Now we will look at Aperture Priority,  designated as Av for Canon,  A for Nikon, and the Mode I use most often.

In aperture priority we set the ISO (how deep we want our bath) and the Aperture (how open our faucet) and the camera works out what shutter speed to use (how long to leave the faucet running) to attain consistent exposure.

As you know the Aperture setting allows us to prioritize the depth of field, or how much of the scene is in acceptable focus. For landscapes, we might want everything from a foreground flower to a mountain range in focus so we would choose a 'smaller' aperture value, something like f16 or f22.  This can result in a long shutter speed and we may need a tripod to get sharp images. On the other hand, if we are taking a portrait and trying to isolate the subject by blurring the background a 'wider' aperture like f1.4 or f2 will work better.

But we can also use Av/A Mode to get our fastest shutter speed in changing light.  For instance, while taking pictures of birds in flight where clouds keep the light values changing if we select our lenses widest aperture (lowest number), then the camera will select the fastest appropriate shutter speed to get a consistent exposure.  This means we can concentrate on getting the framing and focus right and not think about the exposure.

Now a really nice modern twist on camera control and auto exposure is Auto ISO. This works particularly well in the above scenario.  Again we want to take pictures of birds in flight and with the fastest shutter speed to prevent motion blur and the lowest ISO to maintain image quality.  We would set our camera to our 150-600mm lens' widest aperture, f5.6, choose Auto ISO and the camera will set the shutter speed over 1/500 sec because it knows the focal length being used.  Then if the light drops too much, rather than drop the shutter speed and cause motion blur, the camera will increase the ISO to maintain the fast shutter speed.

I'll cover Auto ISO in more detail later but the next installment will be shutter speed priority, Tv for Canon users and S for Nikon users. But in the meantime have a play with Aperture priority, it is amazingly flexible and if I could only use one mode is probably the one I'd pick.