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April 20, 2011 / arthill

Metering


When I started shooting cameras didn’t even have exposure meters.  Basically you guessed, based on your experience.  Or you carried a little card which had suggested f-stop and shutter speed for a given film.  “Advanced” photographers carried a separate exposure meter.  You could point the meter at your subject and see what f-stop and shutter speed to use.  You could even take readings from several parts of the scene and mentally average those to come up with your settings.  Eventually meters were built-in to consumer level cameras.  As you might expect they had two modes: average and spot.  With average mode the camera came up with settings based on the entire scene.  With spot mode only the center of the scene (maybe 5%) was read.

Average metering was often fooled.  If you had a bright subject and a dark background neither was properly exposed.  Rather, the camera settings resulted in an exposure for something in between.  The next improvement was center weighted averaging; as the name implies the center of the scene was given more weight than the rest in determining exposure.  This worked pretty well and came to be the way most serious photographers got used to shooting.  As you can imagine this wasn’t particularly good if the subject wasn’t in the center of the scene but we all learned to center the subject for the metering reading and then recompose the shot to put the subject where we wanted it to be.

Evaluative or matrix: Today’s sophisticated digital camera takes metering to a whole new level.  The main default metering method for your camera is called evaluative or matrix or something like that.  Sophisticated computer algorithms in your camera take light readings from as many as several dozen points within the scene. The computer also notes which focus point has been activated and treats that area as the subject, giving it extra weight.  Then, it compares the information it has obtained to a database of typical scenes and set the f-stop and shutter speed accordingly. It will even apply exposure compensation if it determines that you have a backlit subject.  Whew!  It is truly amazing.  So, most of the time you will be fine with the default evaluative or matrix metering. If you’re shooting in Program or Auto mode with a DSLR or using a point and shoot this is what you’re going to get and it will work well almost all of the time. The gray area in each diagram below shows how much of the scene is used by that metering mode.

image

In this one the camera gives extra weight to the zone where the autofocus point is set.  See the discussion of autofocus points in Are your photos as sharp as you like.  Note that if you’re doing manual focus the metering system uses the central focus point.

Your camera may however support additional exposure modes. My Canon T1i supports three additional ones.

Center weighted averaging: This takes readings from all over but gives extra weight to the center of the viewfinder.  I suspect this is included because so many photographers have used this method or so long that they are comfortable with it.  I never use it.

image

Spot metering: This only uses the information from the very center of the viewfinder (about 4%).  It is useful for getting an accurate reading of a single close-up subject.  I use it mostly for light colored flowers.

image

Partial metering: This takes a reading of about 9% of the viewfinder and is useful when the background is much brighter than the subject (e.g. backlighting). image

My advice is to use evaluative or matrix metering most of the time and switch to spot metering for situations where you want to make sure that a light or dark subject is properly exposed.  Fortunately you can check how well the camera is done after you take a picture by looking at the histogram displayed on your LCD.  I’ll cover that subject in a future post.

And, as good as the metering systems are now there are still situation where they can be fooled.  You’ll want to learn how to recognize those situations and use exposure compensation to get the best picture.  I’ll also cover that in a future post.

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3 Comments

Leave a Comment
  1. Debbie / Apr 21 2011 10:30 am

    Thanks, Art! This is still an area where I lack confidence. I always appreciate a detailed explanation, and hope that someday the facts will sink in permanently and become a part of my working knowledge.
    Informative post!

  2. Howard Hull / Apr 21 2011 5:06 pm

    Thanks, Art. This is very helpful info. I appreciate your expertise!

  3. Maryann Goldman / Apr 28 2011 11:08 am

    I think this is a very clear explanation. I use evaluative most of the time but do switch to center weighted average or spot sometimes. I find birds in branches where there is a lot of clutter may benefit from using spot.

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