Manual-enabled modes give the photographer control over the various parameters of an exposure. There are three exposure parameters – aperture, time (shutter speed), and sensitivity (ISO), and in different modes these are each set automatically or manually; this gives 23 = 8 possible modes. For a given exposure, this is an underdetermined system, as there are 3 inputs but only 1 output. Accordingly, there are many combinations that result in the same exposure – for example, decreasing the aperture by one stop but increasing the exposure time or sensitivity to compensate, and there are various possible algorithms to automatically choose between these.
Most often, ISO is considered separately, being either set manually or set to Auto ISO, and then only aperture and shutter speed need be determined – either determines the other.
The four main modes, sometimes abbreviated "PASM", are:
- P: Program mode has the camera calculate both shutter speed and aperture (given a manually or automatically selected ISO). Higher-end cameras offer partial manual control to shift away from the automatically calculated values (increasing aperture and decreasing shutter time or conversely). The difference between Program mode and Full Auto mode is that in program mode, only the exposure is automatic, while other camera settings (e.g. shooting mode, exposure compensation, flash) can be set manually; in Full Auto mode everything is automatic.
- A or Av: Aperture priority or 'Aperture value' enables manual control of the aperture, and shutter speed is calculated by the camera for proper exposure (given an ISO sensitivity).
- S or Tv: Shutter priority or 'Time value' enables manual control of the shutter speed, and aperture is calculated by the camera for proper exposure (given an ISO sensitivity).
- M: Manual mode both shutter speed and aperture and independently set manually (with ISO sensitivity also set manually), where proper image exposure requires accurate manual adjustment.
Together with setting ISO manually or automatically, this yields the 4×2 = 8 possible combinations of manual/auto.
Exposure is further controlled in each of the above modes with an independent setting for:
- Ev: 'Exposure value', which enables an increase/decrease in image exposure compensation to make the resulting image brighter/darker, typically selectable in steps of whole or partial exposure "stops" (discrete widening/tightening of the aperture). Many cameras offer "exposure bracketing" where sequential images will be exposed at the different compensations selected, so as to increase the probability of a perfectly exposed image.
Less commonly seen modes include:
- Sv: Sensitivity priority or "ISO priority" controls the Sensitivity value (ISO speed), with both shutter and aperture calculated by camera, similar to Program mode. This mode is found on some Pentax cameras; on many cameras (such as Canon and Nikon) this is not a separate mode, but instead is accomplished by using Program mode and manually selected an ISO.
- TAv: Enables the user to control both shutter and aperture values. Sensitivity (ISO speed) is then calculated by the camera. This mode is found on some Pentax cameras, whereas other manufacturers may provide this functionality through automatic selection of ISO speed in manual mode.
- DEP: In DEP (DEPth of field) mode, seen on some Canon cameras, the aperture is set to yield the desired depth of field: one points at the nearest object that one wants to be in focus, half-presses the shutter, then points at the farthest object that one wants to be in focus, half-presses the shutter, at which point the camera sets both focus and aperture so that both objects are in focus. One then reframes the scene and fully depresses the shutter to take the photo. Unlike other modes, this also sets focus, and requires two separate metering/focus stages.
- A-DEP: Canon also offers A-DEP (Automatic DEPth of field) mode on some cameras, which sets the depth of field and focus in a single shot. However, this requires lining up both the nearest and further objects on autofocus points at the same time, which may be difficult.
In cases where there is camera discretion (e.g., Auto ISO), different cameras allow different configuration of how decisions are made. For example, As of 2008, Nikon cameras allow one to set the maximum and minimum ISO sensitivities, and slowest shutter speed that will be used in automatic modes, while Canon cameras will select within the fixed range of ISO 400–ISO 800 in Auto ISO mode. In Nikon cameras, the Auto ISO mode first adjusts the shutter speed, keeping ISO at its minimum desired value, then, when shutter speed reaches the user-defined limit, the ISO is increased, up to the maximum value.
All of the above functions are independent of lens focus and stabilizing methods.
Read more about this topic: List Of Digital Camera Modes
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