The Law of Reciprocity
The quality of a photograph is determined by the amount of light (known as exposure) that comes into contact with the light sensitive material contained within a camera, be this film, or more likely nowadays, a digital image sensor. This exposure is determined by both the duration of the exposure to the light and the intensity of the light entering the lens.
When considering ISO, shutter speed and aperture you must take into consideration the photographic law of reciprocity. This law states simply that there exists an inverse relationship between the intensity and duration the camera is exposed to in order to shoot at the correct exposure.
In short: total exposure = intensity x time, thus the same exposure can be achieved by decreasing either variant and increasing the other by the reciprocal value i.e. if duration is halved, intensity must be doubled.
It is the shutter speed and aperture of the camera/lens which controls the intensity and duration of light and thus the exposure.
(There are cases whereby the law of reciprocity does not directly apply, known as reciprocity failure, but this will be touched on later after other factors have been explained)
The main function of a camera lens is to gather light. The aperture of the lens refers to the diameter of the lens opening controlled by a mechanical iris, thus determining the intensity of light entering the camera. The larger the aperture, the higher intensity the light and thus more light will reach the image sensor (film or digital).
The aperture however is not usually referred to by the diameter of the lens but commonly in ‘f-stops’ e.g. f/4 or f/2.8. The f-stop refers to the length of the focal lens divided by the diameter. Therefore, the smaller the f-stop numerically, the larger the aperture and the more light entering the body. A large maximum and minimum aperture (or f-stop) is preferable to give the photographer more latitude. With a large maximum the camera will perform in low-light situations (and due to the law of reciprocity, for the same exposure, you will be able to use faster shutter speeds – having its own advantages which will be explained further on). With a particularly low minimum, you will be able to again use the shutter speed to give effects such as blur.
The size of the aperture also determines the depth of field. This refers to the range of distances where images will appear to be in focus. With a smaller aperture you gain a greater depth of field i.e. objects further away will appear in sharp focus – good for landscape photography for example. With larger apertures, you get smaller depth of field, also having its advantages such as eliminating a distracting background whilst taking a portrait photograph. If however, it is a sunny day and you are taking a landscape picture, the aperture is unlikely to make much difference to the final product, the effects will be much more noticeable when taking close ups. If you are in bright light and wish to restrict the depth of field but changing the aperture doesn’t give the desired effect, a neutral density filter can be used to counteract the amount of light and in extreme cases these can be screwed together to make the filters’ effect even greater. On a side note, focal length also affects the depth of field; a small focal length increases the depth of field, for example if you zoom, your depth of field will decrease. Due to having only small image sensors, digital cameras have small focal lengths and therefore you will find it hard to obtain a shallow depth of field regardless of aperture.
You can select ‘aperture priority mode’ on your camera in order to be able to manually select the aperture and the appropriate shutter speed will be automatically selected in order to maintain the correct exposure. As an example, exposure on your display may read ‘500 5.6’, the first number refers to the shutter speed (expressed as a fraction of a second) and the second refers to the f-stop (or aperture). The cameras’ meter is usually sufficient in selecting the appropriate exposure for a shot although very light and very dark backgrounds can often cause issues. Every now and then you may have to manually alter the exposure on the ‘exposure compensation slider’ where you can enable the camera to purposefully under or over expose a shot by up to 3 f-stops. An example is that almost all pictures taken in the snow will be too dark due to the brightness of the snow and to correct the exposure, the ‘exposure compensation slider’ should be moved one or two stops over to compensate - you must remember to manually put the slider back to the middle when the situation changes as it will not automatically reset and subsequent pictures will be incorrectly exposed.
Shutter speed refers to how long the shutter stays open whilst taking a photograph. Along with the aperture it determines the amount of light reaching the sensor or film. Shutter speed is measured in units of exposure or EV, sometimes known as stops. These stops have come about following the adoption of a standardised way of representing aperture. Following the adoption of ‘f-stops’ came the creation of corresponding standardised shutter speed ‘stops’ in a 2:1 scale so that each stop either doubles or halves the amount of light entering the lens and thus each stop moved in either direction can be counteracted to give the same exposure if the aperture is moved a stop the in the opposite direction (i.e. if the aperture is doubled by moving from f-stop 2.8 to f-stop 2, then the same exposure is maintained if the shutter speed is moved a ‘stop’ slower e.g. from 1/30 to 1/15).
