# Curves



## gerardo2068 (Jan 18, 2012)

Could somebody explain what you can use Curves for? Do you use Curve more than Levels? Do you use a lot of selective Curves? In my own search online I usually find basic explanations about it. The usual is lift the middle to brighten the image, move the sliders in the corners to add contrast, the S-Curves, I find it easier to use level for those type of things. It there a benefit for using Curves instead of level? I have also seen they use the Red, Blue, Green Channels on the curves to make it look like a faded pastel color. What else I should know about Curves? Thank you very much! You advices will be very appreciated. Thanks again


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## gerardo2068 (Jan 19, 2012)

Any suggestions?


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## EchoingWhisper (Jan 19, 2012)

Yes, I do use curves very often. I don't even use levels.


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## gerardo2068 (Jan 19, 2012)

I see


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## Buckster (Jan 19, 2012)

Here's the thing about curves: As you indicated by your many questions, it can do a lot of things.  Because of that, it's a very deep subject that is difficult to address in a single post or two.  An entire class could be written to explain all there is to know about curves.  It can be used to correct and tweak a LOT of stuff.

A lot of folks use it with a simple S curve to pop the contrast a bit globally, or you could go the opposite direction, if you were looking for that effect for some reason.  Those tweaks can also be done in individual color channels.  Or you can lighten or darken globally.  With more knowledge and ability, you can make those kinds of adjustments that are much more specific to areas of the photo, as opposed to global changes.  Combined with masks and multiple curves layers, you can have very fine control over so many different aspects of an image's color and contrast that the mind boggles.

Google and YouTube have thousands of tutorials that cover the many subjects related to how to work with and manipulate curves, from the simple S curve to the more complex stuff.


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## EchoingWhisper (Jan 19, 2012)

Curves is much stronger than levels.


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## kylehess10 (Jan 19, 2012)

I've recently started playing with curves for all of my photos and I do notice a huge difference. You just have to keep playing with them until it looks just right. Some photos need it, others don't. I think it all depends on the look you're going for. 

Here's a couple comparison photos, with my first edit in Lightroom 3 and my secondary edit in Photoshop CS5 with curves


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## gerardo2068 (Jan 19, 2012)

kylehess10 said:
			
		

> I've recently started playing with curves for all of my photos and I do notice a huge difference. You just have to keep playing with them until it looks just right. Some photos need it, others don't. I think it all depends on the look you're going for.
> 
> Here's a couple comparison photos, with my first edit in Lightroom 3 and my secondary edit in Photoshop CS5 with curves



Wow. I'm starting today to watch tutorials today!

Thanks everybody for your time. I can see how deep curved can be. I was thinking it would be like levels or other filters and that it had a somewhat specific type of use. But for what I can see there a lot more to it.


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## Ysarex (Jan 19, 2012)

gerardo2068 said:


> What else I should know about Curves?



In your post you mentioned the RGB channels so I'm going to assume you're referring to the use of Curves and Levels as applied to RGB photos.

I almost never use Curves on my RGB photos.

Here's what else you should know: If you're sitting in front of an RGB photo and considering using Curves you should first ask yourself these questions:

What did I screw up in the RAW converter that's got me thinking about using Curves now? Shouldn't I go back and get it right before making it worse?

If you're not shooting RAW in the first place then you should ask yourself this question: Why am I considering using Curves on a camera-mangled RGB photo when I could have shot RAW and bypassed this problem in the first place?

After you answer those questions and you still need to use Curves that's fine. But if you're sitting in front of an RGB photo that needs a Curves adjustment the odds are that bad things have already happened to that photo to get it into that state and wouldn't it have been better to avoid those bad things in the first place.

Joe


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## gerardo2068 (Jan 19, 2012)

Ysarex said:


> gerardo2068 said:
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May be I express my self wrongly or we are not in the same page, but I find your post confusing (I'm new to curves). What I mean by RGB channels is that in the curves panel you can choose to adjust the red, green or blue color independently. So I was asking why I would want to do that? or in which situation I would want to use that?. I see many video they talk about using the Curves tool to improve the photo or sometimes to create a desire effect, like a faded pastel color (like old style photo) So I also wanted to know what other styles can you create with curves. Like in the example photos of *kylehess10, *The photo of the baseball player was improved a lot by using curves. I went in to youtube and I found a video on curve and it was well explain. Now I think I understand better. I'm not sure what you mean by RGB photo. Thank you for your input!


