Ginger asked me about LAB curves in a private message. This seemed like a good opportunity for a tutorial on the subject, and Ginger'schallenge candidate seems like a good example image. So in keeping with Ginger and my well established sadomasochistic relationship, here goes.

I'll cut right to the chase. Here is the Ginger's image, very close (exactly?) to what came out of the camera:



And here is my version after only a single adjustment:



I love this image (for reasons that I have tried to make clear on the challenge critique thread). I didn't really want to change it, only intensify it what is essentially already there. I think the sky and the reflections of the sky are the subject of this picture. So I want to add to their drama. If a master oil painter like Turner had painted this, he would have allowed himself a little artistic freedom to strengthen the colors in the sky and I'd like to do the same. I love the sumptious detail in the sky. I don't want to loose this, in fact, I'd like to enhance it and make it stand out if possible. One of the really nice things about this image is that there is no blow out of the brightest part of the sky. The detail is there and I'd be doing Ginger a great diservice if I lost it. By contrast, the other elements of the image (carrier, bridge, swamp), work well as negative space. I want them as dark as possible; they don't have any interesting detail anyway. For these parts of the picture, black is best.

I think I have achieved my goals pretty well. Fine. Now comes the entertaining part.

The single adjustment I made was LAB curves. The curves adjustment is the all powerful global color/brightness/contrast tool. Anything you can do with levels, hue/saturation, color balance, brightness/contrast, you can also do with curves and more. Because it is so powerful and so basic, it may be unintuitive at first, but it is well worth learning how to use curves. I learned to use them by reading Dan Margulis' book Professional Photoshop. This is a great book, but sort of a life study. I'll bet there are more gentle introductions to the subject. Anyone have any suggestions?

No matter what you read, you have to use curves in order for it to become natural. Playing and beating your head against it is the only way this is going to happen. It's worth it. So I'm going to assume that you have a basic understanding of curves, but not any real experience with it.

LAB is another story. Dan's book is also great on the topic, and there is an article of his online. But I'm going to give you my take one the subject as well.

LAB is a colorspace like RGB or CMYK, but less intuitive than either. It has three channels:

1. L - Luminosity or lightness. This is a pure brightness channel. The L channel is exactly what you get if you use Mode->Grey Scale to convert to B&W. Changes in the LAB channel do not effect hue and saturation at all, only brightness.
2. A - Green<->Magenta
3. B - Blue<->Yellow

The A and B channel scales may seem arbitrary, but in fact they reflect some rather deep color theory. Green and magenta are "opponent colors" (and the same goes for blue and yellow.) This means that magenta pigment does not reflect green light. To the degree there is magenta, there can be no green (and similarly, where there is yellow there can be no blue.) At the zero point of the A scale, there is neither green nor magenta. At the zero point on the B scale, no blue or yellow. Thus if both A and B are neutral, the point is also neutral, white, black or grey. LAB is a colorspace because knowing the values for each of L, A, and B uniquely determines a color. End of color theory. In practice, LAB has a huge gamut, too huge, really. Not only can it represent unprintable colors and colors no monitor can reproduce, it can also represent impossible colors that nobody (not even a dog) can see. Dan says this makes it an academic colorspace. I say it makes LAB curves very sensitive. Small moves generate huge results.

Here is exactly what I did to Ginger's image.

1. Convert to LAB (Image->Mode->LAB)
2. Apply the following curves (Image->Adjustments->Curves)
3. Convert back to RGB and save.

Here are the curves I used.







The curves I used for the A and B channels are very simple; I just moved the endpoints inward an equal amount in order to steepen the curves. Steepening the A curve makes greens more green, and magentas more magenta. This is ofen great for vegetation. I used this to make the leaves of the willow tree in the picture of the sculpture in the tree look more alive. Steepening the B cuve makes blues more blue and yellows more yellow. This had a dramatic impact on Ginger's sky.

Notice that I have moved the endpoints inward symmetrically and so have kept midpoint of the curve on the center point of both scales. This means that I have not changed the color balance of the shot. Neutral is still neutral. In this case, I didn't want to change it, just enhance what was alrady there. But you can remove a cast by moving the end ponts of the A and B curves unequally. I often set a couple of color samplers on points in the image that I want to be neutral and then move the endpoints until they have 0 values in both the A and B channels (and are thus neutral). This sometimes entails flattening instead of steepening the curves.

