Today’s Question: During your workshop in the Palouse you said that when you are shooting in Shutter Priority mode you run the risk of “running out” of apertures, while in Aperture Priority mode you don’t have as much risk of running out of shutter speeds. Can you clarify what you meant by this, as the more I think about it after the fact the less I understand! Thank you.
Tim’s Quick Answer: Put simply, with a typical lens you have a smaller range of aperture settings compared to shutter speed options for your camera, providing more flexibility when using Aperture Priority mode compared to Shutter Priority mode.
More Detail: Let’s consider a couple of relatively typical examples for the range of available apertures and shutter speeds.
A typical lens might offer a range of apertures from f/2.8 when the aperture is wide open to f/22 when the aperture is stopped down to the minimum aperture size. In one-stop increments the available aperture settings would be f/2.8, f/4, f/5.6, f/8, f/11, f/16, and f/22. That’s a total range of six stops between the largest and smallest aperture size.
A typical digital SLR might offer a range of shutter speeds that go from 1/8000th of a second for the shortest exposure duration to 30 seconds for the longest exposure duration (with longer exposures possible through the use of a cable release). In one-stop increments there are a total of nineteen shutter speeds to choose from, representing a range of eighteen stops. That’s three times the exposure range (eighteen stops versus six stops) for shutter speeds compared to aperture settings.
What all this means is that when you set a fixed shutter speed in Shutter Priority mode, you are at a greater risk of “running out” of aperture options. There is less risk with Aperture Priority mode, because there are more shutter speed options available.
Of course, it is also important to keep in mind that for a given scene you may want to limit your exposure settings significantly compared to the many options available. For example, for a fast moving subject you may want to use a shutter speed of 1/125th of a second or faster, greatly reducing the total number of shutter speed options available to you.
The key is to be aware of what settings will work best when photographing a given subject, and anticipating when you might run into a limit based on the exposure mode or specific settings you’ve used.
During my Palouse Photo Workshop, for example, we had the opportunity to photograph crop duster aircraft spraying fields of crops. Due to the rotational speed of the propeller, a shutter speed of about 1/250th of a second was ideal. If the ISO setting on the camera had been set to 800, for example, even the smallest aperture size (f/22 in the example cited above) would result in an over-exposed image.