Elements of Exposure - Part III - Aperture

This is the third part of my Elements of Exposure series, be sure to read the first two parts before reading this entry.

Aperture

As stated earlier, aperture refers to the size of the opening in the lens. In this diagram of seven aperture settings the opening at f2 lets in twice the amount of light as the next size smaller at f2.8 or one stop more light is transmitted to the recording medium. Conversely, f2.8 reduces the amount of light reaching the recording medium by half when compared to the next stop down of f2. This opening affects the depth of field or the amount of the scene that is in sharp focus from near to far. Smaller apertures circles (larger f numbers) render the greatest depth of field while larger apertures circles (small f numbers) limit the depth of field. This fundamental holds true for any lens but varies widely by the focal length of the lens. The depth of field at any given f-number will always be greater with wider angle lenses and will always be shallower with longer telephoto lenses. Depth of field is also greatly affected by the distance the lens is from the subject. The closer the subject is to the lens the shallower the depth of field will be at any given aperture. So the primary reason to change our aperture is to change our depth of field. With lenses however, both the smallest and largest aperture of any given lens due to various aberrations and/or limitations of producing a lens causes a reduction in the quality of our images at the aperture extremes while the sharpest and best quality images are typically rendered from the middle aperture settings. In the above diagram that shows a lens with aperture settings from f2 - f16, the middle aperture of f5.6 is likely to produce the sharpest quality image. So the photographer’s compromise is one of effect verses sharpness that affects the overall quality of our images. If a greater depth of field is desired than most photographer’s will not hesitate to use a smaller than optimal aperture to gain more depth of field to render a more three dimensional image by contrasting the near to far relationship of the elements of a scene that are all in focus. Conversely, if you wish to isolate your subject in a scene than most photographer’s will not hesitate to stop down to a larger than optimal aperture to limit the depth of field to a small slice of focus that concentrates the eye on their subject by throwing the other parts of the scene out of focus. This out of focus area is often referred to as a lens’ “bokeh.” A Japanese word used to describe the aesthetic quality of the blur in out-of-focus areas of an image, or the way the lens renders out-of-focus points of light. Differences in lens aberrations and aperture shape cause some lens designs to blur the image in a way that is pleasing to the eye, while others produce blurring that is unpleasant or distracting. In either case the photographer has decided that the effect needed for a particular scene is greater than the need to have the sharpest quality image rendered by one of the middle aperture values.

In my next entry I will discuss the Shutter, the final compent of the exposure triangle. I want to hear from you, please leave your comments, questions or topic suggestion for future blogs in the comments section.