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## Buckster (Jan 19, 2012)

Ysarex said:


> gerardo2068 said:
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> > What else I should know about Curves?
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As opposed to...?



Ysarex said:


> I almost never use Curves on my RGB photos.


What kind of photos do you use them on?



Ysarex said:


> Here's what else you should know: If you're sitting in front of an RGB photo and considering using Curves you should first ask yourself these questions:
> 
> What did I screw up in the RAW converter that's got me thinking about using Curves now? Shouldn't I go back and get it right before making it worse?
> 
> ...


At this point, I think you're thoroughly confused, but maybe it's me.  If you give a bit more explanation, I'm pretty sure that at least one of us is going to learn something in the process, so please do expound upon your thoughts here.


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## KenC (Jan 19, 2012)

I agree that one should do most of the global curves adjustments in the raw conversion and that large global adjustments after that point might argue for going back and re-doing the conversion.  However, there are always global curves tweaks that need to be done after other adjustments, and also there can be a lot of selective curves adjustment applied only to parts of the image, which really is not possible in raw conversion.


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## gerardo2068 (Jan 19, 2012)

KenC said:
			
		

> I agree that one should do most of the global curves adjustments in the raw conversion and that large global adjustments after that point might argue for going back and re-doing the conversion.  However, there are always global curves tweaks that need to be done after other adjustments, and also there can be a lot of selective curves adjustment applied only to parts of the image, which really is not possible in raw conversion.



If I remember right the curves panel in Lightroom may be a little more limited than the one in photoshop? 
Also what about Curves in LR vs ACR?


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## Ysarex (Jan 19, 2012)

Buckster said:


> Ysarex said:
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As opposed to the RAW capture. ACR does have a Curves tool and I do use it. Goal is to get tone response and color right in the RAW to RGB conversion. If you do the RAW to RGB conversion and then you need to use Curves to adjust the tone response, didn't you miss something?



Buckster said:


> Ysarex said:
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I use Curves to adjust the tone response of my RAW files as part of the process to convert them to RGB. Once they're RGB that job should be done.



Buckster said:


> Ysarex said:
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I'm not confused, but I'll agreee I didn't do too good a job expressing myself.

I know it's common for a lot of photographers to shoot camera RGB photos and then to edit those. So I was saying that's not the way to get the best possible result.

Joe


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## Ysarex (Jan 19, 2012)

KenC said:


> {snip}....and also there can be a lot of selective curves adjustment applied only to parts of the image, which really is not possible in raw conversion.



Basically yes if you live in an Adobe centric world and most of us do (I do use ACR). However what you're describing as local tone adjustments is possible if you use a different converter. Capture 1 is very capable of processing a RAW file via multiple layers and tone and color adjustments applied locally.

Joe


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## Ysarex (Jan 19, 2012)

So my first post was badly expressed. I was being indirect and did a bad job of it.

I'll try again: If you want the best possible photo and your goal is a straight photo (no special effects). Shoot a RAW capture and get it right in the RAW to RGB conversion process. If you do that you'll have the best result and, you won't be grabbing Curves in Photoshop to continue editing the RGB photo.

Joe


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## unpopular (Jan 19, 2012)

curved can do everything levels can, but levels cannot do everything curves can.


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## unpopular (Jan 19, 2012)

Ysarex said:


> So my first post was badly expressed. I was being indirect and did a bad job of it.
> 
> I'll try again: If you want the best possible photo and your goal is a straight photo (no special effects). Shoot a RAW capture and get it right in the RAW to RGB conversion process. If you do that you'll have the best result and, you won't be grabbing Curves in Photoshop to continue editing the RGB photo.
> 
> Joe



I prefer to think of the RAW file as the unprocessed film, the TIF file as the negative and the working file as the print. When I am creating a RAW file, I want to get a good capture with as little noise as possible and as much detail as it can hold. In the RAW processor I am looking to compress that data into a gamma curve which will contain all the information I need and the working file renders that information in a way which makes visual sense.


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## KenC (Jan 19, 2012)

Ysarex said:


> KenC said:
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> > {snip}....and also there can be a lot of selective curves adjustment applied only to parts of the image, which really is not possible in raw conversion.
> ...



