Sunday, April 11, 2010

Exposure

I'm still playing catch up. Here is another essay that I wrote for ENG1010 Composition and Language this last session. If you see any mistakes, be sure to point them out to me so that I can learn and correct them for future readers.



Knowing how to balance the three elements of exposure is one of the challenges that a new photographer has to overcome. The ISO number is a guide for how sensitive film or digital sensors is to light. The aperture is simply a means of controlling the amount of light coming in through the lens. The shutter speed is the length of time that the shutter is held open. By varying the ISO number, aperture and shutter speed, photographers can dial in the perfect exposure but each control has side effects to be aware of and use creatively. "The term "stop" in photography refers to a change in illumination, whether the shutter speed or the aperture is changed to achieve it" (London et al. 18).

ISO can vary from as low as 50 to well into the tens of thousands with today's high-end digital SLRs. "The higher the ISO number, the less light is required to produce an image" (London et al. 82). What London, Stone and Upton neglected to point out is that each increase in ISO is one stop faster; e.g. ISO 100 is one stop faster than ISO 200. Rowse says that "ISO 100 is generally accepted as 'normal'" but many digital cameras start out at ISO 200 ("ISO Settings in Digital Photography" 2).

The higher the ISO number, the grainier the images will appear when using film. ISO 800 film is the highest that does not suffer from unreasonable amounts of grain. When using digital, a higher ISO results in more noise in the digital image. As digital camera sensors improve, the maximum ISO number continues to increase. The latest offering from Nikon, the D3S, has a normal ISO range of 200 to 12800 but through digital processing, the ISO range can be extended by one stop at the low end to 100 and three stops at the high end to 102400 (Nikon Corp.).

Aperture, or f-stop, is a rating for the amount of light that the lens will allow in when the shutter is opened. Aperture is not a fixed size, it varies from lens to lens based on the focal length. Regardless of the focal length, the same f-stop will allow in an equal measure of light. "Stopping down" refers to decreasing the size of the aperture. Each stop down in size (up in number) lets in half as much light as the one before it so an f/8 allows twice as much light as an f/5.6 but half as much light as an f/11 (London et al. 24-25).

The aperture, at any given focal length, determines the size of the depth of field or the range where objects in the frame are in focus. Larger apertures result in shallower depths of field whereas smaller apertures result in deeper depths of field; e.g. a 50mm focal length lens at f/1.8 would have rather shallow depth of field while at f/22, the depth of field would extend to infinity.

"Adjusting the length of time the shutter remains open controls the amount of light that reaches the light-sensitive material, " according to London, Stone and Upton (18). Shutter speed is measured in seconds or fractions of a second. According to Rowse, each setting approximately doubles the shutter speed ("Introduction to Shutter Speed in Digital Photography" 1). Each decrease in shutter speed results in twice as much light hitting the film or sensor or one stop so decreasing from 1/15th to 1/30th gives an extra stop of light.

Shutter speed can be used both to show and freeze motion. Slow shutter speeds will show or even exaggerate movement. Fast shutter speeds will stop time for all eternity (or at least the lifecycle of the image). At a speed of 1/30th of a second, an image of a car driving down the road would be a streak with wheels. At 1/500th or faster an Indy car in motion will look like it is standing still.

The intended effect (grain/noise, motion, depth of field) steers the initial decision but adjustments in one value require an equal adjustments in the other two values. This knowledge is the key to the perfect exposure. Exposure is the result of ISO, aperture and shutter speed working together to control the amount of light that hits a piece of film or a digital sensor when a photograph is taken.


Works Cited

London, Barbara, Jim Stone and John Upton. Photography. New Jersey: Pearson Education, 2008. Print.

Nikon Corporation. D3S | D3X - Exceptional Image Quality. Nikon Corp., n.d. Web. 17 Mar 2010. <http://imaging.nikon.com/products/imaging/lineup/microsite/d3s_d3x/en/image_quality/>.

Rowse, Darren. "Introduction to Shutter Speed in Digital Photography." Digital Photography
School. Digital Photography School, n.d. Web. 15 Mar 2010. <http://digital-photography-school.com/shutter-speed>.

---. "ISO Settings in Digital Photography." Digital Photography School. Digital Photography School, n.d. Web. 15 Mar 2010. <http://digital-photography-school.com/iso-settings>.

No comments:

Post a Comment