Shutter Speed

Shutter Speed (SS) is the time that the camera shutter is open and exposing the sensor to light. 

In the above image the SS is shown as 800. This stands for 1/800 of a second. However, the display can be confusing at times because long exposures can be several second long. Many cameras can expose for 30 seconds without using "bulb mode". In that instance the meter would show 30" for 30 seconds and not 30 for 1/30th of a second. That " is important!

Shutter speed serves many purposes:

This image was taken with a faster SS (1/1000) in order to freeze motion.

This image was taken with a slower shutter speed (1/125) to blur the
background and wheels in order to accentuate the sense of speed.

Here I used a variable neutral density filter to cut the ambient light allowing for a 1 second SS and blurring of the water.

One of the most important things SS does is eliminate "camera shake" blur due to you moving while handholding. This brings us to  "the rule of thumb" (ROT) for SS. This rule states that the minimum SS for hand holding is 1/focal length. So if you are using a 200mm lens the minimum SS "should be" 1/200. 

This is strictly a ROT and there are MANY factors which will affect the minimum SS you can use. 

One of those factors is your Holding technique. With good technique and practice you should be able to get well below ROT SS. I can handhold long lenses at as low as one tenth the ROT SS with reasonably consistent results (using the SharpShooter camera mount system).

Another factor is vibration reduction. If your lens or camera has vibration reduction you should be able to reduce the SS even further, often two stops or more (i.e. 1/200 instead of 1/800).

A factor that is not well understood is crop factor. Many say you must adjust the ROT for crop factor. So if you are using a 200mm lens on a 1.5 crop body the effective focal length is 300mm. The ROT SS is then 1/300 instead of 1/200. This is not quite correct. What matters is NOT the crop factor but the megapixels for the image captured. For example, because a full frame (FX) sensor is approximately twice the size of a DX sensor the speed I can handhold a given lens will be the same on both the D800 (32MP FX) as it is for the D7000 (16MP DX). Conversely the speed I can handhold a given lens is less for the D7000  than it is for a D3 (12MP FX). 

Finally, panning (following with the subject) will reduce the SS required. This is due to the law of inertia and the fact you and the camera lens are already in stabilized motion that matches with the subject motion (with good technique).

Using Shutter Priority mode.
Slow SS-  Use a slow shutter speed to emphasize motion. This could be light trails, star trails, panning etc.
Fast SS- Use a fast SS to freeze motion of the fastest subject you can find, This could be your kids or pet running around the yard, or a car speeding down your neighborhood street. 
And practice, practice, practice- start perfecting your holding technique now.

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