It stipulates that one stop increases in aperture is equivalent to the shutter duration doubling. Both increase light by one stop.
Thus, once you have the correct level of light for perfect exposure you can choose to increase aperture by one stop and trade this off with a doubling of the shutter speed (halving of the shutter duration). Twice and much light coming in for half the time = same amount of light.
This allows the photographer to retain the same exposure but change either aperture or shutter for artistic or practical reasons.
In addition to this the photographer may wish to over expose or under expose. Understanding the law of reciprocity allows them to do this in a controlled and intuitive manner, understanding the exposure differences on the image PLUS the different changes to the depth of field or any motion blur increase or decrease.
Sometimes however, the aperture you want and the shutter speed required do not give an adequate exposure with the available light (either natural or including flash). Fortunately there is another variable which effects exposure with the same degree of effect and working in the same measurements of stops of light.This is ISO and refers merely to the cameras sensor’s sensitivity to light.
You therefore have THREE variables at your disposal. Each however alter the image beyond just the brightness of the final image.
The Law of Reciprocity
In summary image brightness is determined by the amount of light that contacts with the light sensitive sensor and how sensitive that sensor is.
This is controlled by the duration of the exposure (the shutter speed), the intensity of the light (aperture) and the sensitivity of the sensor or film (ISO).
When setting these elements you take into account the law of reciprocity which states an inverse relationship between the intensity and duration the camera is exposed to in order to shoot at the correct exposure.
ISO merely acts as an additional method of control.
Exposure = intensity x time so equivalent exposures can be gained by decreasing one variant and increasing the other proportiionally. So if duration is halved, intensity must be doubled.
Aperture How FAST does light get in.
Aperture is the diameter of the lens opening controlled by an iris. The larger the iris the more light gets in over a given period of time.
Aperture is discussed in f-stops’ e.g. f/4 or f/2.8 (the length of the focal lens divided by the diameter) but like all things photographic this refers to stops of light. The smaller the f number the larger the aperture.
Apertures can be remembered with a simple concept, the numbers 1 and 1.4 doubling alternately. So
1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 with each one being an increase by 1 stop.
therefore f/1.4 is one stop higher than f/1… f/8 is three stops higher than f/2.8
Larger apertures are preferable in low light to allow more of the light in. Every lens has a maximum aperture or aperture range (for some telephotos where the aperture closes when zoomed in)
The larger your aperture however the shallower your depth of field.
Shutter Speed How LONG the lens lets light in for.
A cameras shutter normally remains closed. When the button is pressed the camera’s shutter opens for a given period of time. (either set by you or the cameras auto settings). The ‘Shutter speed’ refers to how long the shutter stays open for.
A shutter speed of say 1/100 sec is open for twice as long as say 1/200 second, letting in light for twice as long.
Rather conveniently shutter speeds are discussed in seconds and fractions of seconds and can also be compared in stops of light with doubling of duration meaning an increase of 1 stop.eg.
1/ 100 second is 1 stop of light brighter than 1/200. 3 seconds is 1 stop less than 6 seconds.
Therefore, along with the aperture setting, shutter is controlling the light reaching the sensor or film. How sensitive that sensor or film is of course a different matter.
ISO How ligh sensitive is the cameras sensor or film?
ISO (International Standards Organisation) is an older concept from the days of film. It refers to sensitivity to light and brilliantly all round it works on the basis of again, stops of light.
The higher the ISO the more sensitive and the brighter the final image. ISO can therefore be used to effect shutter and aperture combinations.
If you find the shutter is too slow for the fast action you are shooting and you cannot achieve a wider aperture to allow light in faster to counteract the problem, you can then select a higher ISO making the camera more sensitive meaning you can speed up the shutter speed.
Standard ISO is measured as 100, 200, 400, 800…. and these doubling numbers represent 1 stop of light each time it doubles. 100~ISO is 1 stop of light darker than 200~ISO…. etc….
In summary, once you understand the relationship between shutter speed and aperture, and then how you can utilise ISO to further hone your settings you are safe to use a digital slr in its’ manual mode and not have to rely on manufacturers program modes for lazy photographers. As these program modes make assumptions which may not be what you’re looking for.
Hi, my name is Keith Trigwell. I’m a live music photographer and I also have a passionate interest in most other types of photography, particularly portraiture and Fine Art. My live music shots can be seen at http://www.flickr.com/bigmojo
To pass the time I also write a few articles on photography and technical photographic matters.
