Reflective Metering Primer

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

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Reflective Metering Primer

  1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 10. Metering Basics These examples demonstrate how a reflective meter can be fooled by subject tonality......
  11. 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. 12. Spot Meter off of gray card
  13. 13. Spot Meter off of white card
  14. 14. Spot Meter off of black card
  15. 15. Metering Basics Let’s see what happens when we drain the color information from the scene we have been looking at.......
  16. 16. Luminosity Values RGB Values
  17. 17. Luminosity Values
  18. 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

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