Exposure of a film (or digitized image on digital image sensor) is the correct amount of light, that should act on the emulsion of the film. It should preserve highlight details as well as record details in shadow. Mid tone falls within this range, where maximum color tonality and saturation is recorded. An over exposed film is a washed out image. Light toned area is completely burnt and become white. Shadow area, which should be dark grey and nearly black is recorded as a mid grey with unwanted details.
On the other end, under exposed film is reverse result of an over exposed film. In this case, dark area become jet black without any detail and the whitest part is not white but light grey. Both the results are for dust bin!
Exposure of film depends on four factors:
Intensity or brightness of light
ISO rating of film (or ISO setting on digital camera)
To understand exposure, let us know each factor.
Intensity or brightness of light: This factor is controllable, when we are shooting with artificial lights, in a studio. If we are shooting outdoors, then intensity of light is not in your command. If all other factors are constant then increased intensity of light will over expose the film.
ISO setting: ISO - (International Organization for Standardization) is the sensitivity index of a film towards light. It is determined by sensitometry and it is uniform for all makes of film. Digitized technology also follows the same parameters and standards. If we keep aperture, shutter speed and intensity of light constant, then for given ISO, the exposure will remain same, whether it is a on film or digital image sensor.
Higher ISO number indicates that the film is more sensitive, towards light. So 200 ISO film needs less light compared to 100 ISO film (in fact half the amount of light) to give the same result, as it is more sensitive for light.
Lower ISO films have finer grains. When ISO is higher, the result is grainy. Digital format also has a problem with higher ISO. In this case, "noise" is increased with higher ISO value.
"Noise" is an artifact problem caused at higher ISO. Few pixels randomly get charged to give one solid dot of red, green or blue color spot. Higher the ISO value, more is the density of these unwanted color spots.
If all other factors (intensity of light, aperture and shutter speed) are constant, then higher ISO setting will over expose a photograph.
Aperture:This is the opening of lens, expressed in f-no. f2 has a standard value, which is double compared to the next value f2.8. Aperture (read more) is mainly selected for depth of field but its side effect is on exposure. Smaller the opening, lesser will be the amount of light reaching the film. This has to be compensated with either slower shutter speed or higher ISO or increased intensity of light, or combination of all factors. Again, if other factors are constant, then wider opening will over expose a film.
Shutter Speed: This is duration of time, for which, shutter door remains open to expose the film. The numbers are 1, 2, 4, 8, 15 and so on. Each number is 1/number of a second. Shutter speed (read more) has to be set for its effect for freezing or recording the movement but the side effect is on exposure. As each numerical value is double in duration, if the shutter is remained open for longer time, it will over expose the film.
If all other factors are constant, then slower shutter speed will over expose the film.
By understanding these factors, we can control exposure with correct density, where the result is appealing. Simple comparison of exposure is like filling a vessel with tap water. In this case, pressure of water is intensity of light, capacity of vessel is ISO, opening of tap is aperture and time required to fill the vessel is the shutter speed. When any factor is altered, we have to compensate it with other factors, as the end result should be vessel completely filled with water till its brim and no water should overflow!
Variables and limitations in case of exposure of film in photography is where most of beginners cannot understand what went wrong. When we are on aperture priority mode, we set the aperture and the camera meter will select shutter speed. (ISO and intensity of light is constant). Or, when we are in manual mode, the camera will indicate whether we are over exposing or under exposing the image. However, in both the cases, the camera meter will behave for the way it has been designed and calibrated. All meters are calibrated for an average tonality, brightness and contrast ratio. 95% of photographs an average person will click falls within this calibration. However, in extreme case, when the subject is beyond average, the meter is fooled.
As a camera meter is taking readings of reflected light, when you are shooting snow, the meter will be fooled by the brightness of snow and try to bring it as an average brightness. As it is not designed to understand the whiteness of the subject as white it will suggest underexposure! So in this case, you must over expose the image manually to record snow as white. Similarly, in case of black subject, the meter will try to bring the result as mid grey. So in this case, in order to keep black as black, you must under expose.
In these extreme conditions, you can use 18% grey card to take the reading. This card is calibrated with meter and whatever reading the meter is giving, follow them. You have to shoot on manual mode.
Another way to solve this problem is to take reading of light falling on the subject, and not the reading, which is reflected from the subject. This is incident light mode on external exposure meter. Now the meter has no concern with the nature of the subject and will give indication according to intensity of light.
One more reason why a camera meter fails is because of your setting. The in built camera meter can be set for matrix metering, centre weighted or spot metering. You must know how the meter is going to behave for each setting.
Matrix metering (or multi-zone) takes readings from different area of the image and then evaluate to give best possible result. Refer the camera manual for detail as it may vary for different makes and models.
Centre weighted meter is most sensitive in 20 to 40% area, in the centre of an image. It will measure average reading which works in most of the cases.
Spot metering is most sensitive in 2 to 5% in the centre. It is not influenced by other area in the image. This has a great advantage when you are shooting a portrait and the sun is behind the subject. Though bright light is falling on the camera, spot reading will measure the light only from face.
Depending on these setting, you should set your exposure.
Flash light meter
How to use exposure meter and measure light
How to use flash light meter and measure studio flash light
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