Anyone can take a snapshot, but moving beyond that and making photographs requires an understanding of exposure.  In order to get a good image, you need a good exposure.  But how this process actually happens is a fundamental concept that is difficult for those new to photography.  I’ll try to explain and demonstrate how it can effect your creative decisions.  Much of this is applicable to both film and digital, but I shoot strictly digital these days.

The word “photography” literally means “writing with light.”  Light is the essence of what photography is about.  On a practical level, light is part of the electromagnetic energy, and human eyes have evolved to perceive a range that is referred to as the visible spectrum.  EM energy is measured by its wave length, and human eyes can perceive roughly between 780 and 380 nanometers.  Beyond 380 nm is the ultraviolet region, leading beyond that to x-rays.  On the other end, beyond 780 nm is the infrared region, and eventually radio waves.

When we look at an object, we are not truly “seeing” the object itself.  What we perceive is the energy of the visible spectrum that has been reflected off the object.  Some wavelengths are absorbed by the object and not reflected, so we only detect the wavelengths of that were not absorbed, thus leading to our perception of the object’s colors.

The intensity of light is measured by its luminance.  The range of luminance from the brightest white in a scene to the darkest black is called dynamic range.  The common unit for measuring luminance is the f/stop.

F/stops describe the total light range by powers of 2.  So one full f/stop more is double the light, while one full f/stop less is half the light.  F/stops are often now referred to as “exposure values” or “EV’s”.  The human eye is capable of perceiving a total light range in the neighborhood of 24 full f/stops.  A digital camera can record around 10 f/stops of range.  Depending on the media, film and print are even less, sometimes as low as 2-3.  The important concept here is that the human eye can “see” far more than the camera can record.  These can lead to difficult exposure situations that we must deal with, in order to create a good image.

In a camera, correct exposure is a combination of three values: the f/stop, the shutter speed, and the ISO.  Many concepts and terms in digital photography were coined in the film era, and ISO is one of them.  Film and camera manufacturers needed a standardized way to indicate the films light sensitivity, and be able to set the camera to match.  If the film camera was not set to the film’s ISO (formerly called “ASA, or “film speed”, its meter could not calculate a correct exposure.  The term has stuck, and now we use ISO to describe the sensitivity setting (really the gain setting) for the digital camera’s sensor.  The higher the ISO number, the higher the film’s or light sensor’s sensitivity to light.  High ISO films frequently lead to a grainy appearance because of the larger particles in the film emulsion, and in a close parallel, digital cameras exhibit a similar “grain” effect called “noise” at high ISO’s.  With digital, usually we set the ISO first to match the situation.  The general rule is use the lowest ISO you can under the circumstances.  This will reduce the noise as much as possible.  One advantage to digital is that the ISO can be adjusted on the fly as conditions change, while film shooters are committed to the ISO of the film in the camera until the end of the roll.

The f/stop is also called the aperture, and it’s easy to see why.  If you take an older lens, one that still has an f/stop ring on it, and turn it while it is off a camera, you’ll see the aperture blades open and close.  The blades close to the selected f/stop when you press the shutter release button.  The f/stop value is a ratio of the aperture opening vs. the lens length.  So an f/stop of f/1.0 would have an aperture opening equal in diameter to the distance from the lens element to the film plane (or sensor), or 1:1.  This allows 100% of the available light to flow down through the pipe.  As the apertures get smaller, the f/stop gets higher, until we get to the smallest opening at f/22 or f/32, depending on the lens.  These let through the least amount of light.

To compensate for the amount of light flowing through the “pipe”, we can vary the shutter speed.  Shutter speeds are represented by numbers from 1 to sometimes as high as 12,000.  These numbers are actually fractions, so it would be 1over 1, or 1 second, to 1 over 12,000, or 1/12,000th of a second.  As the “hole” (f/stop, aperture) get smaller, less light can flow through, so we need to allow more time for the same amount of light to reach the film or sensor.  As we use a larger f/stop, we can use faster shutter speeds.  There is no one right combination of exposure settings.  Say we find that f/8 at 1/250th is a good exposure.  We could also use f/5.6 at 1/125th, or f/11 at 1/500th.

Okay, but why would we really want all these possible combinations?  Because it allows us creative choices based on what effect we’re trying to achieve with the image.  As the f/stop gets smaller, depth of field increases.  Depth of field is the distance in front and behind the subject that is also in focus.  For example, landscapes typically want a deep depth of field from foreground to background, so we’d “stop down” to f/22 or f/32, which of course would mean a longer shutter speed.  If we wanted a shallow depth of field to isolate the subject against an out of focus background, we’d use a larger f/stop, like f/2.8, which would result in a faster corresponding shutter speed.  Faster shutter speeds also freeze movement, while slower ones allow the subject time to move in the image, creating blur.  Both of these effects can be used creatively, depending on your goal.  Correct exposure is a balancing act of these variables to achieve the image you’re trying to create.

Cameras use an internal light meter to measure the reflected light from the subject, to arrive at a calculated exposure setting, and usually have a choice of exposure modes.  The simplest is Automatic, or Program, which sets both the f/stop and shutter speed for you.  All you do is press the shutter release.  You should almost never be using this, as it completely takes all the creative decisions out of your hands, but sometimes it can be handy if you just need to get that shot, especially in flash situations.  Another choice is Shutter Priority.  This lets you get to choose the shutter speed you want, and the camera will set a corresponding f/stop.  You’d use this, for example, at a race track where you wanted to freeze the cars as they came around a bend, or get a bit of motion blur to give a sense of motion.  Another choice is Aperture Priority.  This lets you choose the f/stop, while the camera assigns a corresponding shutter speed.  You would use this setting if controlling the depth of field was your most important consideration.  The final mode is Manual, where you set both the f/stop and shutter speed, although usually the cameras meter display shows you how far above and below the calculated exposure you are.  Manual mode can be very useful, and we’ll talk more about that later.  I typically use Aperture Priority, because I’m most concerned about the depth of field.  I also use Manual mode, and even occasionally, I admit it, Program mode.  (Yes, sometimes I have to shoot indoor family events and the kids’ birthdays.  Stick in Program mode, throw a flash, and forget about it.)  I almost never use Shutter Priority in my photography.

Continued in Part Two...