Curve Adjustments

[T Paul]

The PhotoShop Curves command is an extremely handy tool.

Get to know it. You can do so much with it.

When you open an image, it might not be perfect. The Curves command is an excellent way to correct color and exposure, improve tone and revive details.

Curves is probably the most powerful, flexible color adjusting tool around. It allows the user to map any brightness value in an image to absolutely any pixel brightness value. Try it, learn more about it, use it — you'll love it! Curves may seem complicated, but don’t be shy--the only way to really learn is to try.

In short, the curves command is used to amend the range of tones in an image. When the dialogue box opens the tones are displayed as a straight line. Clicking on the line and dragging the cursor changes the line into a curve and tranforms the tones when the OK button is pressed.

As well as modifying the RGB image in one go, each separate color (i.e. red, green and blue for an RGB image) can be adjusted on its own by selecting it from the channel pull down menu at the top of the dialogue box. The best way to discover how tones are modified using the curves command is to experiment with it.

Below are some excellent links to articles/tutorials on the Curves Command

Becoming Familiar With The Curves Dialog Box

Introduction to the Photoshop Curves Command

Essentials of Photoshop Color Correction: Applying RGB Curves

Control Your Density - Adjust Curves

There are many different techniques for using curves, got a particular method you like to use? Got a powerful BEFORE and AFTER example showing the results of using curves? Please share them.

[byRo]

Curves - when to...

T Paul - you've got some interesting threads here. This site has so much , it took me till now to find you here.

OK, here's an opinion on curves..

[QUOTE=T Paul]
Curves is probably the most powerful, flexible color adjusting tool around. [QUOTE]

No doubt about that but...

...it is not user-friendly and not intuitive.
(In other words, everbody loves a slider, and curves doesn't have one)

IMHO curves are best used when you already know just what you want to do. If you want to 'twiddle' things until the image looks right there's usually another way....

Taking an example form the third link in the post (the canoe) - after two pages os Levels measurement, they set up some curves to do the exactly same thing and then put in only one point on the curve. IMHO (again) a waste of time.
Just use Levels, set up the white/black points and then 'twiddle' the middle triangle in each channel until pleased with the result. Quick and user-friendly.

Don't get me wrong - I use a lot of curves when doing 'by-the-numbers' adjustments. But for 'by-the-eye' adjustments I prefer some sliders.

Roland

[Duv]

Hi Roland

I definetly hear what you're saying but if you use Curves often enough, it becomes quite intuitive and fast to use. As you say, I sometimes start with Levels but almost always tweak with Curves because I want the absolute best from my pics.

Cheers
Dave

[T Paul]

I agree that Levels adjustments are much less intimidating than Curves. However, the Curves adjustments really are a much more powerful tool once you get use to using them. The key is experimenting with Curves and basically just getting comfortable with it. I really enjoy the precision and accuracy Curves give you. Basically Curves can do everything Levels can do and a whole lot more!

As for those "Sliders" in Levels...well Curves has them too. Just click on the graph to add an anchor and slide the point any way you want (smile).

When working with Curves I typically add an anchor to the middle (midtones) and raise it slightly brightening the photo. Next I add an anchor along the diagonal half way between the middle anchor and the top. I move this anchor up or down to adjust the highlights as necessary. Then I add an anchor along the diagonal line half way between the middle anchor and the bottom and adjust as necessary. This is basically the same as adjusting the white, black and middle points in levels. However, instead of being limited to dark, mid and light tones, I can adjust one or more points of my own choice along the tonal scale. And I can distort the tonal line (curve) to my heart's content and bring back life to a drab photo.

So start off with levels, and then when you are comfortable start experimenting with Curves...don't be afraid. You will soon fall in love with the precise control that you have when editing your images!

[byRo]

Curves - When to (II)

Hi T Paul and Dave ,

I was just trying to point out that the canoe example is misleading as it gets no benefit at all from using curves as opposed to a more straightforward Levels adjustment.

Actually, I am no stranger to curves, but for those who are I thought best to chip in with a "when to..." opinion

I was thinking more like Dave: I do use Levels first and then fine tune with Curves. In fact I often end up using some preset "inverted" curves (skin-tone to gray) when I need precise flesh tone adjustment.

cheers

Roland

[Duv]

By the way, the canoe example is from (in my opinion) the best book out there on color correction-Photoshop Color Correction by Michael Kieran. It's mostly directed to serious color correction enthusiasts and retouch pros and probably as important to have as Eismann.

Roland, I value your comments and opinions but I really doubt if you work thru the canoe example that you can attain what he has achieved with Curves and finely tune quarter, half and three quarter tone values. By the way, you should notice that there are 2 points on the curve. In any case, I sometimes wish that Levels was eliminated altogether, thereby forcing everyone to use Curves and balance by the numbers.

Cheers
Dave

[Ed_L]

This is a good thread. For those using levels for the first time on a washed out photo, you probably found out the origin of "WOW"! But using curves is not difficult, and I think the only reason people shy away from curves is that they seem to think it's too technical or something. Actually, using curves for tonal corrections is pretty easy, and you can do so much more with it. This very well might be the origin of "DOUBLE WOW"!! Although I use both levels and curves, I use curves much more than levels. If you haven't used curves, do yourself a favor. Learn the basics of curves. You won't regret it.

