Light Metering
Technique - Technique

LIGHT METERING

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The very Basics ... or 18% gray

Metering systems are calibrated to a certain light value in order to gurantee constant exposure settings: 18% gray is commonly accepted because a typical scene seems reflect the same amount of light as this gray value. As a framework for comparison all these colors reflect light like 18% gray:
 

Usually this assumption works pretty good but if you expose a scene with a majority of bright colors/grays without compensation in spot or center-weighted mode the camera will darken the picture to 18% gray - the result is under-exposed. On the contrary a scene with lots of very dark colors/grays will be lightened up to this gray value - the result is over-exposed. The camera cannot differ between a white wall in shade or a gray wall in sunshine because the amount of incoming ligth is identical. Most cameras are blind in regard to colors - with just one exception (the Nikon F5) they analyse the world based on different light values.

Negative Example (w/single segment metering system):

real scene with -say- 
10% gray on the avarage

the camera sees image as if it's 18% gray

------>

resulting image
18% gray

Typical spots with risks for under-exposure:

  • Beach
  • Snow
  • cloudy Sky

Examples with risks for over-exposure:

  • Forest
  • Lakes
  • Dusk or dawn (lots of shadows)

Dependent on the situations you'll have to compensate up to 2 EV to longer or shorter shutter speeds (fixed aperture) or more or less f-stops (fixed shutter speed). Just "shifting the program" doesn't change the amount of light transmitted to the film.

Note: Print films are very tolerant to wrong exposure while slide films require an accurate metering result.  1-2 EV difference compared to the optimal setting doesn't make a difference with prints whereas a slide image is usually quality for the trash bin in such a case.


Common Metering Modes (SLR-cameras)

Metering Modes  Spot/Partial  Center-weighted  Multi-zone/Matrix 
Metering characteristic 
Technical:  This mode limits the metering area to the central part of the viewfinder. Spot metering covers about 1% to 3.5% of the image area (typically 3.5%). Partial metering covers about 9.5%.  Metering is averaged over the entire scene with emphasis placed on the center area (typically 75% center - marked dark gray and black in the icon, 25% outside).  The exposure setting is calculated based on data from matrix/segment elements (3 to 16 or more, typically 6) taking into account such factors as the focusing point in use, subject size, position, distance, overall lighting level, front and back lighting and color. 
Use this mode ...  ... when there're big differences in brightness
(e.g. between foreground and background) or for subjects that require precise measurement, such as close-up photography. 
... when you main subject covers a large portion of your image (e.g. with peripheral shadows) ... for general subjects without larger shadowed or extreme bright areas. Action photography.
Advantages:  Precise metering in your control.  Metering mode for general scenes with relatively easy exposure compensation control. Comfortable with good reliability. 
Problems:  Heavy variations in metering results. Bracketing is surely a good idea for the more extreme situations.  Without compensation risk of over-exposing for scenes with major picture areas covered by sky.  No control of the exposure settings - you simply don't exactly know how the camera weights the elements. 

The following illustation provides an impression about the way modern matrix metering systems work. Basically the camera processes the metering pattern by weighting the data with the selected AF sensor(s) and compares this information with an an internal scene database. Usually it'll try to avoid extreme compensation factors to reduce the risk of a total misinterpretation of a scene. Unfortunate this policy will often fail especially in contra light situations.
 


Note: The exposure center in the pictures below is marked with .

This means I've centered the viewfinder to this spot, pressed the exposure lock key (or hold the shutter release button) and shifted the scene back to the final image before shooting.


Typical scenes for Spot or Partial Metering

You should be aware of the followings points:

  • Don't meter to the brightest spot of the scene - this would lead to underexposed results. Try to find a place which has to be exposed right but always think of the limitation of the metering system.
  • Even spot or partial metering cover a substantial area of the picture (up to 9.5%). E.g. the partial metering in the big picture (slot canyon) above points to a very bright spot but it surely contains some of the shadowed areas - in this special case it leads to a correct result.
  • Normally print films can process a range of around 7 EV=f-stops (slide film: up to 5 EV) so you should choose spots which are a little bit darker than shown in the pictures above. If the amount of contrast in a scene exceeds the limitations of the film your main subject will be outshined or a "skyline"-effect will occure. You can use the latter as a stylistic element like in the picture to the lower left (looks better projected than scanned).

  • Trick:
    You can gain 1 EV by pre-exposing the film:
    • switch to multi-exposure mode (#2)
    • pre-expose with a high-shutter speed and small aperture to a blank paper (e.g. 1/2000s, f/22)
    • expose your picture (non-compensated)

    "Black" will probably not as dense as normal but there shouldn't be any further side-effects.


Typical scenes for Center-weighted Metering

Try to find a larger area which has to be exposed right and -again- think of the limitation of the metering system. In critical situations try to compensate manually which can be well controlled in this mode or change to spot metering.


Typical scenes for Matrix or Multi-zone Metering

Multi-zone metering systems are very easy to use. In primitive cases just POINT & SHOOT. Manual compensation isn't very reliable with difficult scenes because you simply don't know exactly how the camera calculates the settings. After some usage you may get a certain feeling for its reactions in certain situations.


The text above may help you to get a technically correct exposure. 
However for adding contents to your pictures you should follow this: 

    David Muench:

"Prepare thoroughly. 
   Go to the sacred places. 
     Wait. 
       Look. 
         Feel. 
           Then shoot !" 

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