' of the image but nothing happened... only the 'transformation matrix' was 'doing the number thing' so SOMETHING was happening... only my photo was not rotating as I expected (otherwise how do I know what effects my movements are having?)....

I know I am missing a basic task here....

Ray
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Sep 17, 2011, 16:01 (This post was last modified: Sep 17, 2011 16:03 by ofnuts.)
Post: #9
RE: correcting perspective
In "Corrective" mode, there is not muc visual feedback. You position the grid corners to tell Gimp what is a rectangle in the image:
[attachment=668]
and when you confirm, this part of the image is stretched to become the image and so becomes a perfect upright rectangle:
[attachment=669]
Note that to minimize the aspect ratio change I first cropped the image pretty close to what I was going to keep, but for a perfect result you would rescale along one dimension to restore the original aspect ratio. And the secret is... you can't obtain the true original aspect ratio from your picture (you only get a fairly good approximation), but would have to measure it directly on original art when taking the photo), or you include in the picture objects of known dimensions (rulers...).

Since the perspective tool discards the outside of the frame, you can also get the cutout for freeSmile

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