Color Density Filters in Photoshop
This tip was one of the earliest we put on the web. We have left it on the web because the content remains true and because it does show 1) the direct relationship between the density of color compensation filters and Photoshop settings, 2) how deep one has to dig into Photoshop to find color compensation filters, the mainstay of professional color film photography and 3) where we were before we began the systematic mathematical study of digital photography and Photoshop. However, for practical application of the equivalent of color compensation filters - which remain key to preserving color integrity of an image - we suggest that you visit our page Routine Color Balancing and use the methods described there. Most recently we have made available Photoshop plug-ins called ColorIntegrity and AutoColorInt which address the problem directly. Demo versions of ColorIntegrity and AutoColorInt are available. AutoColorInt and the advanced version of ColorIntegrity directly display color compensation (CC) equivalents for the adjustment being made. See our Photoshop Plug-Ins Page for full details.
When experienced color photographers who have done color printing with silver-based materials come to Photoshop the first question is: Where are the color density filters? When making a large presentation color print on silver-based materials, the first print is customarily a test. In shops with a lot of experience and tight controls this test may be very close to what is wanted for the final while in other shops the test print may require considerable correction. By examining the test print it might be judged that the "filter pack" needs to be adjusted by adding in 12R and subtracting out 8M (red at 0.12 density and magenta at 0.08) as actual filters, or dialing in +4M +12Y on a subtractive colorhead or +4R and 8B on an additive colorhead. With an appropriate adjustment to the overall exposure, a final print or another test print is then made.
Since the above activity often goes under the name of "color balance" in photography, the photographer gravitates to Image→Adjust→Color Balance in Photoshop. And becomes thoroughly frustrated when it works nothing like the familiar color density filters. Eventually either trial and error or some kind soul guides the photographer to the "Levels" tool and after considerable experimentation that becomes a workable solution, most of the time. Still, there is the nagging feeling that it is not the same and that there are times when the filters would have offered a more ready control.
The following methods may already be out there somewhere. In fact I would be at least mildly surprised if they were not, but I have yet to find anyone who knows about them. The fact is that color density filters are in Photoshop, but they are buried so effectively that even Adobe may not be aware of it. This unfortunate fact means that to actually be useful, any tip has to be more an explanation of what Photoshop does than a cookbook set of instructions to achieve color density filtering. We have expanded this tip to add a second method and some important details about the use of the Brightness/Contrast tool as part of the methods. Elsewhere we have applied color density filtering, using the second method, to Routine Color Balancing and to Color Balancing Color Negatives.
To use the digital equivalent of color density filters, go to the Image→Adjust→Color Balance dialog in Photoshop. Check "Preserve Luminosity" and check "Highlights." The three sliders then become equivalent to color density filters in the way they operate on the picture. The numerical settings will not correspond exactly to the density numbers of filters and in fact will depend on the underlying "gamma" setting among other things, but the numbers will work in a relative sense. That is, a setting of 20 Red will add very close to twice as much red filtration as a setting of 10 Red, and if a setting of 10 Red actually corresponds to a 5R filter, a setting of 10 Green will be equivalent to a 5G filter.
For the most part, the colors in the picture will change just as they would in response to a color correction filter, but there are a few glitches to watch for. First, absolute black will remain absolute black and absolute white will remain absolute white. Actually, this applies individually to red, green, and blue any pixel in any one of these that is full intensity (255) or zero (black) will stay at full intensity or zero. Second, all corrections are made by making the image brighter and very bright colors may therefore go full intensity. (In a world not made by Adobe, "Preserve Luminosity" might have been expected to take care of precisely these problems, but
If only minor corrections are required, such as touching up after a "Levels" adjustment, the limitations should present no problem. If problems are encountered, there is a workaround, although it is tedious. Use the Brightness/Contrast tool to slightly lower the contrast. This serves to make sure there is no full intensity white or individual color and no zero intensity black or individual color and that the intensities are low enough not to be run off-scale in brightness. Do not overdo these temporary adjustments. Do the color correction filter adjustments as described above, paying attention only to color balance and not overall density, and afterward use the Brightness/Contrast to restore to proper contrast. To use this workaround, it is best to do all these steps in 16 bits/channel mode. This workaround requires practice. For practical applications use the method explained in Routine Color Balancing and Color Balancing Color Negatives.
The obvious glitches pointed out above and the counterintuitive and nonsensical way in which the Color Balance tool behaves in general make me think it was an early and grossly incorrect attempt to simulate the action of color correction filters. For some reason it has never been corrected.
Originally this method of generating the action of colored filters in Photoshop was not included because while the basic concept is simple, it is hard to control except in special cases. The method is too important to omit, as it is an integral part of other techniques and useful in making major color corrections, so we will review it briefly here.
Above we mentioned that the Levels tool is commonly used to make color balance corrections. The graph displayed below is a Levels histogram, showing relatively how often each of the green pixel values between 0 and 255 appears in the image (oddly, Photoshop also reports 16 Bits/Channel data as ranging between 0 and 255):
Color corrections are most commonly made by moving the center one of the three sliders under the histogram. The center slider has several mildly misleading names, such as the "middle gray setting," but it is really the gamma correction setting and can lead to problems which we describe elsewhere.
