What is the optimal "default" aperture value in AV mode?

Question

This is a very basic question.I am trying to learn how to use AV and TV modes with my T3i (after 2 years of shooting in Auto). What should I set my aperture to in order achieve the best results? I know how to blur the background or keep it sharp by changing the aperture value. However, what if I do not really have a preference ? I usually just want to take a picture of my kids. When I use the auto mode the aperture is set for me. How does the auto mode determine the best aperture value for me and how can I determine the best aperture myself in AV mode? Is this basically a creative choice (i.e. sharp background vs. blurred background) or are there specific rules for the "best" aperture value?

The same question applies to shutter priority mode. I understand that high shutter speed freezes the shot and low value will produce a blur. I would usually want to freeze the shot. Does it mean I should choose the highest shutter speed first?

Thanks!

Answer 1

There is no universal default. There cannot be. The exposure-triangle illustrates that ISO, Aperture and Shutter-Speed are related. Given that proper exposure depends on the scene, there cannot be any starting values.

Really if you do not care about what exposure parameters to choose, use Program mode. This is the mode where the camera decides. Based on the scene and lens, it will choose something to give the metered exposure. Even so, most DSLRs let you shift to choose a different combination of exposure parameters.

In Program mode, a camera has the freedom to choose aperture because it can also at least select the shutter-speed and perhaps ISO. It usually decides based on getting a reasonable shutter-speed to capture an image without camera shake. This depends on the focal-length of the lens, assuming hand-holding the camera. Some cameras may take into consideration whether image stabilization is on or subject-movement is detected.

Given there can still be play with ISO on Auto or shutter-speed in some cases, the camera may be able to choose multiple adequate apertures. If this is the case, some cameras select the sharpest aperture of the lens based on MTF data from the manufacturer. This could be a nice default as you allude to, except that it depends on the lens and selected focal-length for zoom lenses.

Answer 2

There isn't one.

Or, if you must have a default, "auto" would work better than any particular value.

Even ignoring the demands of different lighting conditions and focal lengths, the value you most often use will depend on your personal preferences: whether you prefer a small or a large depth of field, whether you prefer fast or slow shutter speed, whether you prefer low noise or low background blur.

Usually, the most useful range of a lens is from its widest aperture through to about f/8 (on full-frame). Smaller than that, ie f/11 through to f/36, usually doesn't give you much benefit and starts to introduce softness through aperture diffraction. If you need to cut down light this much, it's often (but not always) a good idea to use an ND filter instead.

I'll say what I usually say to people who ask what the best "manual" camera settings are: leave your camera on auto. If you don't know you want a certain aperture, or shutter speed, then you may as well use auto and then go with whatever your camera chooses. Photographers set the aperture or shutter speed when they know better than the camera about what aperture or shutter speed they want to use. If you don't have a strong preference, use auto as a starting point and venture into aperture priority when you have a specific requirement, such as background blur, fast speed, or so on. Using aperture priority mode when you don't know what aperture you want doesn't make sense.

Answer 3

It depends what you mean by "best results".

You want to choose an aperture that allows the image to be properly exposed, while also considering your shutter speed and ISO value. If you aren't shooting at the limits of your gear, then you can typically choose an aperture based on creative choices as well.

For example, if you want a blurry background(narrow depth of field, a wide aperture such as f/2.8 will typically give that. If you want everything in focus(wide depth of field), then select a small aperture such as f/11.

Another thing to keep in mind for aperture is that shooting at the widest aperture your lens allows such as f/1.4, f/2.8, or f/3.5(depending on the lens) - usually is not the sharpest aperture. The lens is more or less at its limit so it is a bit "soft" at that point. On the other end of the spectrum you don't want to use too small of an aperture such as f/18 because you will go beyond what is known as the diffraction limit.

So to answer your question, no optimal "default" exists for aperture in any mode. Start with something like f/8, and adjust to either get the correct exposure, for creative choices, or for both.

Shutter Speed is an entire different question, but of course related. Example - If you need to freeze a subject, then you would choose a fast shutter speed, which may force you to use a wide aperture and which could give you a narrow depth of field and a blurry background. It is a set of trade offs and you have to decide creatively what is important to you.

Answer 4

As a general rule a lens will get sharper as it stops down from wide open to f/8 and then get less sharp as it stops down further, due to diffraction. Other than that it is, as you mention, a creative choice and a matter of working with the light you have available. If you choose a large aperture to allow a faster shutter speed to freeze a moving subject you will reduce depth of field and increase your chance of getting the subject out of focus. In this case, increase ISO as it's easier to deal with noise than motion blur.