Understanding Shutter Speed
Shutter speed refers to the length of time your film or sensor is exposed to light. Shutter speeds are measured in fractions of a second but are usually shortened to just their denominator for ease of discussion. So 1/60th is the proper name for the shutter speed that represents 1/60th of a second. But in day-to-day speak, we simply refer to this as 60 and that's also how it's marked on your camera. And likewise for all the other shutter speeds.
Bulb shutter or "B" on the shutter dial refers to holding the shutter open manually, without the built-in timing device of the shutter. When a shutter is set to bulb, the length of time that the shutter release button is pressed is equivalent to the length of time that the shutter remains open. Set a camera to bulb, press the shutter release and the shutter remains open until you take your finger off the release. This is useful for astro photography and cleaning the sensor of DSLR's. Also for shooting on a vintage camera with bad shutter times until it is cleaned, lubed and adjusted!
What you need to understand about shutter speed is that it affects the sense of motion in your photos and can be affected by the length of your lens if you are hand-holding your camera. Shutter speed can either freeze motion or blur it. Blurring motion is called shutter drag or dragging your shutter.
1/60th is generally the slowest speed that you can photograph people or animals without getting blur from their movement.
1/125th is a more comfortable speed for shooting people who are not moving rapidly. It will insure that you won't get blur if they make a sudden movement.
1/250 and up are used for freezing sports or other rapidly moving subjects.
Long shutter speeds, below 1/30th can be used with a tripod for extended exposures for low light and night photography or non-moving objects. 30 and below can also be hand-held with shorter lenses, rangefinders and vibration reduction gear.
Like everything on a camera, shutter speed is measured in stops of light. It's good to memorize your shutter speeds in one stop increments, as they are laid out on mechanical film cameras. Cameras with electronic shutters such as DSLR's or automated film cameras can do fractional shutter speeds in half stops, quarter stops and third stops or are step-less But these over-complicate the choices for a novice shooter (as well as those of us who don't care because we shoot big latitude print film!), so set your DSLR to half or full stop shutter increments (if possible). Full stop shutter speeds are:
Each shutter speed is one stop different from the one before and after it. Therefore, f8 @ 30 = f5.6 @ 60 = f4 @ 125. If you open your aperture 1 stop, your shutter speed must be sped up by 1 stop to make the same exposure, to compensate for the change in aperture. Notice how possibility of motion blur increases as depth of field increases and a reduction in the motion blur leads to a more shallow depth of field.
Something many photographers, even somewhat experienced ones, don't understand is that shutter drag is partly dependent on how long your lens is. The now-common use of zoom lenses has also contributed to this confusion. Most APS-C DSLR shooters that I've asked seem to have no idea what their lowest shutter speed should be with a given lens. So listen up!
For 35mm and full frame digital photography, the equation is simple. Your shutter speed should be as fast or faster than the length of your lens. So if I'm using a 50mm lens, my shutter needs to be 1/50th or above. On a mechanical camera, 1/60th is the closest to 1/50th and should be used with a 50mm lens. You'll see that lens length and shutter speeds seldom are exactly equal, so the idea is to keep your shutter speed faster than the lens is long. This reduces blur resulting from handheld camera shake.
For crop-sensor (APS-C) DSLR cameras, you have to know the crop factor of your camera's sensor. Which is usually about 1.5. So if you have a 50mm lens mounted to such a camera, your shutter speed needs to be 1.5 times 50 because this is the effective length of your lens. So your shutter speed would not be 1/50th but 1/75th.
I know this sounds complicated, and frankly it is. And it's yet another great reason to learn to shoot on a 35mm SLR or rangefinder instead of that consumer grade DSLR everyone's trying to sell you. These cameras just are not good teachers (aside from instant feedback.) But if you use prime lenses or do some experiments with a zoom lens you will come to understand the relationship. It will just take longer than with a simple 100% manual 35mm camera.
That's right, with 35mm film, operating the camera in full manual takes less effort and time than any APS-C DSLR on the market. So while you're saving time with instant feedback, you are losing time with calculating usable shutter speed and other things (poor viewfinder brightness for focusing, more shutter/aperture choices than is easily memorize-able and the zoom lenses that come with DSLR's complicate predicting depth of field.)
So with a 35mm camera or full frame DSLR, here is a list of lenses and their minimal hand-held shutter speeds.
24mm @ 1/30th
28mm @ 1/30th
50mm @ 1/60th
85mm @ 1/125th
105mm @ 1/125th
135mm @ 1/250th
180mm @ 1/250th
200mm @ 1/250th
Understanding these relationships not only helps you get clear, un-blurred photos, it helps you select, purchase and use lenses that are conducive to your shooting situations.