18% grey. Middle grey. Grey card. What exactly does it mean when people refer to middle grey? Is it useful in today's world? That is a topic of great controversy, but a simple concept nonetheless. The concept of middle grey was started in the nineteenth century by renowned photographers Ansel Adams and Fred Archer while developing the zone system. Upon taking a gradient of white to black and dividing it into eleven zones, defining zone five became a problem for them. Middle grey is where you find worn wood, deep sky tones, and darker colored skin tones in black and white photography. Adams used an arbitrary number of 18% to define middle grey.
How does 18% translate to 50% you may wonder. When dealing with whites and blacks you can not find detail in the depths of pure white or pure black. Middle grey is the perceived middle of the spectrum from black to white with visible detail, claimed to reflect 50% of light. It has been said that modern-day digital cameras meter somewhere around 12-14% grey, not 18%. Middle grey at the time was critical in development and printing to ensure accurate reproduction of images from the negatives. We still use the concept of middle grey today in photography, though in my opinion it is completely useless in a vibrant, colorful world.
Today, some photographers will use a grey card printed at 18% grey to either meter their light accurately or to control their white balance. The idea is to take a picture of the grey card alone with nothing else in the frame to gain the data needed. Personally, I can see the benefit in an application for lighting in a studio setting but I find this a poor way to shoot. If you are going to be taking a test shot on a grey card, why not just take a test shot and figure out the lighting? I am a huge fan of spot metering, that is taking a pinpoint reading of light where it matters which will typically be your focal point. There really is no place or need for a grey card in this scenario.
When it comes to white balance, relying on a grey card to find your whites may get you a good starting place, but will not provide the best results. Personally, I set my white balance by Kelvin as I find it the most accurate way to dial in your whites. There are photographers that claim if your light changes, you have to completely redo those settings and it becomes "time consuming." I say to them, learn your camera. Most professional grade digital cameras these days have a programmable quick menu for your most commonly used settings and a quick setting screen which makes changes like these a breeze. The only time you need to change your white balance is if your light temperature, or color, changes. In addition to white balance, you would also need to adjust your tint which a grey card will not assist with. A grey card will neither account for light temperature issues nor creative tuning, like making an image warmer to give it a certain feeling.
The beauty about photography is there are many ways to do just about everything. Every photographer has their own recipe for how they like to mix settings to capture light. Every photographer has their own unique style, both in terms of graphical concepts and preferred way to shoot. While I may disagree with how some utilize tools, as long as they are happy with the quality of work they put out, who am I to question?