Ed

[T Paul]

If Levels are "Wow!"...then Curves are "Double Wow!"

I like that desciption of Curves Ed!

[byRo]

Canoe example

Hi Ed-L, welcome to the discussion.

Ok, let´s get into it......
First let me repeat. I do use curves, quite a lot. But to say they´re the only way to go is like shutting your eyes to a lot of good things.

Now let´s hammer out the example:
I´ll admit I paid more attention to the figure (fig.3) than the text. When I read the text things got worse..

Figure 3 shows a mapping R (0,0 ; 155,255 ; mid-point) , G (0,0 ; 163,255 ; mid-point) and B (0,0 ; 164,255 ; mid-point) - which doesn´t agree with the text. If it had shown values of R (7,10 ; 155,245), G (5,10 ; 163,245) and B (4,10 ; 164,255) as promised, I´d be watching the soccer game and not writing here and (as I said earlier), is a levels adjustment and not curves.

If we read page three, third paragraph..

The measured highlight value in the red channel was 155 (as shown in Sampler #1); it needs to be remapped to the highlight aimpoint of 245, so enter 231 in the Input field and 245 in the output field.

...where did the 231 in the input field come from?
... what is the text for figure 3, talking about?

In other words...
It takes less than five minutes to do a first stage adjustment with levels (identifying whitest/blackest points with Threshold, adjusting white/black and mid with Levels) and (even as an experienced user of PS curves) I just can´t see where the article shows some advantage.

Duv - can´t work through it. It doesn´t seem to make much sense (231?). Anyway, the way he wrote it: moving the 0,0 and 255,255 points to some place else is not a curve (it´s a Level). Now if he had left in those points and added the adjustment points - that would be a curve.

I love this site and admire and respect all of you guys and girls.
I´m discussing this because if we want to help people to get to use curves we need a solid example, and IMHO this one is shakey.

Roland

[T Paul]

First off let me say that Levels adjustments are fantastic and I use them all the time to quickly bring back washed out photos. The purpose of this thread was to bring some light to the wonderful world of Curves (a very imtimidating tool for beginners) and start a discussion on ways to use them.

Roland you have brought up some good points and I will search the web and see if I can find some stronger examples. Also feel free to add your own tips or links.

THANKS

~T

[Ed_L]

Hi Ed-L, welcome to the discussion.

Ok, let´s get into it......
First let me repeat. I do use curves, quite a lot. But to say they´re the only way to go is like shutting your eyes to a lot of good things.

Roland

Absolutely! I totally agree with that. The beauty of Photoshop is that there is usually (always?) more than one way to accomplish something. I wrote a tutorial on very basic curves some time ago. There are still things I don't understand about the power of curves, but if you look at the first image in the tutorial, I think you'll agree that you would have a hard time accomplishing the correction while using levels. To me, this is worth learning the basics of curves.

Ed

[Duv]

Hi Roland

I thought the on line tutorial was verbatim from the book. It is not. There are errors. You've spotted one, the input value should be 155 not 231. There may be other errors so I would recommend the book to get the correct info because I know it's maddening to work thru something that isn't making sense. Keep in mind too that this example is but a small portion in his chapter on understanding how to correct Highlite and Shadow detail.

Cheers
Dave

[byRo]

I thought the on line tutorial was verbatim from the book. It is not. There are errors.

OK, that's a relief. I was starting to think it was 'just me'.

.....but if you look at the first image in the tutorial, I think you'll agree that you would have a hard time accomplishing the correction while using levels.

That's a neat job on the first image. That sure isn't a simple Level adjust.
BTW, that image - with a distinct left/right difference - gave me an idea of a simple example of doing something with curves that wouldn't be better done another way.

Take the second image (the boy), tear it in half(!) and put half original and half the adjusted image. Then use curves, fixing in/out on adjacent areas, to adjust the original half to agree with the adjusted half.
Thinking again, it's like doing one of those 'panoramic' collages...

[Rob S.]

Anybody here ever study B&W photography? Curves are exactly what we learned back then when we were processing film, except our curves had a toe (shadows) and a shoulder (highlights) and weren't depicted as straight lines.
In PS, I've found that curves give you that extra edge in image control that Levels just can't match.

[byRo]

The beauty of Photoshop is that there is usually (always?) more than one way to accomplish something.

That's probably the key to the whole thread here:

Using curves you can do, with just one tool, operations equivalent to a whole list of other tools. Here's a (quick attempt at a) list....

• Brighten
• Contrast
• Levels
• Invert
• Threshold
• Posterize (*)
• Color Balance (*)

(*) Not 100% about this

Here's some pro's and con's of using curves to execute these operations:
Pro
- You can combine number of these tools (or all of them) in just one curve adjustment;
- After applying the basic tool you can tweak it without having to make a new layer;
Con
- Using the original tool you have a user interface optimized for that tool;
- Some tools have 'hidden functions' that won't be available in curves. Example: Levels white/black point adjustment with <alt> or putting in eye-drop markers while in Threshold.

On the other hand, there are some things that can only(?) be done with curves, such as:
- Precise input/output mapping;
- Non-linear and/or discontinuous level adjustments.

My opinion....

- Don't use curves just to mimmick a basic operation;
- Do use curves for detailed fine adjustment;
- Inbetween things.. weigh the pro's and con's.

Comments, very welcome

Roland