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19 Feb 2014 - How to use a Grey Card

This is a tutorial I wrote for my camera club's newsletter.

A grey card can be used to do two things:

1. Set a white balance
2. Set proper exposure

First, what is a grey card?   To state the obvious, it is a card, fold up reflector or even a sheet of paper that is the colour grey.   Wikipedia has this to say:

A grey card is a flat object of a neutral grey colour that derives from a flat reflectance spectrum. A typical example is the Kodak R-27 set, which contains two 8x10" cards and one 4x5" card which have 18% reflectance across the visible spectrum, and a white reverse side which has 90% reflectance. Note that flat spectral reflectance is a stronger condition than simply appearing neutral; this flatness ensures that the card appears neutral under any illuminant.


Sounds very technical.  The important parts of that statement are the 18% and flatness.   In other word it is a flat (non-reflective) grey card.
Set a White Balance

There are two ways of doing this.  You can set a custom white balance in the camera or you can use a reference photo to set the white balance in your editing software.  Setting the white balance in the camera is the easiest because once you do it you are done for all photos until you change the white balance to another setting or the lighting conditions change.   This method also appeals to those who like to get it right “in camera” and prefer to as little post processing as possible.

Setting White Balance in Your Camera

The details on how to do this will vary slightly by brand of camera and even model of camera in some cases.   The basics all remain the same but consult your camera’s manual for details.

1. Take a photograph of the grey card near to the subject you are photographing.   Make sure the grey card is getting the same light as the subject.  If a portrait it is common to have model hold the card in front of their face.  Try to fill the frame with as much of the card as possible.  This helps the camera’s metering system.
2. Select the custom white balance on your camera (Canon often has a button on the right of the LCD, Nikon has the same on the left or can be a menu option on these and other cameras).   Your camera will ask for a photograph to use setting the white balance.  Select the image you took of the grey card.
3. Before taking another photograph, access the white balance settings on your camera and select the custom one.  This will use the white balance set in the previous set.
4. Repeat the steps as lighting conditions change.  Custom really means custom to the light conditions as you set them in the first photo.   If the light moves or you move, change the white balance!

The last step is an important one, especially if you shoot jepgs.  You must reset or change the white balance when the lighting conditions change.  If you are shooting the same thing for several hours outside, the sun is going change position and change your lighting.  If you move to a different location, you need to change the white balance again to match the lighting conditions there.  

If you forget all is not lost because you can fix an incorrect white balance in your editing software, but that defeats the purpose of setting it in the first place.   The warning for jpegs is because the white balance is harder to correct after in your editing software than it is for raw photos.   It can still be done as long as the white balance is not too far off.  

Setting the White Balance in your Editing Software

The way to do this in your editing software varies of course.   I will cover Photoshop, Photoshop Elements and Lightroom.

Photoshop and Photoshop Elements

Photoshop uses Bridge and Elements uses the Organizer to catalogue your photographs.  Open whichever you have and we will start from there.

1. Select the photograph with the grey card plus any others that are lit under the same lighting conditions.  
2. Open all of these images into Adobe Camera Raw (if jpegs you can still do this be using the File Menu and Select “Open in Camera Raw” or Open As Raw, the actual menu option varies depending on your version of the software).
3. In ACR select the photo with the grey card in it.  This photo should be visible in the main window.
4. Select all the other images by clicking the Select All button.
5. Select the White Balance tool (the eye dropper) icon from the menu bar.
6. Having the eyedropper tool selected, click on the grey card.
7. If they are raw files, continue on with any other adjustments you need to make in ACR.  You can also make adjustments to jpegs in ACR as well.
8. Select Done to save the images or Open Images to do further editing in Photoshop or Elements.

This will have set the white balance for all of the selected photos.  

Lightroom

In Lightroom the process is slightly different.  We will start after you have imported the photos.

1. Open the photo with the grey card in the development module.
2. Select the White Balance tool (the eye dropper) from the Basic panel.
3. Click the eyedropper on the grey card in the photo.  The white balance is now set.
4. Select all of the other photos that are lit with the same lighting conditions.
5. Select the Sync button at the bottom of the right hand series of panels.
6. Uncheck all of the boxes except the White Balance check box.
7. Click on the Synchronize button to apply the custom white balance on all of the selected photos.

Using a Grey Card to Set Proper Exposure

Grey cards, for being so simple, are a multi-purpose tool.  Not only can you use it to set a proper white balance but you can use it to set a proper exposure as well.   Often very bright scenes, such as snow, can fool the meter in the camera to underexpose.  The opposite can also happen with a very dark scene leading to over exposure.   A grey card can fix both.

1. Select a centre weighted or spot-metering mode on your camera (the name of this may vary from Canon to Nikon to Sony, etc).
2. Fill the frame with the grey card.
3. If using full manual mode you can adjust your aperture and shutter speed to get the correct exposure from the grey card.

Or

3.  If, like most, you are using a semi-auto mode like Av (A) (aperture priority) or Tv (S) (time value or shutter priority) you use the exposure lock (usually a button marked with * ) on your camera when you point at the grey card.  Check your camera’s manual for full details for your camera.

If you are using manual mode of your camera you can recompose and take your photo.  However, if you are using Av or Tv you will probably need to keep holding the Exposure Lock button while you recompose and take your photo.  

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