Exposure is called in the photograph technique, the collection of the different light distribution of a photographic subject on a photographic film or sensor in a reproducible file. It is influenced by the light sensitivity of the film or sensor, the aperture of the lens and the shutter speed of the camera and is balanced when the lights and shadows in the image have yet duplicable drawing.
The light sensitivity of film and sensor is now given mostly to ISO 5800. The series of current values is:
ISO 100 - 200-400 - 800-1600 - 3200-6400 - 12800-25600
The ascending sequence of numbers is not only computationally a doubling of the previous value, but a film with the next higher ISO rating is twice as sensitive to light as the preceding, in reverse order half as sensitive to light.
The choice of sensitivity depends primarily on the subject contrast. Is the difference between light (not white) and shade ( not black ) in the motif very big, so it possesses a large number of finely differentiated intermediate shades, choose the higher sensitivity because it has a flatter gradation and thus finer details reproduced in the location is. Accordingly, one chooses at low subject contrast a low sensitivity because the gradation is here steeper and comprises less differentiated intermediate tones.
With increasing sensitivity but increases with the manifestation of his films granularity in digital cameras the image noise. Therefore, an average value from the above series of numbers should for most subjects to be sufficient and a low sensitivity can be selected only in exceptional situations, such as when documents scanned or contrast only a few details to be reproduced under design competence point of view or the recording light is too strong, high sensitivity, when the recording light is too weak.
If the sensitivity of the recording medium once fixed, the values of aperture and time just define the exposure.
Aperture is a once formed mostly of thin single slat opening in the lenses whose diameter can often zoom in or out by turning a ring on the outside of the lenses or indirectly via a motor by setting on the camera. Then aperture means the ratio of opening to the focal length of an optical system. A telephoto lens, for example, 50 mm free aperture (D ) and 200 mm focal length (f ) has the diaphragm D: f = 50: 200 = 1: 4, and f / 4 is written. The opening itself is in contrast to other optical devices in the photographic lenses is not specified, because it is for the photographic practice of no further significance. Are mentioned only focal length and aperture, ie: 200 mm - f / 4, where the specified aperture ( here 4 ) is the largest adjustable lens to denote its intensity. Conventional panels are:
F / 1,4 - 2 to 2.8 - 4 to 5.6 - 8 - 11 - 16-22
Here are small f-numbers - ie large openings - only with fixed focal lengths feasible. Between stages are possible, as have many zoom lenses to a maximum aperture of f / 3.5, which corresponds to the level of 2.8 to 4.
The aperture serves not only the exposure, but it also defines the depth of field in the image. If the object to be photographed by blurred background stand out, choose a large aperture. If, by contrast from the foreground to the background in focus are alike, you take a small aperture.
However, the dimming of a lens can not be used arbitrarily, because its optical resolution is proportional to its aperture ratio (D: f). The larger the opening, and the smaller the focal length, the greater the resolution. With the closing of the aperture and the resolving power of the lens is reduced while maintaining the same focal length, therefore. The situation is similar with the resolution of a film or sensor, which corresponds to its granularity or the number of its pixels. The fine-grained, the higher the resolution. If we now compare the resolving power of the lens and the recording medium together, it usually comes at aperture 16 in the area where the resolving power of the lens makes the resolution of the recording material. General image blur is the result.
Exposure Time is the time for which the camera shutter is opened and the recording medium exposed to the projected image by the lens. Typical exposure times are:
1 /8 - 1/15 - 1/30 - 1/60 - 1/125 - 1/250 - 1/500 - 1/1000 - 1/ 2000 s
As with the diaphragm acting on the recording medium, the exposure of step is also here in the order given halved to stage, doubled in the reverse order, so that it also refers to the difference between two values of the time series as aperture or light value.
Short exposure times are used when the recording light is very bright or to stop movement, long exposure, when the recording light is dim or to achieve wiping or flow effects. For exposure times from 1/125 s is downwards especially at long focal lengths due to the magnification effect of camera shake, so use of flash or tripod is required.
Because both the aperture and in the time series, the action of light is halved in ascending order and doubled in descending order, the prerequisite for a correct exposure values for the aperture and the time can be shifted against each other. So you get the same exposure result if, instead of with aperture 8 and 1/ 250 s with more open aperture 5.6 but reduced time 1/500 s exposed. Better cameras offer this displacement of Zeit-/Blendenpaares to as "Program Shift" or with similar names.
For shorter exposure times than about 1/1000 s and at longer exposure times than a second this rule, however, is for recording on photographic film not more fully, because in fact, in both cases for a balanced exposure requires more light than the rule specifies. In the short time range is called the short -term effect in long-term after its discoverer from the Schwarzschild effect.
The determination of the correct aperture and exposure time is carried out with knives.
All photographic recording media and measuring instruments are calibrated on a so-called middle gray, which is why is possible because all colors are reduced from the brightness distribution forth on the different tones of gray. The average is based on light reflection in the inner spaces corresponding to the reflectivity of 18% of the incident light. The difference between this gray and of 90% white, which just represents a little drawing is two aperture values .
In the light meter, there are two ways to measure the light, namely, the light measurement and the measurement object.
The light measurement can be made on the one hand to the subject. In this case you have to go to the subject, which is not always possible. This type of measurement is found only in manual exposure meters, where the incident light is measured at the subject and the meter is held, with advanced dome from the subject in the direction of the camera.
In the reverse direction, in the measurement of the camera to the back motif, is object measurement. In this case, the reflected light from the subject is measured. This type of metering is found in all built- in camera light meters, but is also a possibility of using the light meter in which it is the cap must be pushed to the side.
The object measurement is less reliable because not even the incident light, but the light reflected is measured, which can be reproduced quite different from the individual parts of the subject. The attempt to compensate for the light meter on the basis of mean gray. However, if a subject is not average, but mostly light or mostly dark, set the meter due to their calibration, the Predominant as medium gray ahead and shift the gray scale of the subject in one or the other direction so that light subjects rendered overexposed underexposed and dark motives be. With snow, beach, white house front, deep in the forest and in similar situations, therefore there is a risk to expose wrong.
Underexposed images are too dark and no drawing appears in the shadows more, they run to. Overexposed images are too bright and no drawing appears in the lights more. You seem to be frayed and the colors are washed.
Possible errors in exposure measurement can be avoided in several ways. Simple is an exposure series with at least three to one half or one third aperture value different exposures. Simply it is also to measure a light or dark subject not directly but to the side of the actual subject, because where there is medium gray, and pan the camera with these values on the subject. Reliable and easy it is to combine the above two methods with each other. Reliable, but awkward is the measurement of a gray card.
- Bracketing with histograms
Bracketing: Correct exposure
Histogram of underexposure
Histogram of the correct exposure
Histogram of overexposure
Digital optical devices
With digital optical devices, the metering mode can also be performed by the image sensor itself, with which the image to be recorded. In Live View mode, the user can then even be displayed before the image capture on a screen or in an electrostatic between the viewfinder when overexposure exists by the overexposed pixels are characterized by a bold color or a zebra pattern:
Highlighting the overexposed pixels by a zebra-
Histogram with the corresponding distribution of the frequencies of brightness values - because of overexposure is the white point on the right edge of the histogram very well represented
Exposure without measurement
Makeshift the exposure can be made even without measurement based on experience, as was the case with the old cameras before the advent of the light meter. In the period from two hours after sunrise until two hours before sunset is in ISO 100 and a shutter speed of 1/250 seconds:
In the summer months is to stop down the above information by half aperture value on, undim accordingly in the winter months. Under extreme light conditions may require a correction to an aperture value.