Shutter Priority vs Aperture Priority
Shutter Priority Mode (indicated by the S mode on Nikon DSLRs or the Tv mode on Canon DSLRs) is usually used when you want to freeze motion with a specific fast shutter speed, or to create motion blur by intentionally choosing a slow shutter speed. In this mode, when you choose a particular shutter speed (and ISO setting), the camera will choose a corresponding aperture (F-stop) based on the exposure reading obtained through the camera\’s built-in exposure meter.
Having said that, I do not use this mode at all, because I can achieve the same results with Manual Exposure Mode, or even Aperture Priority Mode. In fact, beginners trying to grasp the principles of exposure should stay away from Shutter Priority Mode. This is because it is very easy to get an incorrectly under-exposed shot (too dark) in Shutter Priority Mode.
This is because every lens has a maximum aperture, which means that the aperture can only open up so much. In this mode, the aperture selected by the camera will depend on the shutter speed selected by you. If you inadvertently choose a shutter speed that is very high (eg. 1/1000 at ISO 100) in an attempt to freeze the motion of a jumping child inside a dimly lit room, the aperture chosen by the camera will likely be the largest possible aperture (typically F3.5 on an entry-level lens) and yet fail to gather enough light to create a proper exposure. This exposure error usually will not happen if you are shooting outdoors in a brightly lit area.
On the other hand, if you were using Aperture Priority Mode for the scenario described earlier, you would not get an under-exposed shot. This is because you would be selecting the aperture, while the camera will select a shutter speed to match your selected aperture.
If you select a small aperture (eg. F22) the camera will select a slower shutter speed in order to let in enough light. This will not be able to freeze the jumping motion of the child. Therefore, you would be selecting the largest aperture that your lens can handle (different lenses have different maximum apertures), so that the camera would select a faster shutter speed. At this point, you can check the LCD to see if the shutter speed is sufficiently fast to freeze the motion.
If it is not fast enough, you can consider increasing the ISO setting, thereby allowing your camera to select an even faster shutter speed. If you have reached an ISO setting that you would not go beyond (for fear of creating excessive image noise) you can then consider using a speedlight in Manual Exposure Mode. This method will ensure that you never end up with unintentionally under-exposed shots.
This is the reason why I prefer to use Aperture Priority Mode over Shutter Priority Mode. It allows me to work within the limits of my gear, and use alternative methods if there is insufficient light.
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