Jump to content

Narrowband

Photoshop - Curves or Levels?


Recommended Posts

Ive been processing DSO images for a few years now so have some experience doing this with PS but I still cant decide whether to use curves or levels for stretching etc. Many folk seem to discuss using levels rather than curves.

Seems to me I do best with curves for preliminary adjustments when in layers and levels for tidying up the final merged image.

Does anyone have any thoughts on this?

Link to comment
Share on other sites

  • Replies 26
  • Created
  • Last Reply

Total processing newb here but I use both ... I use levels initially to set the left end to foot of histogram , then have a few gradual tweaks with curves and then do the levels again every so often but in RGB to keep colour balance ok.

Link to comment
Share on other sites

25 minutes ago, knobby said:

Total processing newb here but I use both ... I use levels initially to set the left end to foot of histogram , then have a few gradual tweaks with curves and then do the levels again every so often but in RGB to keep colour balance ok.

OK thanks for that. And like you I tend to use curves separately on RGB. I guess the question is, does curves do something fundamentally different to what levels does or vice versa?

 

Link to comment
Share on other sites

1 minute ago, knobby said:

Levels stretch is quite linear but curves can be fine tuned to only adjust certain levels of image.

... I think !

Agreed - thats my impression. But somehow on occasion I seem to get a result with levels that I cant get with curves - maybe a false impression.

Link to comment
Share on other sites

Yes I anchor points and tweak the curve in whatever area I feel needs it.

I've seen some of Doug's videos and really like them - trouble is he sounds so much like Sean Lock I can't take him seriously!

Link to comment
Share on other sites

2 minutes ago, knobby said:

There are tutorials on you tube that are pretty good ' astrobackyard' 

I've watched so many of Trevor's videos I feel like I know him personally! 

Sorry for crashing in. I am still learning all the post-processing stuff so have followed this thread. :thumbsup:

Link to comment
Share on other sites

17 minutes ago, Tommohawk said:

Ive been processing DSO images for a few years now so have some experience doing this with PS but I still cant decide whether to use curves or levels for stretching etc. Many folk seem to discuss using levels rather than curves.

Seems to me I do best with curves for preliminary adjustments when in layers and levels for tidying up the final merged image.

Does anyone have any thoughts on this?

I use both for the initial stretch.  I most often go down an LRGB route, so if I have only RGB data then I will create a synthetic lum layer. 

More details of some of my initial processing steps are below, these techniques are mainly taken from Adam Block, you can purchase his excellent video tutorials online see -  https://www.adamblockphotos.com/store/p7/Dimensions_of_Photoshop_by_Adam_Block.html

For the luminosity layer, I generally go down a non-linear route to get the final lum image I want, typically I do this via a Digital Development Process stretch within CCDstack but you can also do this via curves within PS. Here, it's important to check the pixel values of those parts of the image that you want to be colourised.  If you have any pixel values above 200 you'll find it difficult to obtain colour.  Call this result LUM.

For the RGB layer, I take the final lum image I've created and paste it on top of the RGB layer and change the blending mode of the LUM layer to luminosity.  I then select the RGB layer and stretch this using levels by moving the white point slider to the left. Normally, I do this multiple times rather than attempt one massive stretch.  Again, you have to be careful not to stretch this too much - as a guide if you are processing a galaxy then the central core should be no larger than one of your stars. I normally then perform a PS shadows highlight (non-linear) stretch on this layer to boost the colour.  It's quite common for the RGB result to look horrible but it is the blended Lum + RGB result that you should be looking at whilst you are stretching the RGB layer.  Call the layer representing the RGB layer result RGB linear.

If you want get a little more advanced, then you'll find that you can obtain different colour shades by taking the (unstretched) RGB layer but this time performing a non linear stretch, again I do this via a Digital Development Process, rather than curves. Call the layer representing the RGB layer result RGB non linear. 

I then paste the RGB non-linear result above the RGB linear result and set the blending mode to Screen. So, from a PS layers perspective, you now three layers for each of the three results:

LUM - PS blending mode set to Luminosity

RGB non-linear - PS blending mode set to Screen - this is the RGB result obtained by using curves or, in my case, DDP.

RGB linear - PS blending mode set to normal - this is the RGB result obtained using levels. 

It's instructive to switch the RGB layers on/off in order to determine the shade of colour that you want in a particular part of the image. Having determined this, I normally then generate the appropriate mask for the RGB non-linear layer to maximize the overall effect.  

Alan

Link to comment
Share on other sites

I tend to use Levels for the initial stretch.  I NEVER move the white slider as it blows out the stars, only bring in the grey and black slider towards the histogram for a couple of iterations, and then go into curves, anchor the white and black points and stretch the nebulosity in small iterations. 

Have always done it that way after watching a DVD by Adam Block.

Carole 

Link to comment
Share on other sites

I like to use Levels  to set the balance between the various channels and this has the pleasant side effect of reducing the presence of  light pollution. I also use Levels  in the early stages of stretching but I (perhaps controversially!) do this in multiple small iterations. As I approach the point where the background is 'about right', I use the 3 x 3 Eye-dropper tool  to sample the background aiming for a level between 22 and 25. Once I have achieved the background level that I want, I move on to Curves, locking the black point at my chosen level (22-25) by holding down the Ctrl  key and clicking on the background. I then adjust the Curves to suit.

