Reinventing the wheel…
Use the grey point picker (middle one) of the curves or levels dialog and see what happens 😉
To more directly answer Michael’s question: Go to Curves or Levels. Click on the white point picker. Click on the white point of the picture. You have now accomplished what you asked. Next: click on the black point picker and then click on the black point of the picture. Optional third step: If the picture has a neutral mid-gray tone in it, click on the midpoint picker and click on the neutral gray in the picture. If this gray was a bit dark or light, the tonal balance of the picture may be off, so change the gamma by moving the middle slider under the levels histogram or adjust the curve up or down in the middle. If you don’t like the result, alt-click the cancel button to reset and start over.
thanks for all of your help.
Actually, maybe someone can add their expertise here, should it matter whatever gray shade you click on with the gray point picker? I´d think that any color-neutral area would do, white, gray or black.
Mat
wrote:
I´d think that any color-neutral area would do, white, gray or black.
Yes and no… black (or white) are unhelpful if you consider what the tool is doing, but pretty much anything in between will do as long as no colour at that point is 0 or 255. It makes sense to use a mid-tone if possible.
Ofcourse you are right, the extreme ends are worthless, but anything inbetween. Thanks for the confirmation.
Mathias wrote: Actually, maybe someone can add their expertise here, should it matter whatever gray shade you click on with the gray point picker? I´d think that any color-neutral area would do, white, gray or black.
This is true for certain color casts that are stronger in the midtones and fade off as the image gets darker or lighter, but images like that are rare. It’s more normal for an image to have a uniform color cast or a color cast related to brightness – for example blue shadows.
Consider a picture of a gray step wedge that has a uniform blue color cast. Clicking on the middle gray section of the step wedge with the gray dropper will remove the cast from the midrange, but the two ends will retain their color cast.
If the color cast if related to brightness, such as an image with blue shadows, moving the dark end of the red and green curves up a bit will fix the color cast in the shadows and add progressively less color to the midtones and highlights.
Other variations are possible as well, for example when objects reflect light on one another, and when you want to retain color variation because it enhances the look of the image.
Mike Russell
After reading the posts here I’m beginning to wonder if I am missing some of the subtleties of how the white, black and gray point eyedroppers actually work.
Can someone explain precisely what each one does and which range of tones they apply to.
With all eyedroppers the user sets the value unless they use the defaults…Eyedroppers in the Curves and Levels dialog boxes force selected pixel values to the color set in the eyedropper. Also, you can change the Eyedropper setting from "Point Sample" which samples the pixels directly under the eyedropper cursor to 3×3 sample (better for low resolution images under 300MB) or 5×5 sample (better for high resolution images) this allows the Eyedropper to take a sample of the surrounding pixels (9 for 3×3 and 25 for 5×5). This may prevent dramatic color shifting from occuring…
White eyedropper=255 (or set to another value…in print production we tend to clip to 245 so that we get continuous tone dot without a 255 area printing with no ink)
Black eyedropper=0 (or set to another value…in print production we tend to clip to 10 so that too much black ink is not forced onto the paper)
Grey Eyedropper=128-133 (what I do is look at grey values with the info palette open and try to find a neutral grey with at least 2 of the RGB values equaling 128-133. What can happen with the misapplication of the grey eyedropper is to introduce a color cast into the midtones.
Things to watch out for…
Don’t use the white eyedropper in a specular highlight as it tends to brighten the image way too much and can introduce a color cast…
If you select the wrong grey value you will see dramatic color shifts
Don’t use the Levels eyedroppers…the Curves Eyedroppers give better results as their rounding is more precise…
Christine,
Thanks for the information. I ‘m still a little confused with what areas of the image the eyedroppers affect, though. For example, if there is a slight, consistent color cast over the entire image, then it would seem that clicking the white or grey eyedroppers (in the correspondingly correct areas of the image) should both correct the color cast. But would they correct the cast for all tones in the image or would the white eyedropper correct the color cast for the highlights and not correct it for the midtones and shadows. Would the grey eyedropper correct for all tones or just mostly the mid tones.
Gordon said: After reading the posts here I’m beginning to wonder if I am missing some of the subtleties of how the white, black and gray point eyedroppers actually work.
You are right. There’s a lot more to this, but you can pick it up step by step, and each time your images will get a little bit better.
Christine gave a good explanation of how the droppers work, and her warning about specular highlights not being used as a reference is spot on. And I agree with her that Curves should be used in preference to levels. Perhaps the LevelMeister will chime in with a different take 🙂
The eyedroppers have some limitations that should be kept in mind.
Terminology – a neutral is an area of your image that you know, or strongly suspect, should have no color. A highlight is the brightest area in your image with significant detail. It must have no color. A shadow is the darkest neutral area with significant detail. Most, but not all, images have all three of these.
Restricting yourself to one neutral in RGB mode is sometimes too limiting, since a color cast may – and generally does – extend across a range of brightnesses.
A better procedure for setting highlight and shadow is to use Threshold to find the darkest and lightest points with no color and significant detail, note the values, and use curves in individual channels to set the curve endpoints.
For neutrals, drop an eyedropper sample on a couple of neutrals of varying brightness, and adjust the individual curves to make them neutral. FOr a single overall cast, convert to Lab mode and change your curves to set the a and b channels of one of the neutrals to zero. Some images require a combination of the two techniques, using Lab to get rid of a large color cast, and RGB or CMYK for fine tuning.
For more info, Dan Margulis discusses this and more in his Professional Photoshop book.
Mike Russell
Thanks for the great advice.
Gordon
I agree with Curvemeister: "Restricting yourself to one neutral in RGB mode is sometimes too limiting, since a color cast may – and generally does – extend across a range of brightnesses"
Often the worst thing you can do is to set a middle neutral area alone. Most, but not all the time, the colour defect does cross a great range of luminosities. I don’t use the eyedroppers much, but start with adjusting each individual channel’s curves darkest and lightest endpoints upwards or downwards (or right or left) until the colours look right. That keeps the curves linear, not curved. Only then, if the middle tones require a separate adjustment, I will add one or more middle tone points and adjust them. Most of the time, I find after adjusting the endpoints that this is unnecessary. Clicking on the grey eyedropper (after setting the values for this) will generally make the picture look artificial by turning a linear colour defect into a nonlinear defect. Then one generally has to add multiple other points to try to get the colour defects for different luminosities back in line.
One can then change the overall luminosity curve as well in that curve adjustment layer, but I generally add a second curve adjustment layer just to do that (so that the colour correction curve is differentiated from the brightness/contrast curve).
Be careful with shortcutting this by using the white and black eyedroppers. Subtle real colour shifts in a particular dark and light region can throw off your overall correction substantially. That is why I like manipulating the endpoints of the curves and examining the overall light and dark areas much better than trying to do the correction with the white and black eyedroppers.
In Margulis book, he suggests not using 0,0,0 and 255,255,255 for the two eyedroppers, but I think one needs to go further than that, and assign checkpoints to various regions that should be neutral and look at these values while adjusting the endpoints of the curves instead.
