March 23, 2009

Aperture Priority Mode

This is part two in a four part discussion on advanced camera modes.  We will continue to discuss other modes in later posts but this post will deal with aperture priority ... so now its time to get brave and rotate that dial away from the Auto setting and learn how to use the Aperture Priority (A or Av) setting.   

Lets start off with the basics.  Aperture Priority mode lets you select the size of the camera's aperture.  The aperture is the opening that controls how much light reaches the sensor or film at the moment of exposure.  Unfortunately, for most of us, aperture settings are quite hard to understand.  Contrary to what would make sense, the smaller the aperture number the larger the aperture is open and the larger the aperture number the smaller the aperture is open.  There is a reason for this backward looking numbering.   The lens aperture is usually specified as an ƒ number, the ratio of the focal length to effective aperture diameter.  The term "one ƒ-stop" refers to a factor of √2, which is approximately 1.41, change in ƒ-number which in turn corresponds to a factor of 2 change in light intensity.  Wow! Was that way too much mathematical information?  I think so!  So here is the deal … The larger the aperture is open the more light is let through the shutter so you can take pictures in lower light situations.  Unfortunately, and sometimes fortunately, the larger the aperture is open the shorter the depth of field is.  I have supplied some pictures to demonstrate this.

As you may be able to tell, the picture to the left has been shot using a larger aperture opening (ƒ/5 to be exact) and the picture to the right has been shot using a small aperture opening (ƒ/32 to be exact).  Notice how the depth of field changes the smaller you go with the aperture setting.

So what does all of this mean to you?  Well it means that you can set your camera to Aperture Priority mode set your aperture opening and take pictures.  This is a fairly safe way to take pictures, because the camera is still doing half of the work by selecting the correct shutter speed for the ƒ-stop selected.  The best way to get used to using Aperture Priority mode is to experiment with it on a day when you don't need to worry about taking great pictures.  Practice taking pictures with different aperture settings and see how they turn out.  It wont take you too long to figure out how to make the aperture work for you.

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