Yesterday's post threw up a couple of questions on how to use a gray card so I thought I'd answer them in another post so I could do it fully.
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. I use 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 18% gray and non-reflective.
You can use a gray card for two things - setting white balance and measuring exposure. Let's look at these both in turn.
How to Use a Gray Card to Set White Balance
There are two ways to do this. First is to set your camera's custom white balance feature. 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. Fill the frame as much as you can.
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. Come out of menu settings.
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.
The second way to do this is to use it to change your white balance in post-processing when you are editing your photos either in Camera RAW or Lightroom 4 (both work the same on RAW images, but you can also do this with JPEG's too in 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.
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 (more on these modes coming in a following post) 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 (assuming there is one). 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.
Do I REALLY need to do this?!
90% of the time when taking pictures of your children, you will not have the time to do any of this and I rarely do either. However if I am doing some planned pictures, or I am taking pictures in difficult lighting conditions, then I probably will use the gray card to set the white balance, and since it is out anyway it is pretty easy to set your exposure too using the same card. Again, with kids they might stay too long in the spot you have chosen for them so it can be a bit of a hit or miss sometimes!
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 but you do need a good understanding of light awareness to do this (I'm not there yet!) You can always do a test shot first with the camera meter set to 0, and see if the image is correctly exposed - if not manually dial in your settings so you are over or under exposing from what your camera is telling you. (This is pretty much what I was doing yesterday in the middle of the three pictures) You can use your histrogram to help you with this if you know how to read one.
Lastly, you can of course fix exposure issues (both when using RAW or JPEG format - RAW just gives you a lot more wiggle room) after the event in editing but you may find it easier to get it right in camera to cut down your processing time. There is sometimes also the issue of noise to consider - if using high ISO's you will get a lot more noise if the image is under-exposed, even if you fix the exposure later. White balance can be tricky to do in editing even with handy tools like the white balance dropper they have in Lightroom - you still need to have something neutral in the photograph (i.e something that has no colour) to set the white balance without having to do it by eye.
My personal opinion (for what that is worth!) is that for the couple of dollars they cost they are worthwhile having. 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 - things we all struggle with starting out.
I hope this has answered any questions on the use of gray cards. If not, again feel free to leave a comment!
Other Posts You Might Like:
Setting White Balance
Shooting in Manual Mode
RAW Vs JPEG