Sunday, April 5, 2009

Exposure.....Part 2 - Shutter Speed

Today's DSLR cameras use a two curtain focal plane shutter.

At left is an image of a two curtain focal plane shutter part way through the sequence of making an exposure. The image sensor is immediately behind the curtains.
This shutter was set to 1/500th of a second. Both curtains move from the top to the bottom. The front curtain is released first and in this image is near the bottom of it's range of travel below the black horizontal gap low in the image. The rear curtain follows close behind due to the the 1/500th of a second shutter speed.
The gap between the two curtains travels down and is spaced to expose each part of the image sensor for the required 1/500th of a second.
Once the exposure has been made both curtains get reset to the top of the frame so they are ready for the next exposure.
If the shutter had been set to 1/60th of a second the front curtain would travel all the way down before the
 rear curtain was released to follow suit. At 1/8000th of a second the gap between the two curtains would be very, very narrow.

You can watch a stop motion, 70 frame movie of a 1/60th of a second shutter release sequence including the mirror movements here.

Fast Glass

By adjusting the shutter speed we have another way to control how much light reaches the image sensor. If we have a large lens opening that is letting in a lot of light we only need the shutter open for a short amount of time, or a fast shutter speed. Conversely if we have a small lens opening restricting the amount of light we have to leave the shutter open longer, a slow shutter speed, to achieve the same exposure.

If you haven't already you will eventually encounter the term 'lens speed', 'a fast lens', or 'fast glass'.
As mentioned above, the larger the lens opening the faster the shutter speed can be. The term 'fast' refers to shutter speed. Typically, lenses that can be opened to a focal ratio of f/2.8 or larger are considered 'a fast lens'.

Shutter Speed Basics

Shutter speed is typically measured in fractions of a second:

  • 1/1000 s
  • 1/500 s
  • 1/250 s
  • 1/125 s
  • 1/60 s
  • 1/30 s
  • 1/15 s
  • 1/8 s
  • 1/4 s
  • 1/2 s
  • 1 s

Notice that the values follow the same function of 2 as the stops of aperture so we can halve or double the amount of light reaching the image sensor and can speak of shutter speed in terms of stops also. So, 1/8 s is 1/2 the time that 1/4 s is and is 1 stop less shutter speed than 1/4 s, 1/4 s is 2 times longer than 1/8 s and is 1 stop more than 1/8 s.

Most outside exposures will use fast shutter speeds because there is an abundance of light. Indoors there is usually a lot less light so we have to use slower shutter speeds and 'fast glass' or resort to adding light so we can shoot. We'll explore flash photography in a latter blog entry.

Once the Sun has set, shutter speeds approach or even exceed a full second. DSLR's usually have shutter speed settings up to several seconds long or a setting called 'Bulb' that lets a photographer open the shutter for as long as needed. That's getting into specialised territory though we may cover the subject in the future.

Creative Considerations

The trade-offs become apparent if our subject is moving. With a fast shutter speed the movement is 'stopped', there is no blur and a sense of motion is lost. With a slow shutter speed, the shutter may well still be open as the subject moves causing the subject to be blurred. Frequently, it can add to the sense of motion to have at least parts of an image blurry from movement like the waterfall in the image above that had the shutter open for one second.

Images made with fast shutter speeds can show that a running person or a running horse have all their feet off the ground a some point in their stride. Slow shutter speeds can be used to 'paint with light' or capture fleeting moments like a lightning strike.
The image at the left had the shutter open for 15 seconds. Most of the images made that night were also 15 second exposures but didn't happen to coincide with any lightning strikes and were pretty dull very dark images.

Camera Operating Techniques

Hand holding your camera can have it's advantages. It can have disadvantages too if you're not able to hold the camera still enough to prevent camera 'shake'.

As a rule of thumb your shutter speed has to be related to your lens focal length to make hand holding the camera viable. Your shutter speed needs to be the fractional equivalent of the focal length of your lens. If your focal length is 100 mm you need a shutter speed of 1/100 second. For some people this is more then enough shutter speed and for others it's not sufficient and they will need to add a stop or more for hand holding the camera. More and more camera's and lenses come with some type of motion cancelling system, that makes the above rule old hat. Canon calls their system IS or Image Stabilization, Nikon calls theirs VR for Vibration Reduction. These systems can be very helpful for extending the range of shutter speeds that allow hand holding your camera and lens combination.

There is another technique for making images of subjects moving across the field of view. That technique is called panning or moving the camera horizontally in most instances. When you pan you follow the movement of your subject so it remains stationary in your camera viewfinder as you 'pan' the camera with the movement. You trip the shutter release somewhere during the pan and with the proper shutter speed and focus your subject is sharp with a very blurred background which effectively isolates your subject from that background.

When shutter speed falls below the threshold of hand holding the camera must be mounted on some type of stable auxiliary support like a monopod or tripod.

Eventually we'll explore some of the ins and outs of camera supports like monopods and tripods but the next installment of the Exposure series will discuss ISO sensitivity.

See you then,

All images courtesy of Wikimedia Commons.

Where everyone learns Photoshop - National Association of Photoshop Professionals