I've spoken before about using a grey card to either set your white balance or exposure, and today I thought I'd go one step further and explain the steps you take to do that!
First off, a gray card is just a piece of card that represents 18% gray - in other words the middle gray that your camera tries to expose for. Before I got my Expodisc, I used this one - CPM Delta 1 8 inch x 10 inch Gray Card which costs around $10 - but you can get lots of different ones which are pretty inexpensive. (I don't have it but this Pocket-sized one looks good for being a bit more portable). It doesn't really matter which one you choose, as long as they are non-reflective.
You can use a gray card for two things - setting white balance and measuring exposure .
They are so darn handy, particularly when you are starting out, that it's the only piece of equipment I ask you to have for completing the Auto to Awesome ecourse.
How to Use a Gray Card to Set White Balance
There are two ways to do this - one in camera and one in processing. I prefer to set it in camera but exactly how to do this changes with different camera makes and models, but for my Canon you do the following:
1> Take a photograph of the gray card near to where the subject you will be photographing is. This makes sure that the card is under the same lighting conditions as your subject - ideally have them hold the card over their face. Fill the frame as much as you can. (If you find you can't get focus, move the switch on the side of your lens to MF and take the picture of the grey card, then make sure to switch your lens back to AF)
2> Select the custom white balance from the camera menu. It will ask you which picture to use - choose the one you have taken of the gray card.
3> Make sure you change your white balance setting from AWB to custom white balance - it will now set the white balance correctly for you! You don't need to change this again unless the lighting changes.
(Just for reference, this is what happened when I asked my son to hold it over his face...)
Moving swiftly on........
The second way is to use it to change your white balance in post-processing when you are editing your photos either in Camera RAW or Lightroom. To do this you:
1> Take a picture of the scene as normal but with the gray card in it (again making sure that the card is in the same lighting conditions) Remove the card and carry on taking the rest of your shots.
2> In Lightroom / Camera RAW open the image you have taken with the grey card in it. Click on the grey card with the white balance dropper and it will change your white balance to the correct one. You can set all your images taken on the same session with the same white balance by using the Sync button.
How to Use a Gray Card to Set Exposure
You can also use a gray card to get an accurate exposure. You certainly do not need to do this all the time, but in some situations it can be helpful. To do this when shooting manually you:
1) Select centre or spot metering mode on the camera and put your card into the same area as your subject is/will be.
2) Half depress your shutter when focusing on the grey card.
3) Dial in the settings (aperture, ISO and shutter speed) in manual mode until the meter reads at 0.
4) Take away the gray card and ignore the change in the meter reading - the little meter will jump about as you move the camera - and start taking your pictures. You do not need to change your settings unless the light changes.
If you are using AV Aperture Priority it's the same until Step 2, then:
3) Dial in the Aperture and ISO you want and your camera will change the shutter speed to get your meter to zero. You then need to lock in these settings by using the exposure lock facility on your camera. (There is usually a dedicated button for this on the back of your camera - all makes and models vary so you will need to check your camera manual)
4) Once you have locked in the settings you can compose your picture, start shooting, and the exposure will not change. You do not need to change the settings unless the light changes.
It might not be exact, but remember you can manually change the exposure if it doesn't seem quite right, but generally speaking, it should be pretty darn close.
Once you get to know your camera and how to compensate for different lighting conditions by shooting a couple of stops over or under then you might never use it for exposure (and that is something that we cover how to do in Auto to Awesome- so you don't always need to have a a grey card to hand)
My personal opinion (for what that is worth!) is that for the couple of dollars they cost they are worthwhile having - either these or an Expodisc can be really helpful. Firstly, so you have them in situations where the white balance or lighting is tricky, and secondly to develop your eye for correct white balance and exposure - both things we all struggle with starting out!