The Mystery of Exposure - Part 1

Ah...exposure.  That dreaded word that novice and experienced photographers alike hate.  Well, contrary to popular belief, the principles of exposure are really not all that complicated.  It just takes a basic understanding of what is involved.  Exposure is a process of three separate but related functions.

            1. The mechanics / functions of the camera
            2.  How the camera functions relate to each other
            3.  How to use that knowledge when taking a photograph

Blog space does not allow for an in depth description of these elements, but what I'd like to do is break it into three parts and briefly discuss each.  With any luck at all, I hope to clear up some of the mystery and maybe get you to thinking differently about what you're doing when it comes to taking a picture.

Let's begin with the mechanics.  Exposure is made up of four things...or the Exposure Triangle:
                         ISO
                 White Balance
             Aperture  /  Shutter
            
Some people think that white balance is separate, but for arguments sake, I'm going to include it in the discussion because it does affect the visual outcome of the image.

Aperture:  The aperture is what regulates how much light enters the camera. It is a function of the lens. Think of it like using a faucet to fill a glass with water.  Turn the faucet to where a trickle of water comes out and it takes a long time to fill up a glass. Turn it wide open and the glass fills up quickly.  The same thing applies to the lens aperture and exposure.  Allow in just a trickle of light and the exposure process takes longer.  Allow in a lot of light, and the exposure takes less time.  It's really that simple.  But you may be wondering about all those numbers on the lens or the ones that keep flashing on the camera's display.  An aperture is designated in what are call  f/stops...F followed by a number like 16 or f/16.  F stops are actually calculated using a mathematical progression based on a factor of 2...but you don't need to worry about that. All you really need to know is that the larger the f/stop number, the smaller the actual lens opening.  The smaller the f/stop number, the larger the actual lens opening.

Take a number like f/16 or f/22.  These are large f/stop numbers but they represent a rather small aperture setting and allow in a small amount of light.  A number like f/2.8 or f/3.5 are small f/stop numbers, but they represent a large aperture and allow in a large amount of light.

Aperture is also a factor in what is called Depth of Field or DOF.  I'll talk more about that in part 2, but basically DOF is what portion of the image remains in focus from foreground to background.

Shutter:  The shutter is an electro-mechanical devise that regulates the length of time the light is allowed to enter the camera.  It is usually represented in some fraction of a second such as 1/10th of a second or 1/125th of a second.  The shutter and aperture work together to generate the mechanical exposure process.  Pretty straight forward.

ISO:  Understanding ISO is actually quite simple, it's understanding how it affects the combination of aperture/shutter sequence that is confusing (more on this later).  ISO is simply the sensitivity to light setting that your digital camera's sensor is set to...or how sensitive the film is to light if you are using film.  ISO usually begins at 100, but can go lower...and then it progresses upward doubling in sensitivity as you go up the scale.  So, ISO 200 is twice as sensitive as 100.  ISO 400 is four times a sensitive as 100...and so on.

White Balance:  Different kinds of light have different properties and light temperature is defined using a temperature scale called the Kelvin scale.  On the Kelvin scale, very cool light, like cloudy skies, or twilight skies has a very high Kelvin temperature, as high as 10,000 degrees Kelvin. Very warm light has a low Kelvin temperature...down around 1800 or so...kind of opposite of what you might think.  The source of the light also affects the Kelvin temperature.  Candle light is very warm and reddish in nature as is indoor light from a tungsten light bulb. Light from those kinds of sources have very low Kelvin temperatures...like 1800  to 2000 or lower. Middle of the day, blue sky light has a temperature of around 5500 Kelvin.  Outdoor light is different than indoor light and the digital camera must be able to distinguish between the two or your images will not look normal...they could be too blue or too red.

Okay...we've briefly defined all the elements of the exposure triangle.  Next time, we'll talk about how the elements of the exposure triangle interact with each other.  You don't want to miss that part as that is when I get into the nitty gritty of how the camera does what it does.

Keith