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Mark D. Segal has authored a captivating (forgive me-I’m into that sort of thing…) exploration of Curves (and other manipulations) in both Adobe Camera RAW and Photoshop. It is available on The Luminous Landscape site as a .pdf.

Dan Margulis, sponsor of the Applied Color Theory List has taken issue with elements of the methodology of Mark’s study. Mark has responded.

I’d like to make a few observations about the content of Mark’s first essay, which is more than sufficiently comprehensive for a light discussion on Curves.

1. It is absolutely true that the Photoshop Composite Curve can produce a hue shift. Very rarely do the constituent Curves (R, G, and B) coincide in manner that will produce equal Contrast in the three Channels when the composite Curve is manipulated with a single or multiple points. The problem is only aggravated with compound adjustments. This asymmetry is managed by PS algorithms which are designed, according to Thomas Knoll (“The Man” of our PS World), to produce pleasing results. Consider that for just a moment-he didn’t represent that Curves were programmed to give “accurate” results, rather aesthetically pleasant ones (modeled, no less, after film performance). Forensic and technical practitioners will convulse at the very mention of such potential variability and corruption, particularly when they’re denied the programming key they need to disassemble and critique the machinery. Such is the case with PS.

(Click for larger image.) A screencapture of the Curvemeister 2.4.4 RGB Dialogue Box demonstrating asymmetries between an image’s R, G, and B Histograms. Obviously, effects on the composite Curve will be asymmetrically applied to the independent R, G and B peaks evident in the upper left graphic window. Incidentally, the B spike represents the sky, the G spike represents a tree, and the R spike represents the bricks in an architectural image.

2. Color Temperature manifests as hue shifting, specifically of the R and B channels. When Color Temperature extremes are encountered and clipping results either in the exposure or in the manipulation, the image is compromised…potentially with no opportunity for satisfactory recovery. Green compensation has been added to at least on PS plug-in I’m aware of (AGD Color, which evidently no longer exists as a company). AGD Color’s Color Temperature Plug in worked via floating points in both 24 and 48 bit modes and in addition to the G compensation, offered a color-managed preview. However, Mark’s investigation is flavored with preference for correction in RAW, which I agree with if only on the basis of his being bigger than I am (speaking metaphorically) and his quotation of Bruce Fraser stating that the White Balance conversion is less destructive in RAW (speaking specifically of Adobe CR).

3 . Mark references a technique reliant upon the Luminosity Blending Mode, and refers to it as superior to the composite Curve technique. I agree (as if it were important that I do so) with the caveat that the Luminosity Blending Mode is much deeper than is apparent, sitting there innocently as a single element in a Drop-Down Menu (it actually works within an dedicated HSL-type of gamut). The Luminosity Blending Mode will require its own post, and that’s a ways off. I’d actually recommend that unless you understand compensatory techniques for Saturation, you simply experiment with it for the present.

4. Finally, another technique Mark mentions that I’d recommend you look into is the Color Sampling Tool. Open your PS Help (F1) and search on “Color Sampler Tool” (include the quote marks). You’ll need to establish a relationship with your Info Window as well-F8 unless you’ve altered your default Keyboard Shortcuts. Placing Color Sampling points around an image isn’t considered as much an image preparation technique as it is a diagnostic tool. Color Sampling Points carry over into Duplicate image documents.

A rather sloppy and somewhat presumptuous summary of Mark’s opinion of Curves:

• They are similar in Adobe Camera RAW and PS (with the exception of hue lock in RAW)
• They are gamma-like in application and effect
• Composite Curve adjustments are not particularly well controlled through the Curves Dialogue Box (they are programmed to increase saturation within the addition of Contrast)
• There are single Channel, along with alternate gamut and Blending Mode options to consider
• The use of Curves essentially distills down to matters of preference.

Of course, there will always be the distinction between knowledgable preferences and any other type…

L