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  6. P, S, A, and M Modes (Exposure Modes)

Digital SLR Camera Basics

P, S, A, and M Modes (Exposure Modes)

Shooting modes fall into three categories: auto, scene, and P, S, A, and M modes. In auto and scene modes the camera controls shutter speed and aperture. P, S, A, and M modes are known as exposure modes and give photographers a choice as to which elements of exposure—aperture or shutter speed—they wish to control.

Mode P (Programmed Auto)

The camera automatically adjusts aperture and shutter speed for optimal exposure, but the photographer can choose from different combinations of aperture and shutter speed that will produce the same exposure. This is known as flexible program.

Mode S (Shutter-Priority Auto)

The photographer chooses the shutter speed and the camera automatically adjusts aperture for optimal exposure.

Mode A (Aperture-Priority Auto)

The photographer chooses the aperture and the camera automatically adjusts shutter speed for optimal exposure.

  • Note: that in all three modes—P, S, and A—exposure is automatically adjusted for optimal results.

Mode M (Manual)

The photographer chooses both aperture and shutter speed, providing the greatest latitude for creative expression. Choosing the wrong combination could, however, result in photographs that are too bright (overexposed) or too dark (underexposed). We therefore recommend using the camera exposure indicator as a guide when choosing aperture and shutter speed.

Mode
Shutter Speed
Aperture
P (programmed auto)
Selected by camera
Selected by camera
S (shutter-priority auto)
Selected by photographer
Selected by camera
A (aperture-priority auto)
Selected by camera
Selected by photographer
M (manual)
Selected by photographer
Selected by photographer
Choosing the Right Shutter Speed in Mode S
In mode S, the photographer controls shutter speed and the camera automatically adjusts aperture for optimal exposure. Given, however, that the range of shutter speeds available is extremely large—for example, from 30 s to 1/4,000 s, under certain conditions there may be shutter speeds at which no possible aperture setting could produce optimal exposure.
For example, if you select a fast shutter speed such as 1/4,000 s for a dark interior shot, the time the image sensor will be exposed to light will be too short for optimal exposure even if the lowest f-number is used to ensure that the image that falls on the sensor during that time is as bright as possible, and the photograph will be too dark (underexposed). In this case, the aperture display will show “Lo.” On the other hand, if you select a slow shutter speed such as 1 s for a brightly-lit outdoor shot, the time the image sensor will be exposed to light will be too long for optimal exposure even if the highest f-number is used to ensure that the image that falls on the image sensor during that time is as dark as possible, and the photograph will be too bright (overexposed). In this case, the aperture display will show “Hi.”
Underexposed
Underexposed
Overexposed
Overexposed

When shooting in mode S, note the range of apertures supported by the lens to avoid over- or under-exposure.

Sample Camera Displays

camera information display
camera information display

Exposure mode:

Represented by the letters “P” (programmed auto), “S” (shutter-priority auto), “A” (aperture-priority auto), or “M” (manual).


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