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Photographers have their own shooting preferences. Some like using flash, some don’t. Some love to play around with depth of field, while others want motion blur and freeze motion effects more. If you've taken time to understand the use of M A S P, or M Av Tv P on the top dial on your DSLR or bridge camera, then you know that it has something to do with controlling the principal settings of your camera, primarily shutter speed and aperture combinations. And like any other technical setting, we all have a preferred shooting mode.
Among the four settings, Aperture Priority seems to be the most used by photographers. Perhaps it’s because wide depth of field is such a fascinating technique to explore. But also because depth of field seems to be a more acceptable way to generally balance exposure than a slow shutter or higher ISO. This mode is more than just about prioritizing depth of field over shutter speed. Aperture Priority gives you the advantage of shooting speed, control over technique, and the best exposure all at the same time.
Before we get into why the A mode works for many, let us first get things in perspective. All of the four shooting modes require you to manually adjust ISO, white balance, exposure compensation, metering mode, and some others. Therefore our focus is simply on two major things, getting the right exposure combination matched with your preferred technique. There are four basic camera techniques – wide of depth of field, shallow depth of field, motion blur, and freeze motion.
When Is Aperture Priority King?
Aperture priority initiates the best exposure which is not always true with Shutter Priority which is evident in low luminance situations. It also offers versatility with camera techniques that is not common in Program mode. And it offers a shooting speed faster than Manual.
In Aperture priority, if you want the best exposure with the fastest shutter speed, you simply need to set your dial to the maximum aperture. You can do this even without looking at the settings. Remember bigger aperture means more light coming in. If you have a kit lens, it’s usually at f/3.5 at the widest angle. It may seem that having an f/3.5 aperture is limiting, but this limit is actually the biggest advantage of Aperture Priority over Shutter Priority.
Remember that you have three options to increase exposure – a large aperture, a slow shutter, or a more sensitive sensor. A sensitive sensor means a high ISO, setting which also means more image noise. A slow shutter can mean a lot of motion blur which isn't usually preferred for general shooting purposes. Therefore, a larger aperture is used more often.
With Aperture Priority, when you reach the maximum aperture limit, the camera will stop at that setting. It is like the camera saying, “this is the best I can do, this is as far as I can go”. Even if the shutter speed can still go further faster, the camera limits you to go further because your aperture can't go wider. You will further appreciate this limit as we compare it with Shutter Priority.
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Compared with Shutter Priority
With shutter priority, you are not always guaranteed the best possible exposure. Imagine shooting indoors during the day where light is limited. Let’s say that you can have an aperture-shutter combination of f/4 and 1/30th of a second to get the right exposure. You also have a lens with maximum aperture of f/4. What happens if your preferred shutter speed technique requires you to have 1/125th of a second?
If you know how to calculate exposure values, the difference between 1/30 and 1/125 is two stops. Therefore at 1/125, you need to have your aperture set to f/2 to have the equivalent exposure. The problem is that your maximum aperture is at f/4 because of your lens limitation.
(If you're not yet familiar with exposure value combinations, it will be advantageous for you to learn it.)
When you change your shutter speed to 1/125, your camera will allow that change. But because of the aperture limitation, your aperture size remains at f/4. Since you moved your shutter by two stops, something else has to change to compensate for the difference.
If you have your ISO set to Auto, then it will compensate by increasing the ISO by 2 stops. But you don’t want that most of the time do you? Most often, you don't want ISO on auto. With ISO being constant, your camera forces your exposure value to change. The result is that your image will be 2 stops underexposed.
Unfortunately, your shutter speed won't stop when you reach your aperture limit. It will only stop at the minimum and maximum shutter speeds allowable by your camera. Therefore exposure is affected when you try to go beyond the aperture limit. But you won't normally notice it until you reach it. This then gives you more work to do and more settings to change.
Shutter priority works best when the light is strong. On a bright sunny day, you can go with 1/500 at f/11 with no problems. Therefore you don't have to worry about reaching the aperture limit. When the luminance is low you have to consider the possibility of hitting beyond the maximum aperture.
Because of this, Aperture Priority has the advantage of speed. If you want a wider depth of field or if you want to do some motion blur with aperture priority, just move your aperture dial towards a smaller aperture while checking the aperture or shutter speed values, respectively. You don't necessarily have to change modes.
Compared with Manual
Manual is an awesome mode because you have precise control of your camera settings. However, having everything in full manual also means you need to think and adjust everything – exposure, camera technique, shutter and aperture settings, metering among others. This can result in a little delay in shooting.
With Aperture Priority, you only need to focus on a couple of things. First, since the shutter speed automatically matches with the aperture combination, you are already assured of getting the right exposure combination based on the metering system of the camera. Therefore you only need to mind your meter controls and preference in technique.
Manual has the advantage of precision settings and is also perfect for flash photography while Aperture Priority has the advantage of speed. There is an instance however where Manual may have the advantage in speed over either Aperture, Shutter, or Program mode. It is when light is too unbalanced, and metering becomes a little off. The manual shooter in this case, with the right knowledge of exposure values, will only have to compensate by adjusting shutter speed or aperture values to adjust exposure. The semi-auto shooter will have to change exposure compensation, metering mode, or both to do so.
Compared with Program Mode
Program mode is the closest thing to Auto except that it gives you control over several settings including ISO, White Balance, and your on-cam flash. It also allows some control of aperture and shutter speed by giving you suggested combinations of these two prime exposure settings. This makes Program mode the best setting for capturing moments where you don’t have very much time to think. If you can live with simply being able to capture moments with the correct exposure regardless of technique then this mode is for you. Read this article about Program mode to appreciate it better.
But having control over technique is one thing that photographers want. Isn't it that most of us became enthusiasts because we were amazed with background blur and bokeh, light trails from cars, or sea water that looked like clouds? That is why Aperture Priority has leverage over Program mode in that regard, since you still have better control over camera techniques, whether it's depth of field or motion.
Further resources on Aperture Priority and Shooting modes