Exposure

Understanding how the camera captures images is the key to controlling aspects that go into a photograph.  An improperly exposed image results in poor image quality no amount of post-processing work can ever recover.  The better the image is to start with, the more leeway we will have to make fine adjustments to it.

Trying to recover a poorly exposed photograph is time consuming, requires a deep understanding of imaging software, and yields second-rate photographs.  Many factors that go into making a photograph cannot be recovered at all.  Focusing, for example, cannot be corrected using software regardless how hard we try.  These controls affect the way the light is recorded initially.

Using proper acquisition techniques help alleviate these issues.  In this section, we will go over some basic controls and how they affect the photographic image.  Be sure to go over these topics carefully, and repeatedly, if necessary.  Try to fully understand the information provided here, as it will help you become a better photographer.

Aperture

The single most important aspect of a camera for the photographer is the aperture.  It controls the amount of light exiting the lense and exposing the imaging surface.  The aperture is simply a diaphragm consisting of a series of blades (leaves) laid out so they constrict (stop down) in a circular pattern to reduce the amount of light at the exit pupil.  On lenses with actual aperture rings that control the diaphragm, each position clicks as a "stop," hence the nomenclature.

Aperture settings are referred to as f-stops (fraction), or f-number (denominator).  Each full stop changes the aperture diameter by a factor of 2, either halving or doubling the amount of light from of the previous stop.  The result is a logarithmic scaling of light intensity.  Stop values are a function of the focal length of a given lense.  These numbers are therefore expressed as fractions:  f/1  f/1,4  f/2  f/2,8  f/4  f/5,6  f/8  f/11  f/16  f/22  f/32  f/45  f/64,... and so on.  Note the f number is rounded to the nearest 10th. Also note larger f-numbers equate to smaller aperture diameters.

A faster lense, for example, will have a larger maximum aperture allowing for the use of faster shutter speeds.  A slower lense will have a smaller maximum aperture.  Therefore, a 50mm f/1,4 lense has a maximum aperture of f/1,4.  A slower f/5,6 lense requires 4 stops more light to produce the same exposure.

The construction of the aperture also has a direct affect on defocused elements.  Basic apertures are typically straight blades that create a "stop sign" polygon effect in defocused backgrounds.  Fancier lenses employ curved blades that create a smoother, round defocused element.  The quality of this defocused background, the smoothness, consistency and roundness of these out of focus (OOF) circles, is referred to as bokeh.

Circle of Confusion
The circle of confusion is a term used to describe the scattering of light rays reflecting off the subject.  If the circle of confusion is smaller than a single pixel on the imaging sensor, that ray appears in-focus. Should the circle of confusion be larger, covering multiple pixels, that area will appear out of focus (or defocused).

The aperture directly affects this circle of confusion by constricting the spread of light rays.  The net result is smaller apertures produce smaller circles of confusion, yielding a greater depth of field (DOF).

In the above photo to the left, shot at f/5,6, the rhinos are sharply focused, the pandas are a bit out of focus, and as we progress further back the animals become increasingly defocused.  To the right, in the same scene shot at f/22, not only are the rhinos focused, but everything all the way out is sharp as well.

Hence, the larger the f-number, the smaller the aperture, and the deeper the DOF.

Defocusing the background is a powerful tool in directing attention to the subject and not the background itself.  A busy background can potentially detract from the main subject and result in a confusing, uninteresting photograph.  Proper use of DOF allows the photographer to direct the attention to the subject, and even a specific aspect of it.

Generally speaking, mid-range apertures for a given lense tend to yield the sharpest results.  This is typically around f/8 to f/11.