It's finally time for the last piece of the exposure puzzle. If you understand ISO and aperture, which I covered in the last two articles, then you are well on your way to making sense of exposure. All that's left is shutter speed, and thankfully it's the easiest of the three to grasp. It's straightforward, but it also allows for a wide range of interesting creative choices.
Shutter Speed and Exposure
Shutter speed is exactly what it sounds like: it's how long the shutter is open, and therefore how long the sensor is exposed to light. The longer the sensor is exposed the brighter the image will end up being, and like the other two exposure settings, it works in stops.
Full stops are nice and easy for shutter speed. To add one stop and double the light, you just need to double the time the sensor is exposed. To subtract a stop and halve the light, all you need to do is halve the time. This full stop scale looks like:
... 1 sec, 1/2 sec, 1/4 sec, 1/8 sec, 1/15 sec, 1/30 sec, 1/60 sec, 1/125 sec, 1/250 sec, etc...
In most cameras the maximum length is usually 30 seconds, with a fastest speed of either 1/4000 or 1/8000 sec depending on your model. The scale is then divided into 1/2 or 1/3 stops from there.
Using the flexibility of these partial stops, you can dial in the exact exposure you need based on your other settings. This is why I place shutter speed at the top of the exposure pyramid.
Chances are you've experienced blurry photos before. This happens when your exposure is too long and the natural motion of your hands causes the camera to shake, distorting the image. A good rule of thumb to avoid this is to take the inverse of your focal length, and use that as the minimum speed to use.
For example, if you have a 24mm lens the slowest you should handhold is 1/25 sec. With a 100mm it would be 1/100 sec, and for an 85mm it would be 1/80 sec. There are some variables that can change this though.
Based on your technique, and if you have image stabilization, you may be able to get away with using a longer time. On the other hand, with a smaller or high resolution sensor you will need to use a shorter time. Your best bet is to just experiment with you particular combination of equipment and see what works for you.
Beyond exposing your image, shutter speed can also be used for a number of creative purposes as well. Depending on how long your shutter is open, you can control how much movement is evident in the final image.
With a fast shutter you can stop motion completely, which can be good for wildlife photography among others.
Using a slower shutter will cause anything that is moving to begin to blur. You can use this motion blur to create a sense of energy and movement.
If your exposure is long enough, you can cause movement to completely blur out. You need a tripod for this technique, but it can be used to give an interesting effect to running water and blowing clouds. You can also use long exposures to make moving people disappear from your image. This is helpful when you are trying to photograph a busy landmark, and you just can't get a clear shot.
All The Pieces
Compared to ISO and aperture, shutter speed is nice and straight forward. It's simplicity belies the importance of shutter speed, and the multiplicity of creative options it offers. Now that you understand how shutter speed works, you now have everything you need to craft an exposure. Next time we'll put it all together, and set you on the road to mastering your camera.