October 4, 2007
A little while back we posted a story linking to an article on the Luminous Landscape website (Do curves throw you a curve?) in which author Mark Segal offered some interesting perspectives on the nature of curve adjustments in Camera Raw and how these compared with traditional RGB composite curve adjustments made in photoshop. In this feature article for Lightroom-News I have concentrated on analyzing the difference between Tone Curve adjustments made in Lightroom and Photoshop curves. I show you here a method for testing curve comparisons between the two programs and some observations on how and why they differ.
On the face of it, the Tone Curve control in Lightroom appears to work the same way as the Curves adjustment in Photoshop. If you use the Tone Curve controls in Lightroom to edit a color image, a steep curve will increase the tonal contrast but will also boost the saturation as well. The same thing happens when you use Photoshop curves to increase the contrast of a color image. Now in Photoshop you can use the Luminosity blend mode to fade a curves adjustment (or if using an adjustment layer, set the layer blend mode to Luminosity). This, it is argued, will apply the curve adjustment to the image luminance only, without affecting the color information. It is a useful technique to know about and can often be used in Photoshop when you wish to apply a small localized contrast correction without affecting the color. Although the Luminosity curve method might seem to be the ‘correct’ way to go about applying curves, your photographs will tend to look unnaturally dull if you use the Luminosity curve approach when carrying out global adjustments. According to Thomas Knoll, it would be easier to program curves without a saturation boost, but it is included in the Photoshop and Lightroom/Camera Raw processing because the mild saturation boost produces more pleasing, film-like results.
But Lightroom/Camera Raw curves do work slightly differently from Photoshop curves and this is because Lightroom curves have a hue lock. This means that when Lightroom maps the RGB values from the before state to the Tone Curve state, it will map the minimum and maximum RGB values (in the linear Lightroom RGB workspace) allowing the hue to vary. But when mapping the middle RGB value, the hue is preserved. Photoshop curves meanwhile have no hue lock and therefore when you apply a strong curve adjustment in Photoshop the hue values can shift quite a bit from the original before values. This in turn can lead to some noticeable color shifts in the processed image. Lightroom/ACR curves do also produce hue shifts, but these are more tightly controlled so that what hue shifts there are, are usually within plus or minus 3°. As I said, Tone Curves in Lightroom that increase the contrast, will boost the saturation, but from the conclusions I draw later, Lightroom/ACR tone curves are on average about 1–2% less contrasty than curves that are applied via Photoshop in the Normal blend mode. In practice this means that Lightroom tone curve adjustments will have smaller hue shifts and the colors are represented better.
So how can one test this? The following steps show the method I used to create a Photoshop curve that matched a Tone Curve adjustment that had been applied in the Lightroom Develop module.
Important note: these steps for creating matching Photoshop curves will be specific to the RGB space you export the files to. In the following example I exported the images from Lightroom to the ProPhoto RGB space.
1. To replicate a Lightroom tone curve in Photoshop, I made a virtual copy of a color image and converted it to Grayscale mode in Lightroom. In this example I applied a high contrast Tone Curve and exported two versions: one with a neutral Linear point curve and one using a high contrast curve setting (as shown here).
2. In Photoshop I added the Linear Tone Curve image as a layer above the High Contrast Tone Curve layer and placed the layer in a new group. I then added a curves adjustment layer above the linear Tone Curve layer. I then set the layer group blend mode to Difference. The objective now was to open the curves adjustment layer and create a curve shape that matched the Lightroom Tone Curve shown at step 1. Because the layer group had been set to Difference, this meant that the image would appear solid black when the curve adjustment adjusted the linear Tone curve layer so that it matched the High Contrast Tone Curve layer exactly.
3. Once I had got a perfect match I saved the curve as a new preset and named it High contrast.acv.
4. I was now able to test the difference in the hue and saturation response of a Lightroom tone curve compared with a curve applied to an image in Photoshop. I repeated steps 1 & 2, but this time I processed the image in full color. The comparison results are shown in Figure 1 along with a table of measurements in Figure 2.
About the curve comparison creation method
For the first step, I deliberately processed the image in Grayscale mode because at this stage I only wanted to compare the Tone Curve luminosity. Once I had discovered the Photoshop curve setting that would exactly match the Lightroom Grayscale Tone Curve adjusted image, I had a Photoshop curve which when applied to any Grayscale mode image would match Lightroom exactly. Note that it did not matter which image I used to test with at stages 1 & 2, just so long as it was an image edited in the Lightroom Grayscale mode.
Note that when I started putting the curve to use and tested the difference between the Lightroom and Photoshop curves using color images, I confirmed that the brightness values did indeed always match wherever I sampled in the image area. This therefore allowed me to test for just the variations in hue and saturation between the Lightroom and Photoshop curve methods.
Figure 1 In this sectioned image you can see a comparison view of a Lightroom high contrast curves adjustment (top), a Photoshop high contrast curve blended using Luminosity (middle) and a Photoshop high contrast curve blended using Normal mode (bottom).
The tables in Figure 2 shows the Photoshop recorded HSB values at points A,B,C and D in Figure 1. These numbers allow us to analyze the differences between the three curve methods used here and help understand why they each produce slightly different results. The Hue value is expressed in degrees (°) on a scale of 0–360, while the Saturation and Brightness values are expressed as percentages.
