The Mystery of Exposure - Part 2 - B

In part one we defined the four parts of the exposure triangle...Aperture - Shutter - ISO - and White Balance. So now lets expand on that basic knowledge and see how everything works together.

Years ago before the advent of the light meter, photographers used a technique called 'The Sunny Sixteen Rule' to help them with their exposure settings.  The sunny sixteen rule works like this.  On as bright sunny day, the basic exposure is set based on ISO (ASA way back then).  If the ISO is 100, then the aperture is set to f/16 and the shutter is set to the nearest equivalent of the ISO or100...in most cases it would be 125th of a second.  ISO = 100 - Aperture = F/16 - Shutter 125th of a second.  This setting will produce an image basically the way the scene is observed and would be considered the photographers starting exposure from which they could make adjustments up or down depending on the results they wanted and the lighting conditions.

An important concept to understand is the concept of exposure equivalent.  Exposure equivalent works like this: Remember how the aperture scale works...as you go up the scale or use larger f/stop numbers, the smaller the aperture actually becomes...down the scale and the aperture grows larger. The basic aperture settings or f/stops are based on what are called full stop increments.  An f/stop of f/16 allows in half the light of the next lower down f/stop of f/11. F/11 is down the scale from f/16 so it is a larger aperture opening.  F/22 on the other hand is up the scale from f/16, so it allows in less light...exactly half the light of f/16.  Try to visualize this as you work up or down the f/stop scale.  From f/11 down the scale is f/8.  How much light does f/8 allow when compared to f/11....think about it now...well...f/8 allows in twice as much light as f/11.  As you work up or down the scale the same increments apply. Go up the scale, the light is halved with each full stop increment...down the scale, the light is doubled with each full stop increment. (most cameras today allow for less than full stop increments usually in 1/3rd stop, but the basic scale is set for one full stop increments between the f/stop settings.  Here's the basic scale:  f/1.4- f/2.0 - f/2.8 - f/4.0 - f/5.6 - f /8.0 - f/11 - f/16 - f/22 - f/32

(Do you see the pattern? Do you see how the factor of 2 applies?)

The shutter speed undergoes a similar progression.  1/15th of a second is twice as long as 1/30th of a second and using the same aperture the combination of shutter / aperture will allow in half as much light...in other words f/8 at 1/30th of a second allows in half the light as f/8 at 1/15th of a second.  Heres the basic shutter speed scale starting with 1/15th of a second:  1/15 - 1/30 - 1/60 - 1/125 - 1/250 - 1/500 - 1/1000. Again, most cameras allow for increments of less than one full stop between the basic scale settings...for instance between 1/500 and 1/1000 will fall 1/750th of a second or a half stop differrence.

Okay...so what does that mean.  Remember the sunny sixteen rule.  Under equal lighting conditions if you change the f/stop from f/16 to say f/11...what happens to the volume of light?  What happens then to the shutter speed?  To keep the same exposure equivalent, the shutter speed must increase or speed up.  Why is that?  Well, think about it. F/11 allows in twice as much light as f/16, so the shutter speed would have to increase to avoid overexposing the image.  From 125th of a second, the shutter speed would need to increase to 1/250th of a second.  What would happen if you went to f/8...where would the shutter speed have to go?...think about it...F/8 allows in twice as much light as f/11, so under the same lighting conditions, the shutter speed would have to increase again to the next full stop setting.

Sound a bit confusing?  It's really pretty simple once you begin to visualize what is happening.  Open the aperture to a larger setting, more light is allowed in.  When more light is allowed in, the shutter speed must compensate by speeding up accordingly to keep the same exposure.  Keep in mind that most cameras can increment the aperture and shutter in fractions of a full stop...something like 1/3rd intervals.

Maybe this will help.  F/5.6 @ 1/125th of a second will generate the exact same exposure as f/8.0 @ 1/60th of a second.  Why is that?   Exposure equivalents.  They both allow in the exact same amount of light..they are just using two different setting to accomplish it.  One uses a faster shutter speed with a larger aperture while the other uses a slower shutter speed with a smaller aperture...the end results are basically the same.  

The concepts just described were based on an ISO of 100.  If you change the ISO to say 200, what would happen?  Well, conceptually everything is the same, its just that your basic setting must adjust for the more sensitive ISO setting.  The sunny sixteen rule would shift only in that the starting shutter speed would change to the nearest equivalent to 200, or probably around 1/250th of a second instead of 1/125th.

One more quick concept before we move on.  Let's talk about Depth of Field.  Depth of field is a subject better left for a discussion on composition, but because aperture is a key component of determining the DOF I want to at least define it here.  DOF is basically that portion of an image that remains in focus from foreground to background.  Several factors affect DOF...focal length of the lens, distance from your subject, distance of the subject from the background, and aperture.  Simply stated the larger the aperture (f/4.0...f/3.5...f/2.8) the narrower the depth of field.  A small aperture like f/16 or f/22 will provide a very wide DOF.  A narrow DOF is an effective way to isolate your subject (top image) while a wide DOF is preferred when photographing landscapes because you want the foreground all the way to the background to be in focus (bottom image).

Okay...again space doesn't allow for an in depth discussion...but I hope part 2 at least gets you thinking more about what is happening when you start turning dials and pressing buttons on your camera.  The auto exposure process built into your camera basically follows the same rules just described and helps to take away a lot of the guess work.  The problem is, the auto exposure system doesn't always select the best exposure.  Theres a reason for that.  In part 3, we'll discuss how the camera sees light and how that affects the image results.  You just might be surprised to learn that the camera doesn't see light the same way we visualize light...understanding this is a key concept to improving your photos.

Keith Bridgman