Digital Gray Card™ Frequently Asked Questions
Can I use the DGC for setting the exposure?
No. Over the years camera manufacturers have standarized the 18% gray card for setting exposure for film. The Digital Gray Card™ has a higher reflectance, so it cannot be used. The reflectance varies from 30 to 37%, depending on the batch. The higher reflectance makes the DGC unsuitable for setting exposure for film photography, but it is better for digital photography than the traditional 18% gray card. The higher reflectance of the DGC improves the signal to noise ratio of the digital sensor, making the gray balance setting more accurate.
Another reason for not using the DGC for exposure is that digital cameras are not designed to allow it! The only way to set the exposure with a digital camera is in full manual mode, where you set the aperture, shutter speed, and sensor ISO value. While you can do this, how would you know what settings to use? Digital cameras no longer have exposure meters built in that report their information to the photographer. The traditional light meter is set to readings from an 18% gray card, so the DGC would be approximately 1 EV higher in reflectance, so readings from the light meter would be about 1 EV off. If the DGC was exactly 36% reflectance, then the light meter would read 1 EV higher in reflectance and so it would recommend an exposure that would be 1 EV lower than normal in order to compensate. But the DGC can vary so the light meter reading would be in error by the difference between the DGC reflectance and 36% reflectance, the nearest even multiple of 18% and thus an even EV difference.
The exposure might be judged from the histogram display on the camera's back after taking an image of the DGC. But at what position on the graph should the DGC values be? It's position would depend on the exposure and the camera's tone curve. The tone curve is very hard to control in digital cameras. I have not experimented to find a method for determining the tone curve, but I think many cameras are trying to emulate film curves. The question then becomes are they trying to emulate more contrasty slide film or less contrasty negative film? Is there some way for the photographer to control the tone curve?
Additionally, most digital cameras over-expose the image by 2/3 to 1 full EV. They do this deliberately because the small pixel sizes in these cameras produce low detail and high noise in dark image regions. By over-exposing the image they move the dark areas to higher tone values, thus decreasing noise and showing more dark image detail. However, by over-exposing the image the highlight image areas are moved higher in value, thus producing more burned out highlights. Since more information is conveyed visually by the shadow regions than the highlights, most people find this tradeoff more acceptable. But this over-exposure is dynamic, determined by the computer in the camera after analyzing the scene, so how would you know where to set the exposure with the DGC to produce a good image result?
The higher DGC reflectance does work to the digital camera's advantage for neutral balance, also known as "white balance", because the signal to noise ratio is higher with the DGC than an 18% gray card. A signal-to-noise ratio of 1:1 would mean that the sensor signal from the image is even with the sensor noise, so it would be difficult to find the image in the noise. The higher the signal-to-noise ratio the more the image is standing out from the noise. If you had exposure compensation in effect the DGC's higher reflectance works to keep a higher signal-to-noise ratio so the camera can neutral balance on the gray card instead of sensor noise.
Given all of this, if you decide to use the DGC for exposure setting, go ahead. It is your camera and your images. It will probably require some experimentation to determine the right compensation values for your card and camera. Do not be surprised if the camera's automation makes this a difficult experience. Because of all these competing processes in the camera, I cannot recommend a method for using the DGC for exposure setting, hence my refusal to assert that the DGC will work for the purpose of exposure setting.
As for myself, I use the DGC to set the camera neutral balance, then, depending on a test shot of my subject, I set the exposure compensation to change the exposure by -2/3 to -1 EV if necessary. For sunlit yellow subjects I often need to set -1 EV exposure compensation to keep the yellows from being burned out to white. That same yellow that is burned out on a sunny day is recorded perfectly well on an overcast day. So I must judge the exposure compensation on a case by case basis.
What would you say is the RGB value of these cards?
The RGB value is dependent on the exposure (aperture and shutter speed), the tonal reproduction curve used in the camera, exposure compensation, angle of the card surface to the light source, etc. Therefore, there is no correct RGB value for the card. The most important thing is that the RGB values for the card are the same, or within a unit of each other.
Additionally, almost all of the one-shot camera manufacturers over-expose the image by 2/3 EV (i.e. f/stop) to achieve more shadow detail. Again making exact RGB values unachievable.
The only occasion when RGB values can be achieved exactly is when you have complete control over the tonal reproduction curve and use a curve that performs exact tonal reproduction. Very few cameras have this ability and can adjust the exposure precisely in small enough increments to produce an exact result. This requires adjustments of exposure in increments finer than 1/30 EV.
By the way, there is a camera that can achieve these exact conditions (total user control of the tonal reproduction curve and adjustment of the exposure to 1/300 EV per channel), the Better Light digital scanning camera. It does not allow for shooting moving objects, but for fine art reproduction it has no peer.
The traditional 18% gray card was set to approximate the middle of the human perceptual tone range. If you could do exact tonal reproduction, this card would have RGB values of about 117 in a gamma 2.2 compensated curve (this is a curve that does exact reproduction and is compensated for proper display on a gamma 2.2 monitor. The values would be about 98 for a 1.8 gamma compensated curve). The 18% gray presents problems with many digital cameras. First, the reflectance is low enough to make the signal to noise ration (SNR) of the camera poor (this means that noise comprises too much of the signal for highly reliable calibration). Also, many of the 18% gray cards are not spectrally neutral, which means that they have a color cast in many types of lighting. The Digital Gray Card™ has a reflectance in the range of 30-37%, depending on the batch (unfortunately, this is not under our control), which produces a good SNR for the camera (noise is very low) and the cards are spectrally neutral, so they are gray under almost every type of lighting.
If I have not overwhelmed you, my suggestion is to not worry about the exact RGB values, just make certain they are all the same value.
Any problem with reflections when using the Digital Gray Card™?
The Digital Gray Card™ has a matte surface that minimizes the angular reflection problems you mention. However, since the perfect reflecting diffuser (a surface that will uniformly spread light perfectly regardless of the illumination angle) is not attainable, there will be angles that show some specular reflectance. This is minimized, but still there. One method to alleviate the problem is to adjust the angle of the gray card you use (any gray card, not only the DGC) so the illumination to the card surface is at an angle of 45 degrees and the camera views the card from a 90 angle, relative to the card surface (this is the same type of illumination and viewing system used in 45/0 spectrophotometers).
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