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How to use Aperture Priority

You have read my topics on Exposure Triangle, Reciprocity, Aperture, Shutter Speed, and ISO, and everything is as clear as mud. That is not uncommon, as there is a lot of information to be digested. So perhaps a discussion on the application of those principles will help. After reading this topic, if it makes sense, go back to the other topics - re-read them, and see if they become clearer.

Aperture priority means you want to give the aperture the priority in your photo. In that regard, you set or control the camera's aperture. The camera will automatically set the shutter speed to match what you set for the aperture. While there is no mode for ISO priority, the camera will also set the shutter speed to match changes you make in either aperture or ISO. The camera will not however change the aperture or ISO from what you have set.

To enter Aperture Priority, change your camera's mode dial to "A". On Canon cameras, this would be "Av" (Aperture Value).

Next, set the camera to your desired aperture. As cameras have different methods of setting the aperture, you will have to consult your manual.

That's it, you are done! You are now in Aperture Priority, and the camera will automatically adjust the shutter speed to what you set for your aperture.

Ramifications

Now you didn't really think it was going to be that easy, did you? When you set the aperture, you have to be mindful of what the camera sets your shutter speed to. For instance, you want to take a photo of a breathtaking Alaskan vista, and you think that f/8 would give you a nice Depth of Field, with everything in focus. However, depending on the speed of your lens, your ISO setting, and the lighting conditions, the shutter speed could be very low, which means you will need a tripod. And if there are any moving objects in the scene, they could be blurry. So while the camera sets the shutter speed, you still have to keep an eye on what it is setting it to.

If you find that the camera cannot set the shutter speed to your liking, you can increase the ISO. If the shutter speed is set to 1/8th of a second in your photo, and you don't have a tripod, or there are people walking around that you want to capture (and don't want them to be blurry), you may feel more comfortable with a shutter speed of 1/60th of a second. From the Shutter Speed topic, recall that 1/60sec is -3 EV (Exposure Values) less than 1/8sec. Therefore, increasing your ISO by +3 EV will solve the problem. If your current ISO is 200, +3 EVs would be ISO 1,600. Remember, due to the Inverse Square Law, each EV is a doubling or halving of the next.

Perhaps an ISO of 1,600 has too much noise for your liking. In many cameras, that high of an ISO can indeed introduce significant noise. Some cameras do have noise filters that you can turn on, which may help a little, but balancing shutter speed, aperture, and ISO are the trade-offs we must balance as photographers to get that good shot.

The logical step to obtain a better photo is to purchase a camera with better low-light capability - which is more expensive. Now you know why the pros buy $6,000 cameras.

 

Exposure Compensation

Most advanced cameras have some sort of exposure compensation function, but unfortunately most do not have a nice dedicated dial as shown in the photo at the right. You typically have to depress a button combination or menu selection to set this function.

Exposure compensation allows you to change the degree of over or under exposure in your photo without upsetting the shutter speed you have already set. In effect, it tells the camera to adjust the shutter speed a bit more than it thinks is needed.

There are two times when exposure compensation comes in handy. When shooting scenes with a lot of bright background, such as snow, will usually trick the camera into underexposing the scene by -2EV. When this is the case, simply set the exposure compensation to +2EV or whatever is appropriate for the proper exposure. This control is also useful for scenes with a lot of dark background, as they are typically overexposed.

The second time to use the exposure compensation is when shooting HDRs (High Dynamic Range photos). Typically you would set the exposure compensation to -1EV; take a photo, then adjust to 0EV, take a second photo, and finally +1EV, and take the third photo - or in whatever combination you desire. This will give you a set of photos all exposed at different values. Call it "manual bracketing" if you wish.

Do realize that in reality, exposure compensation simply adjusts the shutter speed according to your changes, the ramifications of using too low of a shutter speed or going beyond the camera's capability still exists.

 

Determining the Aperture

The ability to set aperture is one powerful function of a camera, but with it comes the potential to take a really bad photo. Some guidelines to setting the aperture are:

  • For vistas and landscapes, use f/8 to f/11.
  • For great backgrounds in portraiture, use an open aperture; f/2.8 or larger - if your lens supports it.
  • Inexpensive lenses don't perform well at wide open apertures, use f/5.6 or smaller if needed.
  • Avoid apertures smaller than f/11 as they create diffraction (blur) unless you are shooting Macro.
  • Use Aperture Priority for HDR photos whenever possible.

The guidelines are just suggestions, you may find experimentation is the best way to capture those great shots. In an nutshell, open apertures create shallow Depth of Field, small apertures creates extended Depth of Field.

 

Examples


80mm f/2.8 - 8ft distance to subject. Notice the background
is totally blown out. Very nice for portraiture.
 

80mm f/22 - 8ft distance to subject. Notice the background
in this situation is more in focus. Not that great for portraiture.
 
In conclusion, using Aperture Priority unlocks the power of your camera, and makes it possible to take those wonderful "effect" photos by allowing you to control the background. Aperture Priority is the default mode my cameras are set to. Don't be afraid to experiment and practice. Try different shutter speeds, see what works and what does not. Use a notepad and jot down a few notes as you develop your skills and find things you like. There are few rules in photography, and those rules are meant to be broken. So above all else, have fun!