My last blog post, I talked about “P” mode, this time we’re going to explore Av or A, depending on your brand of camera.
Av is aperture priority mode, which means that you set the aperture (the f-stop) and also the ISO. The camera will then set a shutter speed for you so that the picture is properly exposed.
The aperture setting is how you control “depth of field”, which is the blurry or non-blurry background. When you want a large depth of field, choose a high f-stop (aperture). When you want shallow depth of field, choose a lower f-stop. The range of f-stops available is set by your lens. Some lenses will go to f 4, others down to 1.4. The smaller the number, the more blurry the background. The higher the number, the more of the photo is in focus.
Here comes the slightly confusing bit. When the f number is small, the lens diaphragm is actually wide open. So if someone says they are shooting “wide open”, they are shooting with the lowest available f number. Alternatively, if the aperture is a large number, say f 22 then the lens diaphragm is smaller or more closed.
Opening your lens more refers to lowering the f number.
Closing your lens more refers to a higher f number.
Now you know what it means, when should you use Av mode? A lot of hobbyists use Av mode most of the time. Here’s a few times when you may want to switch to Av:
If there's a distracting background that you want to blur out or you want to separate the subject from the background.
If you want to make sure multiple elements in the scene are sharp, for example a landscape scene.
Speaking about blurring backgrounds, I feel I should mention a few other factors. This is particularly helpful if you shoot with a crop sensor DSLR or a phone - basically anything but a full frame sensor. The further away your subject is from the background, the more blur you can create. This will have a greater effect than just changing the f number alone. If you take two photos with a crop sensor at 2 different f factors, with everything else being exactly the same, the difference between the 2 photos may not be huge. (In order to get the really blurry backgrounds, you will need to invest money in better lenses.)
One thing to watch out for when using Av mode, is the shutter speed the camera selects. If your lighting situation is not great, the camera will select a slower shutter speed. If you are hand-holding your camera and not using a tripod, you will get motion blur.
That’s it! Time to go play with Av mode!