How to Use Aperture Priority Shooting Mode
You've decided it's time to forgo the automatic shooting mode on your camera, but aren't quite ready to make the jump to full manual. Shooting in an aperture or shutter priority mode can help you bridge the gap, and gain better understanding of the role each point in the exposure triangle plays.
When using aperture priority, you control the aperture, and the camera automatically sets what it believes to be the appropriate shutter speed and ISO level. How good your camera is at detecting what the other two settings should be varies from camera to camera, but in general, most are at least decent at it.
Unless you're shooting something fast moving, aperture priority is usually where you want to be. It allows you to control the depth of field much easier than shutter priority allows. The reason for this is simple, aperture priority controls the aperture. Apertures control the amount of light that make it into the camera, which directly affects the depth of field in your image. While those nice bokeh effects are partially due to the focus, you can use a wide aperture in tandem to creatively soften the background, focusing all the attention on your subject; resulting in a visually stunning photo.
You'll need to be careful when selecting an aperture though. Depending on what you're shooting, having a depth of field that is too narrow, or too wide can really affect the appeal of your images. If you're shooting landscapes for example, you would likely want to use as narrow an aperture as possible, to ensure as much of the image is in clear focus as possible. If you're shooting a portrait, or something more abstract, a wider aperture, with a shallow depth of field can have a more powerful effect.
What is an aperture though? The aperture is a circular opening, which light passes through the camera towards the sensor, similar to a doorway. While the aperture plays a large role on it's own, it works together with many other aspects of the camera. One of the mechanics the aperture works together with is focal length. The focal length of the lens can be increased or decreased, made longer or shorter. The aperture opening can be increased, made wider or decreased, and made narrower. The focal length and aperture can be adjusted together to adjust the amount and angle of the light that enters the camera.
Aperture opening sizes are measured in f-stops, and since the aperture lives inside of the lens, the amount of f-stops available from lens to lens can be wildly different. Generally, the wider your lens, or the less zoomed in, the wider the aperture you can use. This is due to the cone shape the rods of light form when they enter the camera. If the lens is zoomed in, or using a high focal length, the cone will be too wide when projected, and won't line up properly with the sensor. That's why you'll see the lower f-stop option disappear as you increase the optical zoom. You can zoom in digitally to avoid this, but in general, we advise against this, as digital zoom can have unwanted effects that can be difficult to repair.
We know aperture opening's are measures in f-stops, but what does that mean? The smaller the f-stop, the larger the aperture. The larger the f-stop, the smaller the aperture. This means that f/3 would have much larger opening than f/10.
Apertures are often comprised of either 7 or 9 blades. That being said, there are certainly 5 blade models, or even upwards up 15 blades in some cases. The examples in the image above have 9 blades. With apertures, generally, more blades is better. Part of the reason for this is when you're shooting wide open, you can run into trouble with the lighting in your bokeh. The more blades your aperture has, the smoother the circles of glowing light in the bokeh will be. When your aperture has less blades the glowing spots will become more jagged. Take a look at the example below, the first picture was taken using an aperture with a higher number blades. The second picture was taken using an aperture with a much lower number blades.
The aperture with the lower number of blades is considerably more jagged. Less blades mean the aperture itself is more of a polygon than a circle, resulting in the light passing through the aperture having a more rigid outline. The jagged edges become more noticeable as the image is enlarged, so you shouldn't need to worry about it too much unless you'll be frequently enlarging your images.
If you're not ready to move over to full manual mode just yet, aperture priority can be a great segue to make the jump to manual more manageable. It's clear that the aperture plays a vital role in the overall function of your camera, and when used correctly, can be an indispensable tool when composing your images.