The effect of different shutter speeds is the way in which movement is represented. The shorter the time the shutter is open, i.e. the faster the shutter speed, the sharper a fast moving object will appear. Smaller shutter speeds will tend to blur the moving object. Sometimes in order to blur the background and keep the subject of the image sharp, a technique known as panning is adopted. This requires the photographer to select a slightly longer shutter speed and ‘pan’ the camera following the moving object – this is often used to give the effect of fast speeds. Blurring however, can often be beneficial, for example in removing background ‘clutter’ from an image which otherwise would distract from the subject. Of course if you are taking a picture of a static object then you can leave the shutter open as long as you like, a longer exposure can also be used to capture brief flashes of light such as explosions and lightning.
Another effect of shutter speed is the appearance of unwanted blurs caused by camera shake. If the shutter speed is too slow and you are holding the camera by see smalls signs of blurring – this tends to occur at shutter speeds slower than 1/125, a secure pair of hands may be able to get away with 1/60. If you cannot alter the shutter speed to the desired setting most commonly due to aperture limitations, and you need to eliminate camera shake then a tripod is necessary, image stabilisation can also be used to allow you to use shutter speeds 2 or 4 times slower (8-16 times longer) without the effects of camera shake.
When shutter speed is the priority, you can set your camera to ‘shutter speed priority mode’. It allows you to manually alter the shutter speed and the corresponding aperture will be automatically selected. However as a rough guide you should select the shutter speed numerically closest to the lens focal length in order to best reduce camera shake, for example, if using a 35mm camera with a 50mm lens, 1/60 will be the most appropriate shutter speed in normal conditions.
ISO or the International Organisation for Standardisation is a concept carried over from the use of film and refers to the film’s sensitivity to light. The higher the ISO the more sensitive the image sensor and the less light needed for a picture to be correctly exposed. ISO therefore affects the shutter speed/aperture combinations available in order to obtain correct exposure, for example if you find the shutter speed is too slow, you can’t select a quicker shutter speed, your hands aren’t steady enough to eliminate camera shake and you have no means to stabilise the camera, you can select a higher ISO which will allowing you to use a faster shutter speed.
Standard ISO is measured as 100. ISO 200 will give correct exposure with only half the light and therefore is more sensitive – enabling you to shoot in lower light situations with a smaller aperture or faster shutter speed. ISO ranges between 25 and 1600. But there is a drawback to using a more sensitive image sensor, faster films (higher ISO) have a more prominent grain structure and therefore visible spots often appear when using higher ISO’s due to unwanted light also being included within the image. It is therefore always best to use the lowest ISO possible, particularly if you intend to blow up an image to A4 size or larger. With digital cameras, the phenomenon is known as ‘noise’ – when the light levels are low the sensor struggles to read the scene and pixels of random colour are added to the picture. Some digital cameras have built in noise-reduction technology to combat this and software such as Photoshop have filters to remove or reduce the effects of noise (this may reduce the overall sharpness of the image however). Therefore, the lesson to be learnt is to use the lowest ISO in order to achieve the best quality. An example of ISO restricting the quality of shot is if you were indoors, you would need to use a flash if using ISO100 image sensor otherwise the pictures would be too dark, the sensitivity is so low the shutter would have to be open a long time to correctly expose the shot, if you were shooting indoor sports and you could not get away with this slow shutter speed then increasing the ISO may allow you to use shorter shutter speeds at the correct exposure. Several years ago you would have to shoot 24 or 36 shots of the same ISO film whereas nowadays with the advent of digital technology you can change the ISO manually for each shot if necessary to get the best photograph possible, thus ISO is a useful tool in a photographers inventory.
Now with an understanding of the basics of shutter speed and aperture, you are in a better position to understand the concept of reciprocity failure. It refers to a situation where as the light level decreases greatly the increase in duration, and hence the total exposure, is not proportional as the law of reciprocity states, the result being an underexposed photo. This is because film at very low light is disproportionately less responsive. The particular effect varies between types of film/sensor and some are more susceptible to it than others. The phenomenon also occurs as very fast or slow shutter speeds (speeds over 1 and under 1/1000) – the film speed decreases as the defective photons encountered in forming the image are exaggerated. With black and white film, a longer exposure usually corrects the problem but when using colour, not only is longer exposure needed, but often a colour correction filter of a low value.