Fine, onto the L channel. This is where the real fun lies in this image. The subtle detail in the lightest part of the sky in the center of the image falls on the bottom left of the curve. Steepening that part of the curve enhances this detail. But I don't what the image to end up too dark. So I have decreased this slope a bit when as it reaches the midtones. It is still pretty steep beause I have moved the rightmost endpoint inward. This intentionally makes the dark parts of the images completely dark. Anything originally to the right of this endpoint is now black. The effect is to focus the entire availble gamut on the sky and reflections, spreading them through the avaible range of colors.

Afer LAB curves, I usually use USM. Sharpening the L channel only is a great trick because by definition it produces no color halos. Dan has the example of sharpening a glass of soda water with little bubbles. It's really bad to introduce any color into that image, and L sharpening does a great job as opposed to more standard sharpening techniques. But Ginger's image doesn't want to be sharpened. The non-sky elements are black and their silhouettes will be less strong and more distracting with sharpening halos. The sky isn't going to benefit from sharpening. The color gradations are all too gradual. This sky was painted by Turner, not El Greco.

There is a lot of artistic freedom in the process I have just outlined. Using my L curve but not changing the A and B curves would have kept the original more neutral sky tones, but brought up the detail in the sky and blackened the other elements. Moving the endpoints of the A and B curves unequally or introducing new midpoints in those curves can make huge differences. For example, moving the bottom left endpoint of the B curve in more would warm up the sky by making it more yellow. Go ahead, Ginger and play with it. It's very theraputic.

As Andy would say:

Enjoy (LAB curve) postprocessing.

Rutt, I have been following along with your discussion. I did this one with your LAB technique.

Originally Posted by spockling Rutt, I have been following along with your discussion. I did this one with your LAB technique.

This was the original

Ginger, Thanks for the response. I can understand Rutt's feelings about frames.


Here's how I did it.
Cropped photo
Select: All
Edit : Copy
File : New Make sure you have background color(black and white squares in
tool palette) selected to what color you want.
then size bigger than copied picture you are going to insert.
Paste
Crop giving lots of space( your final crop still to come after)
Layer -> Layer Style-> Drop Shadow (play with parameters to see what you
like, default is okay for now)
Crop to final dimensions keeping sides even.
Layer: Flatten or Merge
File: Save as
I am only framing the final picture, not all of them.

S

Rutt,

Great info. I was aware of LAB sharpening from Scott Kelby's book, but had not yet used it for color work. Looking forward to playing with it.

I also want to note that using the lightness channel is a great way to do a b/w conversion.

Thanks Rutt, interesting and very effective, I didn't even know this existed, I think you mentioned it when you did some work on my swans but I only had elements at the time so didn't understand what you were doing.

Thanks again

Now that I've let the LAB genie out of the box, it's time for some fine tuning.

Ginger's most recent versions still are losing detail in the sky highlights, instead of enhancing the detail that is there. I assume this is not her intention and I am going to focus on this issue first.

Here is a crop of just this section of the sky from the original:



Here is Ginger's most recent version:



Here is a new L curve that I wrote this morning and the cloud crop that results from applying just this curve to the original image:





Can you see the difference in detail and contrast? In my most recent effort, I've recovered a lot of information that was there but hard to see in the original. Ginger has done the opposite. Of course, Ginger probably also changed the A&B curves, but let's try to ignore that for now.

Before I go into much greater detail, there is one issue I'd like to clear up. Photoshop can present curves in one of two ways, either with the highlights on left and bottom or on the right and top. Clicking on the two little arrows in the middle of the horizontal axis flips back and forth. I find it easier to work from light to dark and so I have light on the left and dark on the right. This is a question of taste, but I'm going to assume you have it set up the same way as I do. If you don't and you don't understand what's going on, you won't be able to reproduce my results or even make sense of what I have to say.

There is a very basic rule of color correction. I'll credit it to Dan Margulis, but perhaps it's much older. Still he said it prefectly and often:

Dan's Rule: The steeper the curve the more the contrast

In fact this is the title of chapter 3, a must-ready chapter, of Dan's must-read book, "Professional Photoshop". In this case, the colors in the very bright sky highlight are, well very bright. They thus live on the extreme lower left (highlight) side of the L curve. You can see this directly holding down the mouse button button during a curves correction (on windows, it's left-mouse.) You will see a dot on the curve to show where exactly that point falls. Command-click (control-click on windows) will set a point on the curve that you can then move around. So, because I want to enhance the contrast in this particular section of the curve, I've made it very steep in the extreme lower left in order to bring out the highlight detail and this has worked.

