A look at how reflective light meters work in helping you to set the camera controls for an exposure appropriate to the scene.

1. Metering Basics
This primer on using light metering will guide you through the use of
measure the amount of light
measure the amount of light photographic gray scale and how to
meters, metering modes, the
compensate for subject tonality in any given scene. scene
reflecting from the scene
falling on the
As part of the discussion, we will also look at the responsibility the
photographer bears when pointing his or her camera at a subject.
Michael E. Stern
Photography Education Consulting
626-298-6747
CyberStern.com

2. Metering Basics
Reflective Meters Incident Meters
Measure the amount of light reflecting Measure the amount of light falling on the
from the scene scene
Must be positioned at the subject
Are convenient because they can be used location, or at least in the same light as the
from the camera position subject
Either hand-held or in-camera Always hand-held
Assume light is reflected from a scene Are unaffected by the scene’s tonality
that is 18% grey. This fails to account for because they measure the light before it
the effect of the subject’s tonality or reaches the scene.
reflectivity.

3. Metering Basics
It is important to grasp the concept that in-camera reflective meters average
the reflected light from the subject into a mid-range value. This is called 18% or
middle gray.
The meter does not recognize color but rather the lightness or brightness
value of a color. For example the meter sees light gray and not the light yellow
of a flower. The meter sees a dark gray apple instead of a red one. As such it is
very important to ensure you are pointing the camera at a full range scene
when in evaluative mode or at a mid-tone value if in spot mode. This takes
practice and a trained eye.
What we see How the meter interprets the scene

4. Metering Basics
Metering Modes
There are several metering schemes in most digital cameras, these are the top two:
1) Spot - the meter looks at a small area in the center
of the view finder, ignoring everything else in the scene
2) Evaluative /Matrix Metering - Viewfinder is divided into numerous metering zones
to which all the AF points are linked. After detecting the main subject’s position,
brightness, background, front and back lighting conditions, camera orientation, the
camera sets the proper exposure.

5. Metering Basics
Metering Modes
Spot metering and evaluative are the two choices most often used by
professionals. They are the most accurate because it is clear where in the scene
the light measurements are being taken from, either dead center or off the entire
sensor.
When to choose one over the other? When the sun or other light source is at
your back and the subject has a full range of colors and tones (think group
portrait, crowd scene or landscape) use evaluative.
If the main light source is at a position other than your back, use spot metering
mode and point the camera at a part of the scene that would look like medium
gray if it had no color.

6. Metering Basics
Exposure Modes
The three exposures modes we will concern ourselves with are
Manual, Tv and Av.
Manual - the shutter speed and aperture are set by the photographer.
Av (aperture variable) - the aperture is set manually and the camera
selects the proper shutter speed.
Tv (time variable) - the shutter speed is set manually and the camera
selects the proper aperture.

7. Metering Basics
Exposure Modes
Why work in Av mode? If the image you are attempting to make requires a specific
f/stop (for controlled depth of field), select this mode. Pick a large aperture number
(small opening) and the camera does the rest. If you need a shallow depth of field,
pick a small aperture number (large opening) and the camera does the rest.
Why work in Tv mode? If it’s important to freeze the action, select a fast shutter
speed (1/250 or higher) and the camera does the rest. Conversely if it’s movement
or blur you are trying to create, use a slow shutter speed (one-eighth second or
longer) and the camera selects the appropriate aperture.You may need a tripod to
help steady the camera and your nerves.
These decisions are driven by what you are trying to accomplish. Make the attempt
to know what you’re trying to accomplish beforehand and it will make choosing an
exposure mode a bit less stressful.

8. Metering Basics
Tonal Scale
When in manual, Av or Tv mode, the camera must be set to read off the entire viewfinder grid or
just in the center spot. When in Spot mode the meter is taking its’ light measurement from the
center of the viewfinder/grid. If set to Evaluative (Canon) or Matrix (Nikon) mode, then the entire
viewfinder/grid is measured before setting the camera controls.
In photography the tone/gray scale is an important concept to grasp.
0 128 255
It is crucial to remember that reflective meters only see tone (not color) and want to “average” all
the tones in any scene/subject towards a mid-gray value.

9. Metering Basics
Tonal Scale
0 128 255
Reproduction of shadows and highlights occurs after 0 and before 255. Depending
on your printing device, the shadows can reproduce as low as 16 and the highlights
can reproduce as high as 250. When using a gray card in a scene and using that same
gray card to for measurement after-the-fact in Photoshop, 128 will not be obtained
because of the inconsistencies in the manufacturing of gray card material.
For this reason we concentrate on measuring the white-with-detail numbers
instead. This is a better guide for exposing to obtain consistent reproduction values.

10. Metering Basics
These examples demonstrate how a reflective meter can be
fooled by subject tonality......

11. Metering Basics
Summary
When you point your camera at any subject, it is your responsibility to point
the camera at a color that if it didn’t have a hue associated with it, would be
a middle gray value. If you’re lucky and have a mid-gray, then things just got a
lot easier.
All is not lost if you don’t have a mid-gray value, this also works on black or
white too. But you must make an adjustment to compensate for the steps
away from mid-gray that both black and white are.
This is the adjustment to make: open up two stops if you meter off white
and stop down two stops if you meter off black.
This is the rule but of course you will make on-the-fly adjustments when
applying this rule determined by exactly what you are shooting and what
your aesthetic and end use is for a particular photograph.

12. Spot Meter off of gray card

13. Spot Meter off of white card

14. Spot Meter off of black card

15. Metering Basics
Let’s see what happens when we drain the color information from
the scene we have been looking at.......

16. Luminosity Values
RGB Values

17. Luminosity Values

18. Metering Basics
Summary
Depending on circumstances you may have to adjust the exposure
when measuring a middle gray. Usually this will be less than one stop.
For both black and white values as your target for measuring
reflective light, the rule of thumb is -2 (black) and +2 (white) but you
have to look closely at the tone you are measuring from. Adjust
while using the camera’s historgram as a guide. Be sure it is set to
RGB and not luminosity.
Mid Gray Black or Dark Gray White or Light Gray
Minimal Correction -1 or -2 Stop +1 or +2 Stop
(if any) Correction Correction