AV or Aperture Priority mode does exactly as the name suggests. The aperture is prioritised over other manual settings. When using this mode, you select the aperture number that you wish to stay constant, f8 for example, and the camera will work out a suitable shutter speed and ISO to provide a correct exposure. Likewise, TV, or Shutter Speed Priority mode is when the shutter speed is selected to stay constant. In other words, these modes are semi-automatic.
When to use AV mode As you may know, the aperture affects both how much light is let in to the camera as well as the depth of field. Generally we use this mode when we want to keep our depth of field fixed.
We might choose a mid or high aperture number to give us the sharpest image possible, which looks great on landscapes or portraits. Just make sure that thereís enough light in the scene for your shutter speed to be sitting above 1/60th if youíre not using a tripod, to prevent unwanted motion blur. If youíre noticing that while using AV mode, your shutter speed is dropping below 1/60th, then there may not be enough light and you should either lower the aperture or top up the ISO.
AV mode is also useful when shooting macro subjects or with a shallow depth of field, as long as there is enough ambient light.
When to use TV mode Now letís take a look at Shutter Priority (TV). Just like our aperture, the shutter affects how much light we can get, but also the appearance of motion. Usually this mode is used when we want to lock in a certain speed.
I often use TV mode when capturing sport or event photography. The subjects are moving too quickly for me to catch them with a speed lower than 1/500th, so I can set this as my minimum with TV mode, and ensure that my photos are at least free from motion blur.
Occasionally a lack of light will drag down the aperture until it becomes a shallow depth of field. But this doesnít worry me too much as I find it helps to isolate the subject from the background. If, however, I find that my aperture is dropping as low as f1.8, I would start to top up my ISO or reconsider my shutter speed.
TV mode is also useful for capturing fast moving children and pets as well as wildlife and motor sports.
Why choose a priority mode over full manual mode? Iím often asked this question. Well, the main reason to use a priority mode is because it takes the guess work out of using manual mode. Sometimes we find ourselves in a position where weíre having to move quickly and donít have a lot of time to adjust settings with the changing light. At least in this way we can be fairly certain that our pictures will come out in the best possible quality, while enabling us to work efficiently.
Pro tips Be careful when using priority modes that your ISO is not reaching heights greater than 3500 (or 2500 on older model cameras), as the noise will lower your image quality.
Also keep in mind what metering mode youíre using as this will affect your exposure. You might find that a bright background will return a dark centre subject when shooting with Evaluative or Matrix metering, regardless of what priority mode youíre working with. In this instance, use centre-weighted or spot metering.