It’s the most fundamental question when you pick up a camera: How will I get the right exposure? There’s no default answer, of course, because depending on the photographer, the subject and the lighting, different exposure modes accomplish different things. You may know that Program mode is fully automatic, and Manual mode is fully up to you to get the exposure right, but how do you best put these and other exposure modes to use? Here’s a breakdown of how the exposure modes work and how you can make them work best for you.
Program (P) Mode
This mode is the fully automatic setting so many photographers adore. When Program mode is dialed in, the photographer chooses an ISO and the camera automatically selects the shutter speed and aperture combination to produce what it believes to be the ideal exposure. Most cameras allow you to then adjust either setting on the fly and the camera will adjust the other setting to compensate. This won’t change the exposure, but rather it would allow you to, say, switch to a smaller aperture for greater depth of field. If you want to adjust the exposure in Program mode, exposure compensation is the only way to do so. By adjusting exposure compensation to + or – a given number of stops (or fractional stops) you’re telling the camera you’d like to see, for instance, additional exposure from what it believes to be correct.
Program is a particularly useful camera mode when you’re learning to take pictures or when you just want to be relatively “safe” and know that you’ll get a shot. Given that Program alters two variables of shutter speed and aperture, you’re less likely that either value will max out and allow the exposure to fall out of range. More on that in a moment, as we talk about the next two auto exposure modes, aperture and shutter priority.
Aperture Priority (A, Av) Mode
In aperture priority mode, the photographer sets the aperture (that’s why it has the priority) and the camera automatically dials in the appropriate shutter speed to achieve good exposure. Aperture priority is particularly useful when you want to control the depth of field. For instance, if you know you want to have the maximum depth of field, you dial in the smallest aperture and let the camera choose the shutter speed that will combine with it to create a good exposure. But aperture priority can also be useful when you want to make an exposure decision based on shutter speed. How’s that, you may ask? Simple: If you know you want to use the fastest shutter speed possible, you set your aperture to wide open and then the camera will therefore set the fastest shutter speed that works for the available light. Sometimes you have to think outside of the box this way to get the most from auto exposure modes.
Shutter Priority (S, Tv) Mode
With shutter priority, the photographer sets the shutter speed and the camera reads the light and chooses the appropriate corresponding aperture. So, for instance, you might choose a shutter speed of 1/250th in order to ensure a handheld portrait is tack sharp, then let the camera dial in the aperture that best pairs with it. I tend to use shutter priority in this way, where I want to establish a setting that will ensure I minimize motion blur. But shutter priority is also useful when you deliberately want motion blur. For instance, if you’ve established that a shutter speed of ¼ is perfect for panning with a moving subject, shutter priority allows you to do that while the camera will dictate the corresponding aperture.
In both cases—aperture priority and shutter priority—you’re at a slightly higher risk of an incorrect exposure, either under or over depending on the lighting situation. This is why full program auto is generally safer in that regard, albeit providing less control. For instance, if you set the camera to shutter priority and a shutter speed of 1/250th, if the available light is low and the maximum available aperture (let’s say it’s ƒ/2) isn’t enough to provide adequate exposure, the image will be underexposed. Likewise, if you set the camera to aperture priority and a high ISO with a wide ƒ/2 aperture, under bright daylight the camera may not be able to set a shutter speed fast enough to prevent overexposure. Program mode would solve these issues by adjusting both the shutter speed and aperture.
Manual (M) Exposure Mode
Manual exposure mode allows the photographer to set all of the exposure settings (ISO, aperture and shutter speed) even if the camera thinks it will produce an incorrect exposure. It’s for this reason that manual mode offers the most control and is the choice of professionals in many circumstances. That’s not to say there’s anything wrong with auto modes; I’m a longtime professional and I use aperture and shutter priority frequently. But I tend to default to manual because I want to have control over every setting and I don’t want to rely on the camera to choose on my behalf. Most importantly, though, when a photographer works with strobes—as I often do—manual exposure mode works better because the auto modes can’t accurately predict how a manual off-camera flash will impact the exposure.
Scene Exposure Modes
If a camera has a mode dial, chances are good that dial will have several scene modes to choose from. In some cases, though, scene modes are found in menus. Regardless, scene modes are essentially variations on the fully automatic program mode, which influence ƒ-stop and shutter speed choices based on variables for photographing certain kinds of subjects. For instance, Sports mode will prioritize fast shutter speeds, increasing the ISO as necessary. Landscape mode will prioritize small apertures for maximum depth of field and disable the built-in flash. Macro mode will enable close focusing and prioritize maximum depth of field (a small aperture) and, on cameras with built-in flash, the flash may fire as well. Portrait mode will choose a wide aperture to minimize depth of field and put the emphasis on the subject, while Night Portrait mode will set a slow shutter speed to maximize ambient exposure for the background and then fire the built-in flash to illuminate the foreground subject.