How many items do you carry in your photographic kit that don't need electricity? Not many, I suspect. Even the lens cleaning cloths these days seem to have a battery door somewhere...
One that you can rely upon without power is the grey card. Also known as the gray card in rural Missississippi. It is the photographic studio equivalent of a Powdermilk Biscuit - always there in the cupboard and good for any number of things.
The schools all teach about grey/gray* cards when they talk photography. In some cases I suspect their lecture material was last revised about the time that Ansel Adams invented the 2-zone system...he expanded his practise later...and they make great theatre out of using the things to assess exposure. Some modern students can be brought to understand this and come cannot, but it is still good to see old rituals being enacted.
The standard 18% reflectance grey/gray card was used first with hand-held meters - indeed it may have been seen with extinction meters in the Jurassic period. It allowed the user to have a standard surface colour and contrast to measure to ensure that exposure fell somewhere in the middle of the sensitive range of the plates or films in use at the time. It worked, and let the user make one good guess regardless of whether the scene to be photographed was in contrasty sunlight or flat dark shade. The photographer could then override that advice by merely setting the aperture or shutter speed differently.
This was in the days before camera companies packed the central processing chip with 20,000 images that could be referenced in an instant to give a perfect exposure. You got one bite at the cherry and, often as not, your teeth hit the stone inside. This was in days when you paid for the film, developing, and printing, and did it with your own money. You wanted to get the exposure right the first time.
Your scene might have been all over the place as far as lights and darks went and measuring any small portion could have thrown out the whole shebang. The 18% reflectance was what your meter was set for so it was what you measured. Some of us measured them at home before we went out and cello-taped the controls down on the cameras but that didn't work all that well...
Well... you can do the same thing now with the cards. Use them as exposure guides or use them as white balance guides. You can shoot a custom WB into one and save yourself a lifetime of slider hockey on the computer when you get home. You can take a clear picture of it, compare it to the computer screen and later to the print and see exactly whether you've got the balance right. Like the multicoloured X-Rite calibration colour checker cards, 30 seconds spent shooting this simple little plastic panel will save you 30 minutes in the computer room.
That's good value for money and time. Grey, gray, or grau - they make sense.
* Not to be confused with a noted Melbourne television personality who did crow imitations.
