Shutter Speeds and Apertures
WITH THE AUTOMATIC FUNCTIONS in modern cameras that do a super fine job of making good train photography easy, why is it im- portant for a railfan photographer to under- stand the principles behind using the proper shutter speeds and apertures for achieving sharp, well exposed train photos? Especially if you are new to photography and have al- ways trusted your automatic camera to de- termine proper exposure? Probably even newcomers to photography understand that the long accepted railfan practice for action train photography is to use a shutter speed of at least ¹⁄₅₀₀ of a second. But there are oth- er important considerations besides simply setting this speed while in shutter priority mode and firing away.
Shutter Speed
Whether you are photographing in bright sun or under dark clouds, the required amount of light for good exposure is always the same for any given digital sensor or film ISO. Shutter speed controls the length of time light is hitting the sensor or film. The typical range of shutter speeds on modern cameras ranges from several seconds to B (for time exposures), then one second, ¹⁄₂, ¹⁄₄, ¹⁄₈, ¹⁄₁₅, ¹⁄₃₀, ¹⁄₆₀, ¹⁄₁₂₅, ¹⁄₂₅₀, ¹⁄₅₀₀, and ¹⁄₁₀₀₀ of a sec- ond and faster. Note that these shutter speeds have a doubling or halving relation- ship to each other, and each represents a “full stop” of exposure. Thus, a ¹⁄₂₅₀ shutter speed will expose the light for half the time of the previous ¹⁄₁₂₅ setting (one stop less light); and twice as long as the following ¹⁄₅₀₀ (one stop more light). In addition, electronic cameras can select many intermediate shut- ter speeds, such as ¹⁄₉₀, ¹⁄₁₆₀, ¹⁄₂₀₀, ¹⁄₃₂₀, etc. rep- resenting less than a full stop to fine tune exposures.
Aperture
While shutter speeds control how long the light is exposing the sensor or film, aperture controls the amount of light coming in through the lens. Changing from one aper- ture to another also halves or doubles this light, to create numbered “ƒ/stops” of expo- sure. The full range is ƒ/1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 44, and ƒ/64, although the commonly seen range on most lenses is from ƒ/2.8 or 4 up to ƒ/16 or 22. Beginning with ƒ/1 and ƒ/1.4, all the other ƒ/stops are ob- tained by alternately doubling these two numbers. Thus, ƒ/1 becomes ƒ/2, then ƒ/4, then ƒ/8, etc. out to ƒ/64; while ƒ/1.4 becomes ƒ/2.8, then ƒ/5.6, etc. On some lenses there may also be other “intermediate” ƒ/stops be- tween the full ƒ/stops that change the expo- sure less than a full stop and do not fit this alternate doubling pattern. Modern elec- tronic cameras can also set any degree of in- termediate ƒ/stop. If you have an interchangeable lens with an adjustable ring with aperture numbers, remove the lens from the camera and turn the aperture ring while looking through the lens to see how the aperture opening adjusts to different sizes (you may need to press a prong on the lens mount to make the aper-
A near head-on viewof this Denver & Rio Grande Western DVROF (Denver-to Roper yard [Salt Lake City] Freight) exiting the Fraser River Canyon into Granby, Colorado allowed an acceptably sharp photo of the speeding train even though the shutter speed was only ¹⁄₃₀second (with a tripod) due to ISO 25 film and the low morning light. Y
es, Virginia, there is a Fraser River Canyon in Colorado as well as in British Columbia!
ture opening change). Apertures work in much the same way as the pupils in your eyes get wider or smaller according to the brightness of light. Smaller openings such as ƒ/11, ƒ/22, etc., let in less light and are used to control excessive light on bright, sunny days. Under dark clouds or shade, a wider aperture such as ƒ/4 will let in more light to reach the sensor’s or film’s exposure requirement. Note that when the aperture is set to the smallest number on the lens, the aperture opening will be as big as the interior of the lens. This is what is meant when you hear the term “maximum aperture” or “wide open.”
ISO
There is one other factor in determining cor- rect exposure. ISO (International Standards Organization) film speed or digital settings work in a similar way as apertures and shutter speeds. Each halving of the ISO gives one less stop of sensitivity to light, while each doubling of the ISO gives one more stop of sensitivity. The higher (“faster”) the ISO rating, the less light is needed for proper exposure (smaller
ƒ/stops and/or
faster shutter speeds can be used). Lower (“slower”) ISO settings require more light (wider ƒ/stops and/or longer shutter speeds) for proper exposure.
Putting It All Into Practice
Consider the amount of light on a sunny day. Taking a meter reading at ISO 100 we get a
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