Part 3 - The relationship between aperture, shutter speed, and ISO

 Aperture priority - choosing the correct exposure settings

A proper exposure takes place when the camera's aperture, shutter speed, and ISO settings allow the right amount of light to fall onto the camera's sensor.  We will look into this more in another section.  The aperture is a mechanical device inside the camera's lens that opens and closes much as the iris of your eye does to form the pupil.  Small apertures, represented by high f/numbers, let in little light but allow more depth of field.  Large apertures, referred to as low f/numbers, let in lots of light but create an image with less depth of field.  Picking the right aperture is an important part in good photography.

If you recall from our previous two sections, aperture, ISO, and shutter speed work together to create an exposure.  You can alter each of those three values to obtain the desired results and get the shot you wanted.  Cameras come with automatic exposure modes that make that process easier, unlike full manual exposure mode where you have to manage everything.  The letter "A" on your mode dial is for aperture priority, and by setting the aperture the camera will automatically select shutter speed.  You can manually set ISO or have the camera do it for you (auto ISO).  The advantage with aperture priority exposure mode is that all you have to do is rotate the command dial (or aperture ring on older cameras) to pick the f/number desired.  The camera will pick the shutter speed needed to make a correct exposure.

There is a trick to this though, and it comes in two parts.  First, you have to have an idea of what aperture and focal length do together (ie - how it affects the image), and second you have to understand how altering it changes shutter speed.  Wide open apertures (low f/numbers) allow faster shutter speeds while small apertures (high f/numbers) promote faster shutter speeds.  So first you pick the aperture tnd then check the resulting shutter speed for the situation you are in (frame the shot and press halfway down on the shutter button to get a reading).  At this point you have to make a decision.  Is the shutter speed fast/slow enough to get the desired results.  If you want a faster or slower shutter speed, you have to change something.  You can ...

  • Change the aperture so it is smaller or wider than what you originally wanted
  • Adjust the ISO to alter the shutter speed, remembering the issue with noise
  • Use a different lens, one with a faster minimum aperture (f/2.0 instead of f/4 for example)
  • Increase or decrease the amount of light - this usually means shooting at a different time of day or waiting for the weather to change.
  • Use a filter such as a neutral density filter to enable lower shutter speeds
I recommend that people learning the basics of camera operation use aperture priority exposure mode as it reduces the number of complications that can arise with other modes.  Also, if you want the fastest shutter speed possible for situations like sports and wildlife photography, pick the lowest aperture available.  If the resultant shutter speed is not fast enough, increase the ISO.  Aperture priority is also a great setting for shooting landscapes, portraits, and more as it is relatively easy to get the results you want without hitting a wall (like with shutter priority and manual exposure mode).

Have a look at the photos below; they give examples of different aperture values and why it was chosen.


Thanks for reading.

Eric Svendsen     www.ericspix.com


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