Hmmm...  Capture 1 you say?  Thanks for the tip - I'll have to look into that. I wondered if there was a converter that allowed this (neither DPP nor ACR does).  I've never had any banding or other trouble from doing my selective curves or other adjustments in PS, but even so it would be better to do more in the conversion process.


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## KenC (Jan 19, 2012)

gerardo2068 said:


> KenC said:
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No idea about LR - never used it, only Canon DPP and ACR.


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## gerardo2068 (Jan 19, 2012)

I think in LR you only get dark and light only tonal curve, for like contrast. I think


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## Ysarex (Jan 19, 2012)

KenC said:


> Ysarex said:
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Ken,

Phase 1 has a 30 day free trial. Capture 1 is pricey but if it's your bread and butter nothing's better. It's not nearly as sophisticated as Photoshop's layers but you can basically mask off a section of your photo and then apply local changes to both tone and color. And you can stack multiple masked areas. I don't know any other RAW converter with this ability. There's also a fabulous local color editor that's much better than what ACR provides. My one big gripe about C1 is CA correction. They only provide CA correction for lenses in their database and it's an auto only correction -- no manual hand tweaks. If your lens is supported it's good enough but DPP and ACR are better (OEM often is).

Anyway, C1 is good enough that I can usually get the photo done there except for pixel level edits. C1 will also straighten a photo and make adjustments for convergence which is pretty cool. You should take a look.

Joe

P.S. I agree that applying tone/color adjustments in Photoshop especially on 16 bit RGB files is going to be worth doing if needed and I'll certainly do it, but as you say above, "but even so it would be better to do more in the conversion process." That is my original point -- I think you and I completely agree.


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## Ysarex (Jan 19, 2012)

unpopular said:


> curved can do everything levels can, but levels cannot do everything curves can.



Not exactly, at least not in Photoshop. The midpoint slider in Photoshop's Levels works differently than pulling the midpoint of the curve. The difference is subtle but it can matter.

Joe


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## Buckster (Jan 19, 2012)

Ysarex said:


> unpopular said:
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> > curved can do everything levels can, but levels cannot do everything curves can.
> ...


Interesting.  

First, show us how it can matter.  Second, show us that the result made from levels can't be achieved with curves.


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## Buckster (Jan 19, 2012)

Ysarex said:


> Buckster said:
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Sounds like 6 of one, half a dozen of the other to me.  You appear to be talking about workflow processes and what you feel are best, and they don't include much beyond global adjustments in the conversion from RAW.  I would say if that's what works best for you, great, but if others choose a different path than you to get to their end result, that doesn't mean they're doing it "wrong".

A lot of very skilled and knowledgeable people find curves in Photoshop very useful, and they've obviously already converted from RAW at that point.  Seems awfully pretentious of you to say or even infer that they're doing it wrong.


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## unpopular (Jan 19, 2012)

Ysarex said:


> unpopular said:
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> > curved can do everything levels can, but levels cannot do everything curves can.
> ...



No. Gamma is gamma. Multiply the gamma coefficient(?) by 127 and enter this into the output of level 127 (or whatever scale you are using) in curves. The results will be the same. What levels does do is visually represents the midpoint in a meaningful, though a bit esoteric, way.


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## Ysarex (Jan 19, 2012)

Buckster said:


> Ysarex said:
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Happy to.

First lets make sure we know the question: Unpopular (whom I hold in high regard) posted a graphic above implying that the endpoint sliders and midpoint slider in Levels were effectively equivalent to the end nodes and a midpoint node on the Curves line. Close enough for government work.

I noted that in fact pulling the midpoint slider in Levels isn't going to give you quite the same result as pulling a node placed on the center of the Curves line and that the difference can matter. I noted this is Photoshop exclusive as other software may implement other algorithms.

The difference is this: Place a node on the center of the Curves line and pull it up or down and the photo will lighten or darken. Likewise pull the midpoint slider in Levels to the left and the photo will lighten, pull it to the right and the photo will darken. Make a change with the midpoint slider in Levels and any attempt to make a change of the same degree using a node on the center of the Curves line will produce a higher contrast result: that matters. It can matter a lot.