So fine, I managed to bring up the contrast part of the sky. This was important because it is a large bright spot right in the center of the image. If it is just blank, the image is diminshed. But now what? If the curve continues this steeply through the midtones, I'm going to end up with a very dark picture.



I've turned daytime into nighttime. That's an OK decision for Ginger to make, but not for me in my role as her retoucher. Besides, we know it's not nighttime, becaue of the sun. Anyway I want daytime. In order to do this, I have to make the curve less steep through the midtones. DM has an analogy for this. I have a contrast budget imposed on my by the limitations of the output devices (monitors, printers, &etc) and ultimately by the human eye. So I have to spend this budget wisely. I just spent a big hunk of change on the bright spot in the middle. But now I have to save up a little. I've flattened the curve a bit through the light midtones. This is here the blue sections of clouds live. They don't have a lot of detail in the first place, and so the curve can be a little more shallow here.

But really the whole middle of the curve is very steep. That's because I stole from the shadows by moving the right end point in so far. This has the effect of making things black that were just pretty dark, like the works of man and the swamp. This is fine, we want those in silhouette anyway.

[Exercise for the reader. Start out with an unaltered image in LAB and move the end points of the L curve torward the center one at a time. What happens to the highlights as you move the left point inward? What happnes to the shadows as you move the right point inward? Why is this a direct result of Dan's law?]

Here is the whole image after just applying my L curve.



I haven't touched the A and B curves at all yet. The curve could have been flatter through the midtones to get lighter sky, but it would haved paid for it in detail. All in all, I think I spent my contrast budget wisely. There is some room for artistic freedom in this part of the exercise, but less than you might think. I made the curve steep where there was detail in the image and flatter were there was no interesting detail and dead flat on the upper right were I wanted to obliterate the shadow detal altogether.

Now I am ready to start having fun. There is a lot of room for artistic expression with the A+B curves. When I saw Ginger's first LAB attempts, I guessed that she wanted more red than yellow in the sky and a softer look than I got with my straight steep A and B curves. More Constable than Turner, perhaps? And I still want a lot of blue in the clouds. So I started to deviate from the straight curves of my original post, just as an illustration of what can be done. And I got this:



I used the L curve from the start of the post and these A and B curves:





OK there is one more thing I want to tell you about curves. You can load and save them. So if you want to play with my curves, for example, you don't have to try to duplicate them from the images I've posted. On the right hand of the curves box there are load and save buttons. Saving curves is often a great idea because you never know when you are done with an image. If you save them, they to to a .acv file and you can use the load button to get them back. I've uploaded this set of curves onto my server here, so, Ginger, you should be able to duplicate my result exactly. LOL.

John - I have one question about LAB curves that I do not completely grasp yet -- I understand how to steepen or flatten the a and b curves and how this will affect green-magenta or blue-yellow color realtionships - but I cannot seem to figure out how to watch the changes in color as they occur real time on my computer screen.


When I go to LAB color and select the "a" channel and then to the curves command - I see a black and white image on my screen - I cannot seem to see the colored image real time as I adjust the curves like I can when I work in RGB space. So I adjust the curve and then switch back to the colored image and then again go to the "b" channel and see the black and white image again as I adjust the "b" channel curves. Is there a way around this dilemma? Or am I just doing it all wrong? Is there a way to adjust the a or b channels with curves or levels and see the results in color as I make the adjustments. The same question can be asked about the lightness channel as well. I guess I am more comfortable in RGB because I can see the changes in color as I make them. Please, sensi, tell me I can have it all in color as I work in LAB space.

Originally Posted by pathfinder John - I have one question about LAB curves that I do not completely grasp yet -- I understand how to steepen or flatten the a and b curves and how this will affect green-magenta or blue-yellow color realtionships - but I cannot seem to figure out how to watch the changes in color as they occur real time on my computer screen. When I go to LAB color and select the "a" channel and then to the curves command - I see a black and white image on my screen - I cannot seem to see the colored image real time as I adjust the curves like I can when I work in RGB space. So I adjust the curve and then switch back to the colored image and then again go to the "b" channel and see the black and white image again as I adjust the "b" channel curves. Is there a way around this dilemma? Or am I just doing it all wrong? Is there a way to adjust the a or b channels with curves or levels and see the results in color as I make the adjustments. The same question can be asked about the lightness channel as well. I guess I am more comfortable in RGB because I can see the changes in color as I make them. Please, sensi, tell me I can have it all in color as I work in LAB space.

