Exposure Basics

or, learning to see like your camera – Part 1

Today’s cameras are pretty smart. They know when you’re shooting outside, or in. When you want to shoot a portrait or a close up. They can tell that you need to use a flash, and in some cases, will even find a smile and shoot it for you. But even with all of the advances in digital camera technology, the smartest camera is still not as smart as you are. The human brain is incredibly adept and nimble, and with a bit of training you’ll begin to see things differently when looking through a viewfinder.

Understanding how cameras see the world is not intuitive. Our brains involuntarily adjust our irises to adjust for various brightnesses. Compared to even the best digital imaging devices, our eyes have the ability see in relatively bright or dim conditions. We’ll also subconsciously adjust for variables in color temperature and relative brightness. These are advantages and abilities that film and digital devices simply don’t have.

Most times our cameras do a pretty good job of properly exposing photos automatically. But in order to better express our creativity and our art, more in-depth understanding of exposure is key. A properly exposed image is one in which the appropriate amount of light reaches the image plane. Exactly how much light is needed will depend on a number of factors but the most important to the end user is the sensitivity of the chip (or film.) This sensitivity is expressed as an ISO (International Standards Organization) speed. Typical ISO ratings range from 50 to 3200.   These values are linear in terms of sensitivity, ie, ISO 200 is twice as sensitive as 100. Generally speaking lower ISO speeds have less noise and higher resolving power than higher speeds. Many cameras, when set on full auto, will fluidly adjust ISO to adjust for a given situation. For the purposes of this lesson, we’ll assume the ISO is fixed.

The other two factors to consider in proper exposure are shutter speed and aperture. These are the two factors that will usually be varied from shot to shot in a given session. The classic illustration for proper exposure is that of a glass filling with water from a tap.   The object is to fill the glass all the way, but not overfill (overexpose) or underfill (underexpose.) If the tap is open all the way the glass takes   1/2 second to fill, if the tap is open half way, 1 second.   Likewise, an exposure at f/2 for 1/2 second is the same as and exposure at f/2.8 for 1 second.

Note that each whole stop increase in aperture or shutterspeed increases the amount of light reaching the image plane by 2, each whole stop decrease halves the amount of light. So closing down from f/2 to f/2.8 decreases the flow of light by half. Lengthening shutterspeed from 1/2 second to 1 doubles time.   It is at times counter-intuitive, increasing the f from 2 to 2.8 decreases the aperture, increasing the shutterspeed from 125 to 250 decreases exposure time.   But it’s helpful to remember these values are fractions, f/2 and f/2.8 represent the width of the lens opening divided by it’s length, 125 and 250 are actually 1/125 and 1/250 of a second.

Most beginning photography courses restrict students to shooting black and white. There are a number of reasons for this however one key reason is that it eliminates one big variable associated with color photography – and that’s color temperature. We’ll address that in part 2.

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