Here's a visual example. I started with this image:







I made a Levels change and pulled the midpoint slider to a value of 1.45. Note the inset histogram:






I went back to the original, brought up Curves and placed a node on the center of the line. I picked a reference point in the photo and matched it between the Levels and Curves versions. It doesn't matter how you try and test this. There is no position to which you can move that node in Curves that will produce as low a contrast image as you were left with making the Levels change. Note the inset histogram and compare it with the one above.






In Photoshop, the midpoint slider in Levels will lighten or darken your photo and give you a lower contrast end result than anything you can get using a center line node in Curves; and it matters because it can really influence what you'll ultimately get as you continue on.

Joe


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## unpopular (Jan 20, 2012)

That is significant, and I am thinking its an interpolation error, either intentional or not. The more I think about it, the more it really does make sense.

Regardless though how inaccurate curves renders gamma, the basic principle is maintained nonetheless.

Still, just for the sake of the experiment, try entering 184 into the output field of level 127 in Photoshop's curves. I will do the same in photoline under different interpolations.


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## Ysarex (Jan 20, 2012)

unpopular said:


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This could end up being one of those cases where we're talking about the same elephant form different perspectives and in fact we agree its an elephant. I didn't say Curves can't replicate the same result you get from Levels. I said you don't have a midpoint to midpoint equivalence. For example in the photo I just posted you can't place a node anywhere near the midpoint of the Curves line and get a result equivalent to pulling the midpoint of the Levels slider.

So I was reacting to the original graphic that drew an equivalence between the Levels midpoint slider and a node mid-line in Curves.

Joe


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## unpopular (Jan 20, 2012)

Well, you have to consider how both tools work, one works using a lookup table, interpolating the values between points, the other uses arithmatic. So it makes sense that they might render a bit differently. Adobe may also interpolate the curve in a way which is more accurate.


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## Ysarex (Jan 20, 2012)

unpopular said:


> That is significant, and I am thinking its an interpolation error, either intentional or not. The more I think about it, the more it really does make sense.
> 
> Regardless though how inaccurate curves renders gamma, the basic principle is maintained nonetheless.
> 
> Still, just for the sake of the experiment, try entering 184 into the output field of level 127 in Photoshop's curves. I will do the same in photoline under different interpolations.



It's late and here I am a couple beers down, but I think I got it. First off I reacted to the graphic and, no offense intended, but I've seen that before -- the line between the Levels midpoint and the Curves midpoint -- I knew from practice they didn't do the same thing. In fact I think they would be the same if the histogram distribution were a theoretical bell curve. That just never happens in practice.

So I've run into photographers before telling me they're the same. Well, no, not if the histogram weights to one side or the other, and it does in the photo I grabbed. This is really a discrepancy between theory and practice. You have to expect Curves could replicate the Levels result. But it's not going to work from the midpoint on the Curves line.

Joe


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## gerardo2068 (Jan 20, 2012)

Good stuff


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## Ysarex (Jan 20, 2012)

unpopular said:


> Well, you have to consider how both tools work, one works using a lookup table, interpolating the values between points, the other uses arithmatic. So it makes sense that they might render a bit differently. Adobe may also interpolate the curve in a way which is more accurate.



Look at these two curves that I applied to the same image of the flower from above. I worked pretty hard to make the curves the same. The visual results are dramatically different. It depends on where you pull the node from. The image in which the node is pulled from the midpoint (left curve) is much higher contrast than the other. The shape of the curve isn't all that matters. By pulling the node from near the shadow end I was able to get a contrast result that approximated the earlier Levels change that I made, although the color saturation remained different.

Joe


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## Ysarex (Jan 20, 2012)

Buckster said:


> Ysarex said:
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I never said "wrong" and I don't want to sound pretentious. I know a lot of people edit either camera produced RGB photos or already converted RGB photos. I'm trying hard not to make it black and white by just saying something like "wrong." I said I want the best possible result. And my experience tells me you get that by getting as much right up front in the RAW conversion process as you can. I'll stick by that.

Joe


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## unpopular (Jan 20, 2012)

I hear you, my tests are indicating the same results. But I am not sure you completely understand what curves are actually doing. In the first example you did not adjust the middle point. I can't see your input value, but you've translated another level, and not level 127. This is kind of an arbitrary adjustment.

