What curve in Curves?

[curiousgeorge]

I've always been using the S curve but I noticed the auto levels button (this is in Paint Shop Pro XI) and this produced a very different and more dramatic effect - the top of the line was moved left and the bottom to the right - the line in between was straight.

What do you tend to use?

[Damo77]

I always use the auto levels (or equivalent) first. Then, if still more contrast is needed, I might apply an S curve.

[agedbriar]

I also try auto levels first. That forces the picture to evenly use the whole tonal range available and yields the highest contrast before local "sacrifices".

If the picture is a little more demanding I use the Levels tool proper, with Curves being my last.

You may also try Clarify if you have that option. If you haven't, you can get the same effect (local contrast enhancement) with Unsharp mask set to something like

Amount = 15-20
Radius = 50
Threshold = 0

but watch out for possible halos over flat backgrounds.

[toneyw]

Play around with a reverse S curve. It can create some great results.

[curiousgeorge]

Thanks for the replies. Just to make sure we're talking about the same thing (I'm not sure if you have the same thing in PS) this is what I mean:

240718

I've edited hundreds of images using just the S curve. I feel I have to revisit them and use auto levels!

PSP has clarify but I tend to prefer tools that give you more control.

Thanks for the Unsharp Mask tip (I have Clipping instead of Threshold). Seems to work really well in a couple of images I've tried. Is this what "local contrast" refers to, or is there a "local contrast" tool in PS?

[Damo77]

I don't know anything about PSP, but your screenshot shows me that yes, it's very similar.

One thing concerns me - see how your auto levels adjustment has clipped a bit of data at both ends of the histogram? It's most obvious at the shadow end - the point has come in a couple of millimetres past the start of the data in the histogram. This is potentially a problem.

In Photoshop, you can adjust the clipping of the Auto setting by going into Options. I can see you have an options button there. Does it allow adjustment of the clipping? (In Photoshop by default it's set at 0.5% - waaaay too high in my opinion. I set it at 0.01-0.03%)

[agedbriar]

Yes, local contrast enhancement is the general term for the effect, mostly used when doing it with Unsharp mask.

As far as I know, Clarity is a dedicated tool for the same thing without resorting to Unsharp mask.

[curiousgeorge]

Yes, there is the same option is PSP and the default is also 0.5. Thanks again.

Thinking about this again, isn't it odd that you have the same function (auto levels) in both Curves and Levels? It does exactly the same thing in both.

[Damo77]

Perhaps that'll tell you how important/handy it is ;)

[curiousgeorge]

Yes, local contrast enhancement is the general term for the effect, mostly used when doing it with Unsharp mask.

As far as I know, Clarity is a dedicated tool for the same thing without resorting to Unsharp mask.

Thanks, that's clarified it for me (pun half-intended).

[agedbriar]

I don't have Clarity as a dedicated tool, but I created a macro which runs Unsharp mask with the local contrast enhancement settings. The macro runs in a separate layer and then I adjust opacity as needed.

You might want to try the same approach with Clarity to avoid setting the Unsharp mask parameters every time again (and having to remember them).

[curiousgeorge]

In PSP you can save the settings for any adjustments so I can use that.

Problem is, Unsharp Mask is not available as an adjustment layer in PSP, so I can't do it the way you do, I'll have to adjust the strength.

[Damo77]

USM isn't available as an Adj layer in PS either. I think AgedBriar means to duplicate the layer and run USM on it.

[curiousgeorge]

OK, thanks. I'll try that.

[agedbriar]

USM isn't available as an Adj layer in PS either. I think AgedBriar means to duplicate the layer and run USM on it.

Yes, you are right. Or a Copy layer, as my PhotoPlus would call it.

[StMarc]

My response to your question is, "Mu."

This is a Discordian term which means, "Your question cannot be answered because it contains at least one implicit assumption which is incorrect." (It is, for instance, the correct answer to the old setup line, "So, have you stopped beating your wife?" Assuming you do not now and have never beaten your wife.)

There is no "correct" shape for the Curves tool, any more than there is a "correct" shape for the exposure histogram. The Curves tool does something very simple. It increases or decreases the brightness of pixels in the range in which you raise or lower the curve from the linear preset.

The real question is, "How do I look at the picture and know which ranges of pixels should be so treated?" The answer to that is, "Whichever ones make the picture closer to your ultimate vision of how it should look."

Typically, one tries an S-curve first - this brightens the upper midtones to the lower highlights, and darkens the lower midtones to the upper shadows. As another poster pointed out, sometimes you also get results with an inverse-S curve, which does the opposite. Sometimes you just raise or lower one end of the range. Sometimes you raise or lower the center of the range. And sometimes you do it to all three channels and sometimes you do it to just one channel and sometimes you do different things to all three channels at once.

The way you find out which one you should do is by doing it. You can always hit the "Cancel" button and start again, or do multiple layers of Curves Adjustment and turn them on and off to see various results. Play with it! Monkey! Tinker! You can't hurt anything. See what happens!

Personally, I rarely hit Auto Curves or Auto Levels, but I often use the White Point sampler in the Curves adjustment dialog to see what the computer thinks the picture should look like color-corrected. Sometimes I like this and go from it, sometimes I don't and start all over. Also, I frequently play with the opacity slider for the Curves layer to see what I'll get if I just do a partial adjustment. This is often handy when using the White Point sampler for auto-color correction, because if you slide it down to two-thirds or so, you frequently get a picture with sufficient color-correction to suit your purpose while avoiding the blown highlights that you might get with a full-opacity adjustment layer.

M

[curiousgeorge]

Marc, you put "correct" in quotes but I never actually asked what the correct curve is. I was just asking what method(s) people tend to use.

You told me your method and that's what I was after. Thanks for your reply.

[agedbriar]

This Photoshop (& compatibles) plugin may be of interest to Curves tool lovers. It's Curves on steroids.

http://www.curvemeister.com/

I've had it since it was still free (if I remember well?) but I haven't mastered it because I do all my tone and color adjustments in the Bibble RAW converter.


EDIT:: No, this is the free one

http://free.pages.at/easyfilter/curves.html