Figure 2 This table shows a comparison between the Photoshop HSB values for points A,B,C and D in Figure 1. These figures show the differences between the original values and those for the image after it has been processed via Lightroom and via Photoshop using a normal blend and a Luminosity blend mode curve.
Compare the Lightroom adjusted Hue values with the original image and you will notice that there is only a small difference. Now compare these with the Photoshop Normal blend mode values and you will see a much wider variance in the Hue values. However, when the Luminosity blend mode is applied to the curve in Photoshop the Hue values will always be preserved exactly.
If you now look at the Saturation values you will note that the Saturation is almost the same with both the Lightroom and Photoshop curves, except I would comment that in the testing I have done, the Lightroom curve saturation is on average always slightly less than a normal mode Photoshop curve (by about 1–2%). Sometimes it is higher, but mostly is less. But what is interesting about the Photoshop luminosity curve is its tendency to suck some of the saturation out of an image and the figures in the Luminosity curve Saturation column back this up. Compare these values with the Saturation values for the Lightroom and Photoshop normal blend curves.
The Brightness columns confirm that the luminance values are identical for both the Lightroom and Photoshop normal blend mode curves, but the Luminosity mode curve values do end up being different.
Figure 3 The HSB values shown in Figure 2 were noted down via the Info palette in Photoshop. I went to the Info palette preferences (via the flyout menu) and altered the coordinate second color readouts as shown here.
It is all very well running involved tests like the one described here to calculate the numeric differences between Lightroom and Photoshop curves, but at the end of the day, the only way to judge anything is by using your eyes and comparing the results visually. The differences between the two curve methods can be quite subtle, but where it is noticeable, I would say the Lightroom/Camera Raw curve result always looks more pleasing. The testing data kind of backs this up, because as I mentioned earlier, the figures show that there is a much finer tolerance in the amount of hue shift you get with a Lightroom curve and this is why the colors tend to be better preserved.
Here are the three versions of the image shown as separate images:
Lightroom Tone Curve version
Photoshop Normal mode curve
Photoshop Luminosity mode curve
It is interesting to examine what happens when you apply a Luminosity blend mode to a Photoshop curve. The received wisdom here is that a Luminosity curve will only affect the luminance values of the image and the color values will be preserved. At least that is what I have written in the past! If you compare a contrast increasing Luminosity curve with a Normal mode curve, the Luminosity curve will look flatter in color, which one might assume is because the curve has been filtered to target the luminance only. While a Luminosity curve will always preserve the hue, it can still have a marked effect on the saturation values. And whereas the luminance values will always match between a Lightroom curve and a Photoshop curve in Normal mode (using the test method described here), the Luminosity mode curve brightness values can actually deviate a lot from these two other curve methods. The conclusion I draw here is that Photoshop Luminosity curves are useful for preventing unwanted hue shifts and taming any saturation boost, but one should be aware that the saturation values can shift upwards or downwards and there will not be an exact tonal match between the effect of a Luminosity and a Normal mode curve. This last point should not necessarily represent a problem. It should not matter if the curve outcome is different because you can always manipulate a curve in Photoshop to get the luminance balance you do like. For Photoshop users it could be argued that what is needed is a slider control in the Curves dialog going from a Normal mode to a Luminosity mode curve adjustment. That way, one could tweak the way the curve was applied to the image from within the Curves dialog. That might work, but if we go back to the subject of Tone Curves in Lightroom there is already an easy method for taming the saturation boost, namely the Vibrance and Saturation sliders. One can easily take the Vibrance or Saturation down if you find that the Lightroom curve is making a photo look too colorful (see Figure 4).
Figure 4 If you want to achieve the Luminosity curve desaturated effect, it is easy enough just to turn down the Vibrance and Saturation in Lightroom. In this example, the left half shows the image treated with a -25 Vibrance and -15 Saturation. The right half shows a version created using a Photoshop Luminosity curve. The hue and saturation values can be made to almost match, but the tone luminance characteristics will always be slightly different. In this example the Lightroom adjusted Tone Curve has more tonal contrast in the highlights than the Luminosity curve version.
Mark has carried out some very thorough research here. He does reach a slightly different conclusion about the saturation differences between Lightroom and Photoshop curves. But as I have discovered, Lightroom curves can indeed sometimes produce stronger saturation values, so what I have concluded here does not necessarily disprove his observations.
Martin Evening has worked on the development of Adobe Photoshop as an alpha tester from the program’s earliest beginnings. The Adobe Photoshop Lightroom Book describes all of Lightroom’s features in detail, with photographers in mind. Photographers who routinely work with raw (and even jpg & tiff) images will find Lightroom–and The Adobe Photoshop Lightroom Book–an indispensable tool in their digital darkroom.
Lightroom-news has a free PDF download of Chapter 1. (click here to download-4.6MB PDF).
Free Lightroom 1.1 PDF update
You can also download a free PDF update for the Adobe Photoshop Lightroom book. Go this link for the full instructions on how to access the Adobe Photoshop Lightroom Book 1.1 update.