No one seems to be aware that the slider on the far right, sometimes called the highlight slider, acts precisely like a colored filter. Slide it to the left and it applies a filter with exactly the same effect as a colored filter would have in silver-based photography, green in the illustrated case. That is, the green in every pixel in the image is changed by the same percentage. The filter effect is achieved by increasing the green rather than decreasing both blue and red as would be the case with a physical filter.
The image must be in 16 Bits/Channel mode rather than 8 Bits/Channel for satisfactory results from the multiple actions usually required by this method.
The method can be hard to control, primarily for three reasons. The first problem is that the numbers generated by moving the slider will have no resemblance to corresponding color correction filters, and moving the slider 20 counts will not result in double the filter density of moving it 10 counts. The following table shows the counts corresponding to various filter densities for the most commonly used image gammas (2.2 for PC.s and 1.8 for Macs).
It is possible to do successive color correction filter adjustments using the above table; that is, working in gamma 2.2, an adjustment of the Levels red highlight slider to 230, followed by invoking the Levels tool again and adjusting the Levels red highlight slider to 230 a second time will result in a color correction of 0.1 + 0.1 = 0.2, or equivalent to a 20R (positive) color correction filter.
The second problem may be obvious from the tables. To get a large correction, it is necessary to move the slider way to the left. If the slider is moved past where data start to show in the histogram, the image will start to lose highlight detail in the filtered color, as all pixels with starting values to the right of the slider will be fully saturated.
There are several ways to deal with this, but before launching into a complicated explanation, it should be made clear that very often the corrections below will not be necessary. Typically when filter corrections are needed there will be sufficient space above the histogram data to make the required adjustment. In fact, just removing the empty space or most of the empty space in all three channels will often lead to the required filter color correction. It is widely realized that this action sets the tone of the white areas of the image, but not that it is the exact equivalent of applying color correction filters to the entire image.
The easiest way to provide some headroom for the filter adjustment is to first use the Brightness/Contrast tool. Lowering the image contrast will pull both tails of the histogram toward the middle, and lowering the brightness will shift the entire histogram to the left. The Brightness/Contrast tool is one of the many tools in Photoshop that is not programmed to operate accurately with gamma-adjusted images (even though gamma-adjusted images are the primary target of Photoshop Those few working with images that are not gamma adjusted will know that they are). To make a contrast adjustment which will preserve image integrity, proceed as follows:
The leftmost histogram is of an original image. From the menu, Image→Adjustments→Brightness Contrast. Adjust the Contrast slider left. We have selected 25 as sufficient to get some work room. Using the Image→Adjustments→Levels tool, we find the middle histogram. Here we have moved the leftmost slider over to find where the image data starts, shown as 25. This number will not always match the negative of the contrast adjustment as it does here. Be sure to Cancel out of the Levels tool. Now activate the Brightness/Contrast tool again and set the Brightness to minus the tail position measured in the previous step, 25 in this case. Go back to the Levels tool and the rightmost histogram shows the graph to be slid over to the left. There we also have used to right slider to measure that the upper limit of data is now 195. The filter density tables above show that working with a gamma of 2.2, correction filters of up to about 0.25 (that is, 25R, 25G, and/or 25B) can be applied without affecting highlight image detail. Obviously, this is a complicated operation in Photoshop.
As well as being necessary to the topic, the above diagram is a nice example of the sort of perception problem that complicates any imaging work. The two blue "Brightness/Contrast" banners are identical despite the one on the right appearing to be much darker.
In a previous version of this page we wrote that only red, green, and blue color correction filters are individually possible; no cyan, magenta, or yellow, which is not strictly true. We wrote that because moving the left slider, sometimes called the shadows slider, to the right may seem to give the effect of adding the opposite filter; magenta rather than green in the illustrated case.
Unfortunately, the effect is not that of a pure colored filter. However, it is possible to get a magenta filter action by moving the right Output Levels slider to the left in the above (green Levels) or by moving the right Highlight slider to the left the same amount on both the red and the blue histograms. Calibrated use of the right Output Levels slider would require another set of tables similar to the above, particularly as the Output Levels right slider effect becomes much greater than expected as it is slid further to the left. The tables really were for illustration purposes and we doubt anyone will actually use them in practice, so including more tables would be pointless. Practical application of color density filters will be found in Routine Color Balancing and Color Balancing Color Negatives.
The cautions we have outlined here are necessary to preserve the color balance integrity of an image. From a practical standpoint, most digital images will have been compromised as an accurate light intensity record before you even start to work on them. There are gamma adjustments and shifts of gamma adjusted images hidden in all manner of equipment and software. In the end, all these things are just tools. Our attitude, to paraphrase Duke Ellington via Peter Schickele, is: If it looks good, it is good.
As these methods are original with us, you are welcome to use them in an article, a course, or a book but please credit C F Systems and www.c-f-systems.com
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