In our recent article on metering modes, we touched on the option of using a grey card as a reference for getting accurate exposures, writes Jon Adams. Here, we’ll explore the way such a tool is used for this purpose, and also how it can be used as a white balance reference to make sure you get accurate colours, too.
A ‘grey card’ is rather boringly named, but it’s at least an accurate description of what it is! They’re available in all shapes and sizes, and aren’t necessarily ‘cards’, but the grey tone on its surface isn’t any old grey – it’s a very specific grey that reflects 18% of the light falling on it. This level of reflectance is exactly halfway between black and white, so is an official ‘midtone’ in photographic parlance. Midtones are useful, as they are what lightmeters use to measure the brightness of a scene or subject, and give a correct exposure.
Grey cards come in all shapes and sizes but reflect 18% of the light falling on them. This is the reflective property of an official ‘midtone’.
Of course, when we’re just talking about reflectance, the colour doesn’t matter at all, so if you want to measure the luminosity reflecting off a midtoned area, provided it reflects 18% of the light, it doesn’t matter if it’s red, green, blue or grey. But grey is useful for more colourful reasons we’ll get to in a moment.
Getting your exposure spot on
When determining the correct exposure for a shot, all you have to do is place a grey card in the same light as your subject, and then spot meter from it. This will give an accurate exposure, so if you set aperture priority mode, dial in your preferred aperture value and point the spotmeter at the grey card, then the shutter speed you’re given is what you should set for that scene in that light. This grey-card function was important for critical exposures on slide film back in the day, as it allowed you to take an objective exposure reading from an independent reference point that wasn’t influenced by the content of the scene.
By spot-metering off a grey card held in the same light as the subject, acccurate exposures can be achieved.
A tool for better colours
Nowadays though, with a screen on the back, a histogram display at your fingertips, and no cost for shooting extra frames either side of your best guess to hedge your bets, this grey card function isn’t so important. Where it DOES come into its own is in getting your colours right. The grey of a grey card is perfectly neutral. This means you can use it as a neutral reference point for colour. You can do this by setting a custom white balance in camera, using the grey card as the reference point for the setting, or – much more commonly – you can do it later in raw conversion software like Adobe Camera Raw or Lightroom.
Place your grey card in the scene and take a reference shot using your camera’s RAW format. Now, remove the card and shoot your pictures, and when you’re making your raw conversions, you can use the grey card shot as a white balance reference. Just select the white balance eyedropper in the raw software and click on the card, and your custom white balance will be set for the scene.
By taking a raw reference shot with a grey card in the scene, you can ensure accurate colours by clicking on it with the WB eyedropper and transferring the Temperature & Tint settings to subsequent shots taken in the same light.
Now, all you have to do is transfer the white balance setting (the colour temperature value) to all other the other shots you’ve taken in the same light, and your colours will be spot on. This can be important where colour fidelity is critical – for example, when shooting product shots or getting accurate skin tones in portraits – and is especially useful when you’re inconsistent or variable light sources.
Grey cards always seem a bit more expensive than they should be (they start at around £30), but it’s worth investing in one if you need to maintain accurate colours in your shots, or want an objective reference for exposure settings in tricky conditions.