Don't select a channel before you bring up the curves. You can select then select which channel to work on from within the curves dialog box and you'll the the changes is real time. This works in all color spaces.

Great tutorial rutt!

Thanks for posting it

Originally Posted by rutt Don't select a channel before you bring up the curves. You can select then select which channel to work on from within the curves dialog box and you'll the the changes is real time. This works in all color spaces.

Duh!!!! I guess I've been doing it the hard way only pulling up the curves dialog box after clicking on the a or b channel!!

Thanks - Now I KNOW I'll be more interested in pursuing things via LAB color. That is the stumbling block that has had me flumoxxed!! Thanks again, John.

"This works in all color spaces" I know this in RGB, but somehow I missed it when I wandered into LAB color. Sometimes the simplest things are so obvious, that they are the hardest!!

Originally Posted by ginger_55 I can't load them. Even with knowing the file name, my photoshop does not recognize the file name. I can't get more detail into the sky, I don't know why, you got a tad more than I did. ginger

Make the L curve very steep in the highlights. Be sure NOT to move the leftmost endpoint inward; that will obliterate the lightest highlights. You could try moving it upward a little, which will darken the lightest highlights, but there really is no reason to waste your contrast budget that way.

You should be able to load my curves. I tested this on both mac and pc, both PS 7 and PS/CS. Try this. Save and load your own curves. Can you do that? If so it's probably only a question of being able to navigate to whereever it was that you saved my curves. They were called pp3.acv. Put them somewhere easy to find and then after you click on the load button, navigate to there. Or you could cheat a little. When you save your curves, give them a distinctive name like 'cantfindme.acv'. Then use the find command to find this file. Move my curves into the same folder. That might make it easier.

and you all know my name and the title of the picture is, as always,

The Storm











which does anyone like?

Ginger, you're still not getting the detail in the highlights. Go back and compare the bright spot to mine. I have two assignments for you:

1. Figure out how to load my curves. Be as stubborn about that as you have been about trying to win the challenges. Get local help if necessary.
2. Start with the original and do a curve adjustment in just the L channel. Forget A&B for now. Can you get a result as good as the one I posted yesterday? It's here if you need to look. Using my L curve is allowed. You need to be able to do this before you start playing with the A&B curves. For the purpose of this assignment, "good" means that there is a lot of detail in the brightest part of the sky. It doesn't mean that you like it more. It's objective, not subjective.

Originally Posted by rutt Ginger, you're still not getting the detail in the highlights. Go back and compare the bright spot to mine. I have two assignments for you: Figure out how to load my curves. Be as stubborn about that as you have been about trying to win the challenges. Get local help if necessary. Start with the original and do a curve adjustment in just the L channel. Forget A&B for now. Can you get a result as good as the one I posted yesterday? It's here if you need to look. Using my L curve is allowed. You need to be able to do this before you start playing with the A&B curves. For the purpose of this assignment, "good" means that there is a lot of detail in the brightest part of the sky. It doesn't mean that you like it more. It's objective, not subjective.

I may not be getting the detail in the highlights, but I did use your curves.

There are a lot of good photographs out there, I did like this, and I do appreciate everything, but I can't do more than use your curves and present it how you like it, naked.

g And I did it twice to make sure I was not getting it wrong somehow.

Originally Posted by ginger_55 I may not be getting the detail in the highlights, but I did use your curves. There are a lot of good photographs out there, I did like this, and I do appreciate everything, but I can't do more than use your curves and present it how you like it, naked. g And I did it twice to make sure I was not getting it wrong somehow.

OK, the mystery now seems to be solved. Ginger edited her post and the middle image was done with my curves and looks like my result. (Ginger, I think it is a better idea to make a new post to correct this kind of thing. It leaves a record that for others who are following along and just won't get what happened. I do edit my posts, but only to correct typos and the like.)

