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Author Topic: How To Remove Vignetting And Gradients Using Photoshop  (Read 2289 times)

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Offline stealthtech

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How To Remove Vignetting And Gradients Using Photoshop
« on: 19:11:21, 04 November, 2011 »
Hi All,

I'm just starting out into the world of DSO imaging, and recently learned a good technique for removing vignetting and gradients using Photoshop. Obviously, it's better if you can eliminate these at the point of capture, but if they do appear in your images removal is fairly easily with a few simple steps. I posted this technique, recently, in reply to a post by Andy (G4THI) and have used his same image for the pictures here - hope you don't mind, Andy. It was suggested posting the technique here may also benefit others in the future, so with that in mind here's a brief description of the process, followed by 4 screen shots of the steps. My version of Photoshop is CS4, and hopefully other versions should be a similar layout to this. Remember, each image is different so use the settings here as a starting point and experiment to find ones that suit your image.

Open the image in PS, on the top bar select Image > Duplicate and title this 'copy' or 'blur', something to distinguish it from the original, as in pic 1.

Select the copy image and then, on the top bar select Filter > Noise > Dust & Scratches. This will open the dialogue box you see in pic 2. In the dialogue box set the Radius to something like 30 - 50  Pixels (try a few settings) and click 'ok'. This will really blur the image, as in pic 2. Notice the blue halos from the stars are still visible. Next step is to tone the blueness down a bit.

In the left hand column select the healing brush tool and select a medium brush size, to cover more area.....something like 34 (in the top section under the word 'Edit'. 'Alt' 'click' an area of the brown muddy colour and begin to smooth out the blueness little and often. If you make a mistake just undo it by clicking 'Edit' then 'Step Backwards'. Try and get rid of the blueness so it just leaves the muddy central part in the background and the vignetting around the edge, as in pic 3. Next step is to subtract this from the original image to get rid of of the vignetting.

In step 4 the first thing to do is to now click on the original image, so you adjust the correct picture. Once done select Image > Apply Image and this will open another dialogue box as in pic 4. It's import to get these settings correct. The Target Image should read as the original (M45_1) and the source should read as the adjusted version (M45 copy). In the dialogue box set the 'Blending' drop down menu to 'Subtract' and this removes the muddiness from the image and darkens the background more evenly. Only thing left now is on the bottom right of the box you will see 'Offset'. It may be set to zero, but increasing this number will brighten the image. This is personal taste, but you can see on this image I have set it to 30, as in pic 4. Try a few settings to see for yourself.

If you download the images it's good to flick between 1 and 4 to see the difference before and after. Here's the pics, anyway. Good luck, and I hope this proves helpful to a few of you  :D!!!

Regards Martyn




Martyn
SW200P (f/5) Newt, ST80, WO ZS66, ED80 Pro, Skymax 150 Pro,
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Offline PaulB

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Re: How To Remove Vignetting And Gradients Using Photoshop
« Reply #1 on: 21:05:20, 04 November, 2011 »
Cheers Martyn,

Have some K
Paul.
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Offline Carole

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Re: How To Remove Vignetting And Gradients Using Photoshop
« Reply #2 on: 10:10:50, 05 November, 2011 »
Hi Martyn, I have tried this method on a friend's image and I just cannot seem to get the source box to come up with anything but a previous named layer I used in a prevous attempt, the drop down box does not give me the current named source layer as an option.

Does any-one have any thoughts on this.

Carole
Mount: HEQ5,  Telescopes: SW ED120 Pro and SW Evostar ED80 pro, WO ZS71 APO,  SW130PDS
Cameras: Modified Canon EOS 1100D, Atik 460EX, Atik314, DMK21, DBK21, Dual rig options. Observatory: SkyPOD & PZT
https://sites.google.com/site/caroleastroimaging/home

Offline stealthtech

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Re: How To Remove Vignetting And Gradients Using Photoshop
« Reply #3 on: 16:33:38, 05 November, 2011 »
Hi Carole,

Bit strange this, not sure if I can be much help, but.....

For practical purposes, and because I'd worked on it before, I just downloaded the original image again and opened it in PS. Made a copy with todays date (so now 2 images in PS), and then opened 2 more images (that I still had saved on the laptop) of edits I made in the original post to Andy. In summary, 2 images open in PS from today, and 2 more previously edited images from last Saturday (see image below)

With the 'Apply Image' dialogue box opened up, the 'Source' drop down file shows the 4 images currently open in PS, and I can select any of them to use. It appears that the 'Source' image has to be from an image currently open in PS (at least that's what I'm experiencing).

I'm not sure whats happened. The only thing I can suggest is to start from scratch and open the image to be worked on, then make a copy of that and see if that cures the problem. Not much help, but I can't seem to replicate the problem you're having.

In the pic below, you can see 4 images open in PS (the first 2 on the left are new and the other 2 are old edits) and the drop down box shows all of them as selectable.

Sorry I can't be more help, Carole.
Martyn
SW200P (f/5) Newt, ST80, WO ZS66, ED80 Pro, Skymax 150 Pro,
SPC900NC, QHY5V Mono, QHY8 OSC, EOS 500D (modded)
NEQ6 Pro, EQMOD
An infinitely expanding box of accessories
Roll off roof obsy with warm room

Offline Carole

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Re: How To Remove Vignetting And Gradients Using Photoshop
« Reply #4 on: 16:57:35, 05 November, 2011 »
I'll re-check this.
Carole

Mount: HEQ5,  Telescopes: SW ED120 Pro and SW Evostar ED80 pro, WO ZS71 APO,  SW130PDS
Cameras: Modified Canon EOS 1100D, Atik 460EX, Atik314, DMK21, DBK21, Dual rig options. Observatory: SkyPOD & PZT
https://sites.google.com/site/caroleastroimaging/home

 

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