Elements of Exposure - The Shutter - Part IV

This is part four of my elements of exposure series.

The shutter is the curtain in front of the recording medium. The shutter speed is the amount of time the curtain in front of the recording medium is left open to the light. The shutter speed is expressed in seconds or fractions of a second. The typical DSLR has many shutter speeds and will typically include those diagramed below.


1/2000•1/1000•1/500•1/250•1/125•1/60•1/30•1/15•1/8•¼•½•1•2•4•8•15•30

<------------- Fast ---------------------- Moderate -------------------- Slow -------------->

In addition to these standard settings there is typically a Bulb setting that allows for an infinite amount of time for an exposure as decided by the photographer. In the standard shutter speeds illustrated above as we move from left to right we are doubling the amount of light for each successive speed or we are increasing exposure by one stop. Conversely, as we move from right to left we are cutting in half the amount of light or reducing exposure by one stop for each successive move in that direction. Shutter speed controls our ability to freeze action or reduce motion blur associated with a moving subject or blur as a result of camera movement or camera shake. A photographer would employ a fast shutter speed if he/she wishes to freeze action and/or reduce motion blur associated with camera shake. On the other hand, if a photographer wants to accentuate motion blur he/she would employ a slow shutter speed. The ability of the photographer to take images without noticeable blurring by camera movement is an important parameter in the choice of slowest possible shutter speed for a handheld camera. The rough guide used by most 35 mm photographers is that the slowest shutter speed that can be used easily without much blur due to camera shake is the shutter speed numerically closest to the lens focal length. For example, for handheld use of a 35 mm camera with a 50 mm normal lens, the closest shutter speed is 1/60 second. For a wider angle lens of 28 mm the shutter speed would be 1/30 second and for a telephoto lens of 200 mm the shutter speed would be 1/200 second. This rule can be augmented with knowledge of the intended application for the photograph, an image intended for significant enlargement and close-up viewing would require faster shutter speeds to avoid obvious blur. Through practice and special techniques such as bracing the camera, arms, or body to minimize camera movement longer shutter speeds can be used without blur. If a shutter speed is too slow for hand holding, a camera support, usually a tripod must be used. Image stabilization can often permit the use of shutter speeds 3–4 stops slower (exposures 8–16 times longer). The shutter speed compromise is deciding how much we wish to reduce motion blur of a subject or blur associated with camera shake versus our intention to artistically blur the action or increase camera blur to accentuate the feeling of motion in a scene. But, slower shutter speed, those typically longer than a couple of second, can introduce noise as well. Generally this is not a big issue due to most modern digital cameras ability to employ a noise reduction algorithm to counteract long exposure noise. However, longer exposures will have more noise than moderate or fast exposures.

Next we will take a look at dynamic range and what we can do when the dynamic range of a scene exceeds the camera's ability to capture it.

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.