Instead of moving the middle point up and down, try moving it left and right:






As you can see, the two are much closer, such that I am guessing that what any slight variation is carelessness on my part. If this is the case, what this indicates is that the gamma operation in levels is being performed on the input value and not the output value - as I initially assumed. Which makes sense, as the levels being adjusted are labeled the "input levels".


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## unpopular (Jan 20, 2012)

Also. I have no idea why I thought simply multiplying the gamma value by the target would work...

I am working on a conversion atm.


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## Ysarex (Jan 20, 2012)

unpopular said:


> I hear you, my tests are indicating the same results. But I am not sure you completely understand what curves are actually doing. In the first example you did not adjust the middle point. I can't see your input value, but you've translated another level, and not level 127. This is kind of an arbitrary adjustment.
> 
> Instead of moving the middle point up and down, try moving it left and right:
> 
> ...



I do understand what Curves is doing. I've suffered through this before. I've tried everything I can to make the Curves result match the Levels result and that includes holding the input value at 127. I've tried starting with a node on the line at 127 and then pulling that node. I know that can change the input value if you pull the node in multiple directions. I've tried that and you get a higher contrast result from Curves. I've tried it by the numbers. Type in 127 as an input value and don't change it. There is no output value that you can enter that will produce the same result that you get from Levels -- the Curves image will always be higher contrast. I've even tried it backwards (hold the output to 127) and that really doesn't work. Again I stress that this is specifically Photoshop behavior. There may be some characteristically ideal photo in which this doesn't happen; I haven't found it. 

I'm describing what actually happens in Photoshop -- not what should happen. I learned this years ago as a matter of practice. In fact I ran into first in a book where they had a similar illustration that drew the same relationship between Levels end and midpoints and the ends and middle of the Curve. I tried it and wound up scratching my head. I put it down to Adobe's implementation.

Joe


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## unpopular (Jan 20, 2012)

That is REALLY pretty odd. I don't have a copy of photoshop right now. But heck, I'm tempted to download the demo just to try. I have no idea what to think really. Photoline seems to translate the curve similarly to the Levels. I wonder about GIMP. I'm almost inclined to say it's a flaw within Photoshop, or an attempt to make curves more user-friendly.

It's almost as if Photoshop is applying the curve non-linearly.


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## Ysarex (Jan 20, 2012)

unpopular said:


> That is REALLY pretty odd. I don't have a copy of photoshop right now. But heck, I'm tempted to download the demo just to try. I have no idea what to think really. Photoline seems to translate the curve similarly to the Levels. I wonder about GIMP. I'm almost inclined to say it's a flaw within Photoshop, or an attempt to make curves more user-friendly.
> 
> It's almost as if Photoshop is applying the curve non-linearly.



I've been tempted more than once to pick up a copy of Photoline.

I've never assumed that Levels and Curves *should* behave the same way, but I'm completely happy with that. If so then Adobe is puttin' some English on the ball so to speak. Anyway I wanted you to see one where you could read the numbers yourself so you know I'm not nuts.

I started with this photo (unedited) because of the pot on the bench. I filled the pot with a solid grey value 127.







Made a dupe of the photo and then took the first one into Levels and pulled the midpoint slider to the left until the pot value was 158. Then I took the dupe into Curves and set the input to 127 and the output to 158. While I was at it I again tried to free-hand that Curves node to a position where the two photos had the same contrast and density -- not possible.

Joe


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## unpopular (Jan 20, 2012)

Try entering the output to 127 and the input to 158 in the above example.

Photoline is cool, there are some really cool lower level features that Photoshop doesn't have. It takes some time to get used to.


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## Ysarex (Jan 20, 2012)

unpopular said:


> Try entering the output to 127 and the input to 158 in the above example.
> 
> Photoline is cool, there are some really cool lower level features that Photoshop doesn't have. It takes some time to get used to.



Yep, tried that long ago -- that just gives you a darker and still higher contrast image.

For what it's worth it may be Photoshop's implementation of Levels that's "off." Frankly I don't care that much. I don't dig into the numbers so much as I pay attention to what I see. The key in all of this is contrast. Photoshop's midpoint slider in Levels doesn't raise contrast while the midpoint node in Curves does. I'll gladly leave it to you to come up with a numerical explanation -- I know how to manipulate them to advantage.

