Exposure. Exposure correction.
Exposure is dosed by fixed assets:
ISO sensitivity value
Flash (or other lighting)
Basically, in digital photography, it is customary to say that exposure depends only on shutter speed, aperture and ISO, but in fact everyone who says that lowers the flash operation.
Exposure is measured in units of exposure E.V. (Exposure Value).
Important: if you fix the ISO value and flash power (as, for example, on old film cameras), you will only be able to change the shutter speed of the camera and the aperture of the lens. In such a case, the shutter speed and aperture are called the expopairs. They are a couple, because when you change one, the other adapts to the second. This adjustment is done by the camera.
The most important thing: changing one of the parameters of the main four, you need to change and one of the other three to save the current exposure. So, changing the shutter speed, you will need to either change the aperture value or the ISO value to save the current exposure value.
For the correct exposure in the camera responsible exposure meter. The exposure meter is a special sensor in the camera that measures the amount of light “absorbed” by the lens and calculates the “correct” exposure parameters, aperture and ISO values, and sometimes the flash output to create the “correct” exposure.
Under the correct exposure usually imply a balanced filling of the image with light and dark areas, usually, the exposure meter tries to make the “correct” histogram. In general, as an exposure meter does, it is very difficult to explain on the fingers.
Metering in automatic modes
In automatic modes, in virtually all modes except P, A, S, M, the camera completely determines the settings for shutter speed, aperture and ISO. If the flash is on, the flash power is also calculated. Depending on the shooting mode, the priorities of one or another parameter in the exposure are calculated.
Usually the camera (camera) has the ability to shift the total exposure of the image to the left or right along the histogram (either add the amount of light or reduce the amount of light). A special button is responsible for this, which is very easy to find, it has the designation “+ -“. For example, on Nikon cameras, the “+ -” amendment can only be used in P, A, S, M creative modes, in all other modes the correction will be unavailable.
Exposure compensation with flash off and auto ISO off:
1. In A mode (aperture priority) – when using “+ -” the camera will change the shutter speed to compensate the exposure
2. In S mode (shutter speed priority) – when using “+ -” the camera will change the aperture to compensate the exposure
3. In M mode (manual mode) – the camera will not respond to the “+ -” amendment if the AUTO ISO function is turned off. If AUTO ISO is on, the camera will change the ISO value.
4. In P mode (software mode) – the camera can change both shutter speed and aperture
If the flash is on, then compensation occurs with a change in the flash output and other parameters. Especially, when the flash is on (in the TTL auto flash output mode), the “+ -” correction is strongly felt in the manual M. mode.
The camera’s exposure meter does a good job with some tasks.
The camera’s exposure meter does a good job with some tasks. And the amendment does not need to use
Flash power correction
Another complication when working with “+ -” is a separate flash correction function. This function has the same icon as “+ -“, but the lightning bolt icon is also added. This feature directly affects flash compensation. Changes the same way, by a certain amount of E.V. It is very difficult to calculate the flash power and exposure correction simultaneously. For example, if the usual exposure compensation “+ -” with a value of + 0.3EV and a “lightning + -” with a value of + 0.7EV is on, then the total exposure will have a shift of + 1EV.
And even worse, the flash power when using external flashes changes in two places – on the camera and on the flash itself. The camera may have a flash power correction of + 1E.V., While the outermost flash may have a power correction of -0.7E.V. eventually the photo will get + 0.3E.V. amendments to the total exposure of the standard, which will offer camera automation.