Link to comment
Share on other sites

Quote

Hi Carole, do you mind me asking what this means? (and how)

There is a small tutorial on my website, but it only shows the anchoring of the black point in the pictures.  So basically, you are in curves, click on the white part of a star, or a very white bit on the image and you will see a small point pop up on the curves straight line.  You need to anchor that so it doesn't stretch white areas when you stretch the nebulosity.  An easy way to do this is to press Alt Gr on the keyboard whilst clicking on the white star core.  Then do the same with the darkest part of your sky background, (again you can use Alt Gr), and this will fix those points so they don't stretch with the nebulosity.   HTH

https://sites.google.com/site/caroleastroimaging/home/processing-in-photoshop

Looks like we are doing similar things Steve. 

Carole 

Link to comment
Share on other sites

4 hours ago, Redscouse said:

I've watched so many of Trevor's videos I feel like I know him personally! 

Sorry for crashing in. I am still learning all the post-processing stuff so have followed this thread. :thumbsup:

Funny thing - in all my firkling around on the net I hadn't seen this website and tutorials!  Looks useful!

4 hours ago, alan4908 said:

If you want get a little more advanced, then you'll find that you can obtain different colour shades by taking the (unstretched) RGB layer but this time performing a non linear stretch, again I do this via a Digital Development Process, rather than curves. Call the layer representing the RGB layer result RGB non linear. 

I then paste the RGB non-linear result above the RGB linear result and set the blending mode to Screen.

I've done Lum layers using both L and synth data, but never tried non-linear stretch or screen mode. Another thing to try! Thanks for the tip! Not sure what Digital Development Process is though?

4 hours ago, carastro said:

I tend to use Levels for the initial stretch.  I NEVER move the white slider as it blows out the stars, only bring in the grey and black slider towards the histogram for a couple of iterations, and then go into curves, anchor the white and black points and stretch the nebulosity in small iterations. 

Have always done it that way after watching a DVD by Adam Block.

Carole 

Thanks Carole and for tyour subsequent post. I use anchors, but don't do this in such a specific way - I just anchor where it seems about right. I've reprogrammed my ALT GR key to be the CTRL key because that key failed and in n years of PC use I've never used / needed the ALT GR key once! Couldnt see how to anchor the point from the image without it. Do I use the dropper tool?

4 hours ago, knobby said:

And remember to crop the black edges caused by stacking as this effects the levels.

Thats a good point - never thought of that!

4 hours ago, steppenwolf said:

I like to use Levels  to set the balance between the various channels and this has the pleasant side effect of reducing the presence of  light pollution. I also use Levels  in the early stages of stretching but I (perhaps controversially!) do this in multiple small iterations. As I approach the point where the background is 'about right', I use the 3 x 3 Eye-dropper tool  to sample the background aiming for a level between 22 and 25. Once I have achieved the background level that I want, I move on to Curves, locking the black point at my chosen level (22-25) by holding down the Ctrl  key and clicking on the background. I then adjust the Curves to suit.

So its looking like most folk do levels then curves. And I'm doing the opposite - no surprise there!

So the summary seem to be that levels does a linear stretch, where curves does a non linear stretch?

Link to comment
Share on other sites

Quote

Couldnt see how to anchor the point from the image without it. Do I use the dropper tool?

If you can't use Alt Gr, then just watch where the point pops up on the diagonal line in curves and then click at that position and it will place an anchor right there.  That's what I used to do before I was shown how to use Alt Gr.

Carole 

Link to comment
Share on other sites

1 hour ago, carastro said:

If you can't use Alt Gr, then just watch where the point pops up on the diagonal line in curves and then click at that position and it will place an anchor right there.  That's what I used to do before I was shown how to use Alt Gr.

Carole 

OK Carole that works when doing curves from top menu as Image/Adjustment/Curves (in fact it seems to work with CTRL (rather than ALT GR))  - but cant make it work when doing curves as a layer. Is that possible?

Link to comment
Share on other sites

3 hours ago, Tommohawk said:

Not sure what Digital Development Process is though?

It's a type of non-linear stretch that was invented to simulate photographic development in digital images. It's a popular technique that tends to be included in quite a few software processing packages.

Alan 

Link to comment
Share on other sites

1 minute ago, alan4908 said:

It's a type of non-linear stretch that was invented to simulate photographic development in digital images. It's a popular technique that tends to be included in quite a few software processing packages.

Alan 

OK thanks Alan - but can you do it in PS?

Link to comment
Share on other sites

Quote

but cant make it work when doing curves as a layer. Is that possible?

Seems to work on my CS3.  You were definitely working on the correct layer when you tried?  If bottom layer did you have the top layer switched off?

Carole 

Link to comment
Share on other sites

15 minutes ago, Tommohawk said:

OK thanks Alan - but can you do it in PS?

Well, you can approximate the effect and you could develop PS actions which attempt to mimic the process - for example, see http://darkhorizons.emissionline.com/PSDDP.htm Personally, if you want to explore this option then I'd suggest you trial a software program that supports this. I use CCDstack - http://www.ccdware.com/products/ccdstack/ but it's also available in Astro Pixel Processor, Pixinsight etc.

Alan

Link to comment
Share on other sites

I recently wrote up my approach(es) to colour preserving stretches in Photoshop here:

http://www.markshelley.co.uk/Astronomy/Processing/Colour_Preserving_Stretch/colour_preserving_stretch.html

You might find it useful, you might not.  Typically they are applied after performing background subtraction.  You never want to be subtracting background from data that have been stretched in a non-linear manner.

Mark

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.