When will your camera's light meter get the exposure wrong! 

The photos below are an experiment to demonstrate how any in-camera light meter (i.e., reflective light meter) can be fooled into under or over exposing a scene If the objects being metered in the scene are not middle grey in tone.

For this experiment I used an old Kodak 18% grey card, and 8 different colored sheets of simple construction paper. The 8 colors were 1) white 2) light brown 3) blue 4) light green 5) orange 6) yellow 7) red 8) black.

The lighting was exactly the same in all of the shots, and I used a Pentax spot meter to get the indicated exposure. (this spot meter is highly accurate and measures the scene with a 1 degree circle). Each colored square has the exposure written along the bottom so you can see the exact exposure used. I kept the aperture constant at f 2.8 so only the shutter speed varies.

The first square is the 18% grey card.   The spot meter indicated an exposure of f2.8 at 1/80 of a second. 

The next 8 shots were all taken at the same exposure - f 2.8 at 1/80 of a second (i.e., the exposure indicated by taking a reading off the Kodak grey card).  All the colors are accurate because we used the exposure obtained by metering off the grey card.  Notice that the white sheet looks white and the black sheet looks black.

 
For the 2nd group of shots, I took a reading with the spot meter directly off each colored sheet instead of using the grey card. 

When you look at these shots, you will see that the exposure varies from the "grey card" reading of f 2.8 at 1/80 on ALL of the shots. 

The white sheet now looks grey.  This is exactly what happens when you meter off a white wedding dress.  It's now 2 1/3 stops underexposed and it middle grey in tone.

The brown and blue sheets are only 1/3 of a stop underexposed but you can clearly see that the colors are a bit darker when you compare them with the first set.

The color of the green sheet is 1 stop underexposed.  The color has gone from a teal color to true green.

The orange sheet is 1 1/3 stop underexposed and now looks more rust colored.

The yellow sheet is 1 2/3 stops underexposed and the color has changed dramatically.  

 The red sheet is 2/3 of a stop underexposed and the color is a bit darker and richer.

Finally, the back sheet is 1 stop overexposed, and as expected, now looks grey.

So when we metered directly off the colored sheets (instead of the grey card) why did we get the wrong exposure?

Because each of these colors has a different reflectivity property from the grey card. In fact, all of the colors reflect more light that the grey card with the sole exception of the black colored sheet. That is why all of the shots in the second group are underexposed, except for the black sheet.

If an object reflects more light (than middle grey), the meter will measure the brightness (luminosity) as being greater than it really is. If the meter sees the scene as brighter, it will select a faster exposure and your shot will be underexposed. For example, if you meter off a white wedding dress than you will be about 2 stops underexposed and the dress will look grey in the final shot.

In this experiment, we found out that all of the colored sheets except of the black where more reflective than the grey card.  Since they were more reflective, the meter's assumption that colored sheets had the same reflectivity as middle grey was wrong.

Here is a Table of the exposure differences that occur when you meter off each colored sheet:

White:                            - 2 1/3 stops
Light brown:                - 1/3 stop
Blue:                              - 1/3 stop
Light green:                 - 1 1/3 stop
Orange:                         - 1 1/3 stop
Yellow:                          - 1 2/3 stop
Red:                               - 2/3 stop
Black:                            + 1 stop

One of the challenges that often occurs, particularly in landscape photography, is to identify the middle grey tone on the scene that you are framing.  We can see from this experiment that the light brown and blue sheets were close to middle grey in brightness, but this was far from obvious by just looking at the sheets.

 All of this leads into the discussion of the Ansel Adams Zone System.  Stay tuned.