So which one of the three do I like? I like mine. But that is only because of what the L curve did for the highlights. Ginger, if I were you I'd try to use my L curve with your own A+B curves. Or you can use my A+B curves, I'd take that as a compliment, but wouldn't be hurt if you did something different. The job of the retoucher (as opposed to the photographer-retoucher) isn't really artistic at all. I'm trying to help you find the image you want.

Do you think the sky in my version ended up too dark? That would be a reasonable thought. You can muck with my curve by making the L curve less steep in the midtones (but don't change the extreme left side.)

I wrote a new L curve that does just this. It gets the desired effect in the sky highlights, but doesn't really darken the rest of the sky. It does darken the foreground on purpose. Here is what the curve looks like:



Here is the result (notice NO A+B changes at all):



And the curve that did it is here.

I want to see them

I always start over with the same print, the second one, that I always start with. I start with the second one as the first one had to be rotated a bit.
These are crazy IMO, but what I do up in ps is not exactly what I see here, I think someone else had that experience if I remember right. No, I don't have time to calibrate anything. I have to take a shower before tomorrow........etc.








I think I have photographed WW2 readiness in Charleston harbor with the equipment available to me at the time. Above the scene of the day, which was always portrayed in black and white, above that are two photos typical of this period, one of which hung in my grandmother's drawing room.

I wonder which one that was..............

Mary Rogers Jones

Originally Posted by rutt OK, the mystery now seems to be solved. Ginger edited her post and the middle image was done with my curves and looks like my result. (Ginger, I think it is a better idea to make a new post to correct this kind of thing. It leaves a record that for others who are following along and just won't get what happened. I do edit my posts, but only to correct typos and the like.) So which one of the three do I like? I like mine. But that is only because of what the L curve did for the highlights. Ginger, if I were you I'd try to use my L curve with your own A+B curves. Or you can use my A+B curves, I'd take that as a compliment, but wouldn't be hurt if you did something different. The job of the retoucher (as opposed to the photographer-retoucher) isn't really artistic at all. I'm trying to help you find the image you want. Do you think the sky in my version ended up too dark? That would be a reasonable thought. You can muck with my curve by making the L curve less steep in the midtones (but don't change the extreme left side.) I wrote a new L curve that does just this. It gets the desired effect in the sky highlights, but doesn't really darken the rest of the sky. It does darken the foreground on purpose. Here is what the curve looks like: Here is the result (notice NO A+B changes at all): And the curve that did it is here.

__________________________________________________

I think that new one is too blue, but that is just my opinion. I never tried to change the A and B curves, if I could have figured out how to send mine, I would have this afternoon, before I received your curves in my documents.

I think they were the same, but there would have been a slight variation.

On the photos I just did, I have no idea what I did. No clue. I don't know if I started over, I must have, but I tried to make sure that at no point was the sky actually blown. These were the only two color images that were printable. The others did not have enough contrast. I guess that is why I think that one you just gave me the curves for doesn't work, is I don't think it pops.

And I am not trying ever, to win The Challenge, would be amazed if that were to happen, now and in the past. I try to get in the top ten, that is my main goal. It leaves me room to play. In my head, anyway.

I have learned a lot, and I thank you for all of it, including the knowledge that these pages on this site must be printed as landscapes.

And, curves, of course, I am sure I will continue to mess with it.

I am going to chill a moment.

ginger

Originally Posted by ginger_55 __________________________________________________ I think that new one is too blue, but that is just my opinion.

The L curve doesn't change blue, only lightness. It looks bluer because it is lighter. You can make it less blue with the B curve. That's the point:

1. Write an L curve that uses your contrast budget wisely to bring up the detail where you want it and supress it where you don't. Don't think about color at all when you are doing this. Writing the curve to achieve your goals here is scientific and objective instead of artistic and subjective.
2. Then use the A+B curves to make the colors the way you want them. Your goals here as the photographer can be very different from the goals of a retoucher, who is probably only trying to make it look real and "avoid impossible colors", as DM would say.

I'm really not trying to torture you anymore, Ginger. But I want to tutorial to help people to know the right way to do this.

A compromise, I think, between the retoucher and me. I had to frame it to be happy with myself. By looking carefully I see as little totally blown areas in this
workup as are in many photos being considered, some of which I like and some I don't.

I understand that this tutorial is for everyone, so you all make sure you get all of yours true.............

Myself, I went with this, and I thank you Rutt for opening a whole new world for me and the others.

ginger