Wednesday, July 29, 2009

How to Choose a Shutter Speed Part Two

As I mentioned in Part One, one of the main factors that should influence what shutter speed you choose when photographing is your focal length. (Focal length is the number indicating how far you are zoomed in or out. For example, 18mm or 50mm.) Focal length is important because it affects how much our inability to hold a camera still influences our photographs.
No matter how hard you try, you can't hold your camera completely still. So if your shutter speed is too long (or slow), your entire image will be blurry because of the camera's movement during the picture capturing process. Unlike motion blur caused by subject movement, this type of blur is almost always bad. It looks something like this:

So how fast does our shutter speed have to be to prevent this type of blur? It depends on focal length. The longer your focal length (the more you are zoomed in) the more your lens magnifies or exaggerates your hand movements. You can see this when you zoom into 200mm or so and notice that everything seems wobbly. So the more you zoom in, the faster your shutter speed needs to be.
It is tough to set rules for preventing this kind of blur because different people can hold their cameras more or less still. Also, you may use a shutter speed that would be considered "dangerously slow," and still happen to get a sharp picture. But in general, you can use this equation to determine a safe shutter speed:

1/(focal length x 1.5) = slowest safe shutter speed*

So at 50mm, 1/(50 x 1.5) = 1/75 or faster.
At 200mm, 1/(200 x 1.5) = 1/300 or faster.
At 18mm, 1/(18 x 1.5) = 1/27 or faster.

Remember that this is a starting point, not a law. You may find that you have particularly steady hands and can go a little slower, or that you have shaky hands and need faster shutter speeds. But if you notice motion blur in parts of your image that were not moving, you need a faster shutter speed.
Note, this does not account for Vibration Reduction/Image Stabilization.
* This equation is for DX cameras which include most DSLRs under $2000. For full frame digital cameras, or film cameras the equation would be 1/focal length.