When it comes to making the leap to manual, many photographers may be pleased to know that auto-exposure modes can actually serve as the ideal entrée. Aperture priority and shutter priority, for instance, are great ways to get comfortable with a single manual control at a time. In aperture priority mode (A), the photographer dials in an aperture (also called an ƒ-stop) and the camera chooses the appropriate corresponding shutter speed to produce a correct exposure. In shutter priority (S), the photographer sets the shutter speed and lets the camera choose the ƒ-stop. A little practice with these two modes and you’ll gain a better understanding of the impact aperture and shutter speed have on an exposure. Switching to Manual mode—most often represented by an M on the mode dial—simply puts these two exposure controls together at the same time.
When trying to understand these two controls, it’s important to distinguish how they each impact the light entering the camera. The fundamental difference is simple: The aperture dictates the amount of light let in, while the shutter speed dictates the duration.
Considering these exposure controls, I find it helpful to think about a tangible analog to stand in for light—something like water. A drinking straw, for instance, doesn’t move as much water as a garden hose, which conversely doesn’t move as much water as a fire hose. The bigger the pipe, the more water (or light) can move through at any moment. That’s aperture.
Similarly, the length of time that faucet is turned on is equivalent to the shutter speed. The longer the duration, the more water is delivered—just like a longer shutter speed allows more light into the camera.
So, continuing with water as a proxy for light, if what you want is to fill up a bucket, you can get there with a big pipe and less time or a trickle through a straw and more time—but in each case, the bucket fills up. It’s the same with exposure. You’ve got to pair the two controls correctly to fill the bucket right to the top—no more, no less.
The benefit of working with a digital camera is that exposures can be checked immediately on the back of the camera. Shoot a picture and take a look. Is it too dark? That’s called underexposure (a bucket that isn’t full) and you’ll need more light. That light can be provided by a wider aperture (represented by a smaller ƒ-number) or a longer shutter speed (which are measured in seconds and fractions thereof). It’s your choice; either way will get you there.
Once these basics of manual exposure are understood, photographers can transition from simply trying to make a correct exposure to making an exposure with the depth of field and sharpness desired. In some cases, shutter speed will take priority in order to freeze fast-moving action. In others, it will be aperture based on the need for a shallow depth of field, which comes from a wide aperture. Whatever your priorities, with an understanding of manual exposure controls you’ll know how to adjust the aperture and shutter speed to produce the effects you’re after.