I hear nothing but good about Photoline and the price is insanely cheap. I hate to add new software to a stable system, but soon enough I'll probably add that one. Wow! now there's a thought. I already rely on Capture 1 as my primary RAW converter. If I went from C1 to Photoline to finished output I'd be Adobe free. Wow! Adobe free -- is it possible?!

Joe


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## unpopular (Jan 20, 2012)

I am proud to say that I am Adobe Free, but i am not professional either. I still haven't found a good prepress workflow, and I don't know enough about the demands of a propho to really say if Photoline is good enough. I have downloaded the Capture 1 demo, I haven't tried it yet. I'm using Raw Photo Processor (mac only) right now, it's _extremely_ precise and produces the sharpest images I have seen yet. It's REALLY clumsy to use, it's like using a film processor.

Anyway, the big thing I like about Photoline is that you can edit RGB, LAB or HSL without changing the mode of the image layer. Working directly in HSL is just AWESOME in itself. You can make saturation adjustments on a curve. So you can actually make less saturated colors even less saturated, and more saturated colors more saturated. Or you can make warm colors warmer and cool colors cooler by adjusting the Hue curve. This all doesn't sound like much, but it _really_ adds to the vibrancy and depth of the image.


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## unpopular (Jan 21, 2012)

I edited your photo using the techniques mentioned. As you can see contrast is implied through color, rather than tone, making for a subtle contrast adjustment without affecting shadow/hilight detail.






(not saying it's an improvement, just as an example)


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## gerardo2068 (Jan 21, 2012)

Can you give me example where I would want to you use Curves adjustment in a specific color channel? Like on the drop down menu you can choose red green or blue. I understand the RGB setting. You could use it to make it brighter or darker, add contrast and things like that. But beside a few tutorials where they use the other setting to make the photos looks pastel color, I can't think of any other situation. I'm beginner on PS.


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## Ysarex (Jan 21, 2012)

unpopular said:


> I edited your photo using the techniques mentioned. As you can see contrast is implied through color, rather than tone, making for a subtle contrast adjustment without affecting shadow/hilight detail.
> 
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Drifting off topic here, but what the heck.

That is a very intriguing feature! I achieve a similar result with saturation in Photoshop by accessing the Lab color channels independently, but that would be much more direct and convenient, I'm heading over to the Photoline website again to study up.

Joe


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## unpopular (Jan 21, 2012)

gerardo2068 said:


> Can you give me example where I would want to you use Curves adjustment in a specific color channel? Like on the drop down menu you can choose red green or blue. I understand the RGB setting. You could use it to make it brighter or darker, add contrast and things like that. But beside a few tutorials where they use the other setting to make the photos looks pastel color, I can't think of any other situation. I'm beginner on PS.



There are SO many situations that you'd want to do that, it's hard to think of just one. Toning a black and white image, for example.

But also if the shadows have a blue cast, it's easy to correct by dropping the shadows in the blue channel, making them more yellow. Any time you have a _color_ correction in a specific _tonal_ region you'd use RGB curves.


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## unpopular (Jan 21, 2012)

Ysarex said:


> I achieve a similar result with saturation in Photoshop by accessing the Lab color channels independently



HLS and LAB are very similar in a lot of ways. I usually prefer HLS over LAB. I'm curious how you adjust relative saturation in LAB?

In either case, to access LAB adjustments in Photoline you don't have to convert the whole image first, you just switch to LAB in curves (etc). Which is nice.


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## gerardo2068 (Jan 21, 2012)

I see. Most tutorials just explain the basics and most focus on the brightness and contracts. I would like to see more showing situations like color tonal correction. I will keep searching.


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## MTVision (Jan 21, 2012)

gerardo2068 said:
			
		

> Can you give me example where I would want to you use Curves adjustment in a specific color channel? Like on the drop down menu you can choose red green or blue. I understand the RGB setting. You could use it to make it brighter or darker, add contrast and things like that. But beside a few tutorials where they use the other setting to make the photos looks pastel color, I can't think of any other situation. I'm beginner on PS.



Some people use curves to fix skin tone by adjusting the individual colors as well.


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## unpopular (Jan 21, 2012)

^^ much better example.


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## gerardo2068 (Jan 21, 2012)

Thank I will practice that


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