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Lens distortion corrections (Was: New photographer in town!)


Hanne

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I am a Norwegian photography artist which have been using photoshop a lot for the past 9 years. I have quite good skills and have developed artistic expressions which haven´t been used much before, since it demands a lot from photoshop.

The reason I joined this Forum is because I always think there is more to learn and share.

Currently I am entering new territory with my photographs, since I will make a series of images converted to black and white.
I also wonder how to choose in lens corrections when the image is going to be cropped to a different format.
I hope to get some tips about the best ways! :)
 

IamSam

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Hello Hanne and welcome to the forum. I'm certain that you will receive help on any questions you may have. Looking forward to seeing some of your projects. Enjoy.
 

Tom Mann

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A big welcome from me, as well! Like you, I have a very strong interest in photography, having sold my first photo back in the 1960's.

Although this is not the correct forum to answer technical questions, the answer to your question about lens corrections seems to be very simple: Apply lens corrections right at the start of your workflow (eg, in ACR), not at the end, after cropping, and all should be well. OTOH, if you are talking about geometric corrections (eg, minimizing perspective distortions such as "keystoning"), while these should also be applied at the start of the work, if one forgets, you can apply them after cropping and they will work just as well. However, in this case you have to do it on a case-by-case basis.

If you want to discuss this at more length, either start a thread in the photography section, or let me know, and I'll move this post over there and we can have a full discussion over in that sub-forum.

Again, WELCOME!!!

Tom M
 

Hanne

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A big welcome from me, as well! Like you, I have a very strong interest in photography, having sold my first photo back in the 1960's.

Although this is not the correct forum to answer technical questions, the answer to your question about lens corrections seems to be very simple: Apply lens corrections right at the start of your workflow (eg, in ACR), not at the end, after cropping, and all should be well. OTOH, if you are talking about geometric corrections (eg, minimizing perspective distortions such as "keystoning"), while these should also be applied at the start of the work, if one forgets, you can apply them after cropping and they will work just as well. However, in this case you have to do it on a case-by-case basis.

If you want to discuss this at more length, either start a thread in the photography section, or let me know, and I'll move this post over there and we can have a full discussion over in that sub-forum.

Again, WELCOME!!!

Tom M
Thank you for making me feel so welcome!! And also for taking time to discuss my lens correction question. It would be super nice if you wanna move it into a discussion. Cant wait to learn more. Many thanks!

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Hanne

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I have photographed the images with a canon 5D with a 36x24 format (like the 35 mm film), now that I crop the images to 10x8 format the difference is quite big. What I see in the lens correction is that when I choose it as Mamiya (which can have a 6x7 format) the image seem to become more square. So my question is if this is a better selection for my images since they will be cut almost like they have been originally shot with a Mamiya? Does anybody have experience with this?

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Tom Mann

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Hi Hanne - The purpose of the lens distortion correction tool is to correct for the distortion introduced by a particular lens, be it barrel distortion, pin cushion distortion, or more complex distortions such as "mustache" distortion. For example, if you use a lens that exhibits barrel distortion to take a perfectly centered and squared up photo of a rectangle, the image recorded by the camera will come out looking a bit like a barrel. The purpose of the lens distortion tool is to move pixels around slightly to turn the barrel shape back into a rectangle. In other words, it counteracts the original distortion (eg, barrel) by introducing a different distortion (eg, pincushion) that is the mathematical "opposite" (aka, inverse transformation) of the original.

For this process to work properly, (a) you must use the preset for that particular lens; and (b) the correction *must* be applied to the entire frame, not a cropped portion of the frame. If you need to crop the image you ALWAYS do it after the lens distortion correction has taken place.

In fact, if you are referring to the lens distortion correction tool that is in ACR, you will notice that the program reads the EXIF data in your image, and if it determines that you have already edited the image (eg, cropped it), the lens distortion correction tool will be grayed out, so there is no easy way to, for example, apply a Mayima lens correction to an cropped version of the image taken by the same lens, and certainly not to an image taken using a completely different lens, like your Canon glass.

That being said, there are other distortion correction tools besides the one in ACR, and it's easy to force some of these to apply the wrong distortion correction -- you simply specify the wrong lens. The problem is that doing this just stacks one distortion on top of the other, so unless this is being done for purely artistic effect (eg, exaggerating existing barrel distortion to make a lens act more like a fisheye), one should never specify the wrong lens.

