What a minefield this subject can be.
According some luminaries on the topic, the Kodak Grey Card 18%-R27 (new: Grey Card Plus) was designed to have a neutral colour cast and render an 18% light reflectance measurement when measured with a reflectance light meter.
The trouble with modern digital cameras, apparently, is that they measure 12% Grey, a supposedly ANSI Standard for Meter Calibration. Which begs the question, What do our light meters, meter? (An excellent read and counterpoint to the 12% Grey Argument.)
Now, conventional wisdom dictates that it costs less in the long run to buy a reference card like Kodak’s Grey Card because this allows for standardization across the board, one standard, one expectation, no confusion. Yet, confusion reigns supreme! On top of this, in some parts of this world, there is no easy access to such nice, simple things like 18% Grey Cards (or color charts, or printer/monitor/camera/workflow calibration devices, etc…)
Do you NEED to buy one? No. But you might like to look at the Wiley range of Digital Field Guides for your camera – far more useful, and it may come with a free Grey/Color Checker card inside, mine did.
Can you DIY a Grey Card? Well, Yes and No. Welcome to the equally murky world of Workflow Calibration. By this I mean, calibrating your camera, your monitor, your printer, your printer inks and your printing paper. If you can get this consistent across the board so that the colour you see and photograph is the same colour your view when looking at the final print, then printing a Grey Card (or color checker) should not be a problem.
But just for the exercise, what is 18% Grey? Now there is a lot of discussion and debate as to what is the true RGB values for Neutral Grey or 18% grey, just have a read of some of the above linked articles. You could pick one of these numbers if you like. However, if I were to go about Printing an 18% Grey Card, I’d start with matt photo card and a colour swatch that was filled with Pantone Grey82. According to December.com’s Color Codes this would correspond to CMYK (0,0,0,18) or RGB(209,209,209) which is RGB%(82,82,82). Now this last figure is the one of interest, for me at least.
When I took a photo with a Grey Card in the image, and later did an automatic WB correction and auto exposre correction, I ended up with approx. RGB%(64,64,64) which from the same color chart, listed above, would be 36% grey. To bring this closer to 18% Grey I needed to increase the image exposure in Post by +0.82EV. (I mostly use Adobe Lightroom 3 for post-processing).
Now, all that’s quite nice and academic, but what about printing it. Well like I said before, I’d start with RGB(209,209,209) and print that onto matt photopaper, making sure I had fresh, fully charged ink cartridges and that the printer had had a cleaning cycle run through it first. Why such a high, bright, light, grey? Pick a colour space and gamut, then work from there, and see what works, for you. This is what corresponds to 18% black ink. It’s also the perceptual grey that was closest to the grey card I got in my Digital Field Guide book.
Continuing on, I’d redo the swatch with the nearest Hex Safe Grey, which is #CCCCCC which corresponds to 20% Grey, print that and then run some Zone Ruler Trials on them (see an earlier post) to see what colour casts might show up. If one of these cards work out fairly clean, then I’d get on with learning how using this “Exposure Guide” influences my photography, and the results my camera and work flow produces.
The Grey Card issue is quite a murky world but in the end, you will need to evaluate, just how useful a Grey Card would be for your style of shooting.
For today, that’s what’s in my
Line of Sight.