There are a lot of common mistakes to make in photography, but possibly the most frequent is to shoot in Aperture Priority mode when it’s not the best choice.
There are many situations where shooting in Aperture Priority is ideal, such as when you are using a tripod and your shutter speed does not matter, when you are in strong sunlight, when you are shooting a scene with a wide-angle lens and nothing is moving, or when you want a very shallow depth of field for aesthetic purposes.
However, in most other situations it is beneficial to shoot in Shutter Priority mode (or manual mode).
I see this problem most often when people are traveling. They will stick their camera on Aperture Priority for the entire day, forgetting that the light sometimes will not be strong enough to yield a shutter speed fast enough to offset camera shake or to freeze moving subjects, particularly if their lens aperture does not go below f/4 or f/3.5.
The result is that they will come home and many of their images will be blurry, when they didn’t have to be.
Your shutter speed is the first line of defense for creating a sharp image. The rule of thumb is that the shutter speed should be at least one over the focal length of the lens to offset handheld camera shake. I like to add a little extra speed to that to be safe, so if you are shooting at 50mm on a full frame camera, I would keep the shutter speed at least at 1/80th. On a cropped sensor however, such as an APS-C camera, a 50mm might be the equivalent of an 80mm view, so I would shoot at least at 1/100th of a second.
You can see, especially if you are using a telephoto focal length, how quickly and easily your shutter speed can go below this threshold when on Aperture Priority and not in strong, direct light.
When there are moving subjects in a scene such as people, 1/320th or 1/250th is often a good shutter speed to freeze their motion. So when traveling, I use 1/320th as my baseline shutter speed and will lower it for the situations where there is no motion or when the light is extremely low. This guarantees that no matter what I capture, it will be sharp, including the spontaneous moments. Often, because I shoot this way and prefer to also shoot with an aperture of f/8 or above, I will raise the ISO to 800, 1600, or even 3200 in areas with very low light. This will add more grain/noise to the image, but especially on the newer digital cameras you will quickly notice that the technical quality and sharpness of your images will actually be improved, despite this grain.
Now you might be saying that you are able to shoot in Aperture Priority mode (or Manual mode) all of the time and can pay attention to the corresponding shutter speeds so you never have a problem. That’s great, and in that case, please keep shooting in Aperture Priority. I do not want to stop you from shooting in a way that works well for you. If you know your camera settings well then you can effectively shoot in any situation with any of these modes.
However, I teach a good amount of students and I would guess that about 80 percent of them come in shooting in Aperture Priority. Then, when I look at their photos, for a good portion of these students, way too many of their photos turn out blurry, or have some sharpness issues. They will pay attention to their aperture but then zoom for a shot and not notice that the shutter speed is lower than their focal length, or not notice that is too low to freeze the motion of a subject. I find that switching them over to Shutter Priority for these situations puts the shutter speed in their mind first and then they can pay attention to the corresponding aperture to achieve the largest depth of field possible (if you are purposely looking to create a shot with a shallow depth of field, then Aperture Priority would be ideal.)
Particularly for travel photography in cities or in areas with people, often when I explain to students to try photographing around 1/320th of a second and a higher ISO, they quickly remark about how much sharper their photos become.
Give it a try and see if yours do too.