The Mystery of Exposure - Part 3

One of the biggest mistakes novice photographers make is to believe that the camera will always give you a correct exposure for every lighting circumstance.

In part 1 we looked at the elements that make up the exposure triangle and followed up with a discussion in part 2 on how those elements work together.  In part 3 we're going to look at how the camera sees light and how the metering system applies the exposure values.

Have you ever returned from a vacation with a stack of photos to share only to have your friends quickly thumb through them..."Those are very nice..." they say.  Sort of deflating...as you say "Well...you had to be there...the pictures just don't do it justice."  There's a reason for that.

What we see visually can often be spectacular because of how our eye and brain work together to create an image in our mind.  We visually are able to distinguish between a wide range of contrasts, colors, and depths and are able to filter out all the clutter.  Contrary to popular belief, the camera does not see light the same way we visually perceive light.  A camera can only evaluate the intensity of the light that enters through the lens...then based on that evaluation make a programmed guess as to what the exposure should be.  That programmed guess is based on what is call the 18% neutral gray value.

The Thru the Lens or TTL metering systems built into cameras today are sophisticated computerized processes and will for the most part do a pretty good job of determining a workable exposure solution.  There are variations in the way they work, but all metering systems fundamentally do the same thing...They simply  take an average of the metered light and sets exposure values that are shifted toward the middle.  Remember the black to white light scale where the middle is a neutral gray.

There are three basic metering modes that TTL systems use:  Matrix or Evaluative, Center Weighted, and Spot.  Matrix/Evaluative meters the light across the entire spectrum of the view.  Center Weighted does the same thing except it places a higher value on an area concentrated around the center of the view.  Spot metering allows the photographer to meter off a specific area...or spot...like a face, or band of a distant hill...without  having the rest of the view skew the reading.  All three have their advantages and disadvantages.

Ninety percent of understanding the mystery of exposure is based on understanding how the metering systems work.  Using the strengths and weaknesses of each will give you an advantage when it comes to capturing those great shots.

Let me give you an example.  Take for instance photographing snow.  Snow is very bright and white.  Visually, even on cloudy days snow still looks white to us because our brain is able to recognize it as such.
But, the camera responds to bright white circumstances differently that what we see visually.

Allowing the camera to select the exposure will tend to make snow look a dingy gray (top image) as a result...think about it for a moment.  Why is that?  Well the camera doesn't know the difference between a snow field and coal field. The TTL meter wants to move everything toward the middle...or neutral gray.  Visualize the scale again...the white value of snow falls toward the outer white edge of the scale.  The camera wants to set an exposure that moves toward the middle...which is gray...thus white snow will often look gray as a result.

How do we get around that?  By using the Exposure Compensation (EV or AV +/-). This tool is something you need to understand and use.  On the back or top of most cameras you will see a button marked with +/-.  This button is used to trick the camera into shifting the exposure it wants to make, either more toward the lighter (+) or more toward the darker (-).  Going back to the snow scene...the camera wants to shift it toward the gray...you want it to look white...so...what would you do?

The plus (+) sign adds light to the exposure and the minus (-) removes light from the exposure.  To make the snow appear more white...you would have to add light to the exposure by using the + compensation.  Just how much depends on the circumstances...snow may require as little as +0.3 of a stop to more than one full stop higher (+1.0).  (bottom image)  Too much can create an over exposed situation...so be careful.

This concept applies to almost any bright (or dark) object including beach sand, buildings, birds, animals, even the sky..and to any color...not just black or white.  A dark red barn will have it's color shifted to a more neutral lighter red color...green grass is already a neutral color and will often remain unaffected...Are you beginning to get the idea? Just for grins...what would happen if you took a photo of a pile of coal ...very black.  You figure out what would happen and how you would compensate.  Understanding how the camera sees light and how to use the exposure compensation gives you a huge advantage.

This three part series was designed to get you to thinking about what the camera is doing and to use its capabilities to your advantage..not as a full blown workshop on exposure.  Even so, if this information helps you move forward, then the series accomplished its goal.  Todays cameras take a lot of the guess work out of the exposure process...but leave enough gaps open so you can begin to use the artistic flair that resides inside every photographer.  Knowing how to blend that artistic value with technical understanding is what separates the ordinary snapshot from artistically expressive images.

Keith