Note, all of the above pertains to just lens distortion, not the geometric distortions that arise when the plane of the subject is not perpendicular to the optical axis and/or is not centered on the optical axis. These latter distortions will turn rectangles into trapezoids, circles into rounded tear-drop shapes, etc. These have absolutely nothing to do with the use of a lens. These occur even in pin-hole cameras. These "geometric" distortions can be corrected using other tools in PS, NOT the lens distortion correction tools.

HTH,

Tom M
 

Hanne

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Thank you for such a long and well written reply!

There is one more question I would like to ask, if somebody can help.

I just installed cs6 on a macbook pro, but there is a problem. When I open raw files to convert to photoshop I adjust them and they look fine, but once they appear in photoshop they dont look good at all. I have choosen correct color settings and profile in ps, and cannot figure out what is the problem?

Does anybody have experience with this?

Hanne

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Tom Mann

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Hello again, Hanne -

It's good to hear from you again. Welcome back!

We will be glad to try to help you, but we need a lot more information.

Specifically, can you give more detail than your statement, "...once they appear in Photoshop, they don't look good at all..."? In what way don't they look good? Have the colors changed? The brightness or contrast? The resolution or sharpness? Is the image distorted in some way? Etc.etc.

Also, can you please post two files:

(a) a copy of the raw file that you started with, ie, EXACTLY as it came out of the camera; and,

(b) a copy of the Photoshop file (ie, the *.psd file) Immediately after you have converted the raw file, but before you do anything to it in Photoshop. These files may be too large to post here in the forum, so, in that case, please use one of the on-line free file transfer services like Dropbox, yousendit.com, etc. etc.. One of the services that doesn't require the recipient to sign up just to retrieve the file would be best. Once you have done this, just post the URL where we can pick up the file.

We look forward to hearing from you.

Sincerely,

Tom M
 

Hanne

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Hello again,
I took a screen shot of the raw file in ps raw converter and one screen shot of the same image after its been converted to ps as smart object 240 dpi prophoto. I did not make any adjustments to the file/image.


in photoshop copy.jpg

in Raw converter ps copy.jpg

All the best, Hanne


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Tom Mann

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I'm sorry, but screen shots simply are not very useful in a situation like this. If you would like us to help you, please re-read my previous post and respond with the specific items I requested from you in that post, ie, (1) a written description of the problem, and (b) two files, NOT screenshots.

Cheers,

Tom M
 

Hanne

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I am sorry, because I didnt think the images would display straight in the forum, but come as link to download from db.. on my screen the difference is very much evident that there is a gradient color shift happening especially on the blurry area of the photo. I also noticed this particular thing with images of water. My laptop was just calibrated and I had the same issue before. I will try find another way to share the files thank you.

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Tom Mann

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Just to get a rough idea of the magnitude of the effect you are seeing, I quickly (ie, approximately) overlaid the two screen shots and made an animated GIF that compares the two.

To be honest, the differences between the two look fairly small. Is this the effect that you are talking about, or do you see a larger effect on your monitor?

Tom M
 

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Hanne

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Yes I believe so. When you focus in on the center of the image where the constrast is bigger I see it fairly well. But my point is, that the pictures change from the raw converter to photoshop, so whatever I do to fix them before ps changes a bit. And this hasn't happened before. So I am wondering if there is any settings I could have missed?

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Tom Mann

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Will you be sending the files? Without them, there is really no way I can even begin to diagnose color space problems, or, for that matter, try to distinguish color problems embedded in the file data vs problems at your end caused by hardware or hardware specific settings (eg, display device profiles).

Tom M
 

Hanne

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I will, but to where? The dropbox link i posted made them appear directly to the forum wall which wasnt my intention.

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Tom Mann

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I'm not sure what you did, but once you load the two files into Dropbox, it gives you the option to copy the URL of each file to your clipboard. Do so. Then past the contents of your clipboard into an ordinary forum post.

Also, don't forget that (as per post #9 in this thread), I need a raw file (eg, an NEF, CR2, or similar file), as well as a PSD file, NOT screen shots.

T
 

Tom Mann

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Hello, Hanne - Good to hear from you. Unfortunately, I am on vacation about 800 miles from home and my Photoshop computer, so there is no way I can respond until I return next Monday. Perhaps one of the other particiipants in this forum will want to take up this conversation.

See you in several days,

Tom M
 

Hanne

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Hi Tom, thats fine by me. I am also occupied these days and dont need to use ps next week.
Have a good vacation!

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