This article will cover the relationship between ISO (film speed or sensor sensitivity), Shutter Speed (how long the sensor is exposed) and Aperture (the size of the hole in the lens). These 3 parameters truly have a three-way relationship because changing one affects the other 2. This article will cover primarily natural light photography as the addition of strobes causes further complications but the use of strobes can be found in another article in the Knowledge Base.
Film Speed, ISO or ASA. ISO is short for International Standards Organisation. Its comes from the film days when standards were laid down on how film would react to light and is short for ISO 5800: 1987 (colour negative film), ISO 6:1993 (black and white film) and ISO 2240:2003 for (colour slide film). The basis for the ISOs is that doubling the film speed halves the amount of light required to expose the scene correctly. However, there is a downside to making the film or sensor more sensitive to light so that you can deal with lower light levels and that comes in the form of grain or noise. This means that if you want to shoot a scene with low light, you need to accept a certain amount of noise. In addition, as you make the sensor smaller, the chances of noise becoming apparent is higher. Therefore a scene shot with a new dSLR at an ISO of 800 or 1600 would have far less noise than an image on a compact with the same ISO setting. In addition, increasing the size an image is printed or cropped, increases the amount of apparent noise because the noise is enlarged. Noise is more apparent in uniform mid-tones.
Choosing how much grain or noise you want is, to a certain extent, personal choice but it is also subject dependent. Most of my images are scenic images and the addition of noise isn't too much of a problem, in fact, the addition of noise can add ambience. However, close-in shots such as macro do not benefit from noise and so a lower ISO would be recommended. The best way to work out what you noise can accept is to shot a series of scenes with varying ISO but changing the aperture and shutter speed (the relationship is explained below) and printing the images to a size you would want to view them. Once you have got to a level of noise you are happy with, make that your max ISO and base your shutter speed and aperture combinations around that (details below).
Image of the left shot with an Olympus 5050 and the one of the right with a Nikon D200, both at ISO 400
Shutter Speed. Shutter speed is normally stated as 1/x e.g. 1/60, which is 1 sixtieth of a second. Most cameras can cover shutter speeds from 5-10 seconds through to 1/4000 sec but the majority of these fast shutter speeds are unachievable in the underwater environment due to the lack of light. The most common selectable shutter speeds have a 0.5x or 2x relationship with the next step e.g. 1/30, 1/60, 1/125, 1/250 and so on. A faster shutter speed has 2 advantages; it reduces the amount of camera shake apparent in the image and reduces movement in the image of the subject. Choosing a shutter speed to reduce camera shake is approximately half the focal length e.g. 1/60 sec should reduce camera shake on a 50mm lens to acceptable levels. Underwater, you can have less than this because there is damping in the aqautic environment (but you maybe moving in swell or current). There is no real easy equation to cover subject movement but sometimes it can be used to good effect to show movement in the scene. Changing a shutter speed by halving it, only means half of the light gets to the sensor/film.
This image was shot with a low shutter speed to show movement but part of the image has been frozen by a strobe
(operating at around 1/10,000th of a second)
Aperture. Aperture is the size of the hole in the iris in the lens and is stated as something like f5.6 or f22. Typical values for SLR lenses are f2.8 through to f22 and for compacts f3.5 to f8.
The f-value is the ratio of focal length to the diameter of the hole in the lens. A lower f-number e.g f2.8 defines a large hole than f22. The 'stops' that people talk about are the 'steps' as you make the hole smaller and the ratio between these steps is the square root of 2 or approximately 1.41. Again, these changes mean that as you change the stop, you double or half the amount of light reaching the sensor. In addition to changing the amount of light reaching the film or the sensor, changing the aperture changes the depth of field of the image. Depth of Field is the distance from the focal point in the image (front and back) which is considered to be sharp. For a fixed focal length, a large aperture has a shallow depth of field, whereas a small aperture has a large depth of field. However, to complicate matters, a wide angle lens e.g. 15mm with the same aperture will have a large depth of field than a longer lens such as a 60mm; I will cover depth of field in more details in another article.
An image showing a narrow depth of field
A correctly exposed image is created when the required amount of light falls onto the sensor or film to stimulate the sensor pixels or film material. Therefore, if you reduce the aperture by one stop (making the hole smaller and thereby halving the light onto the sensor), you have to either reduce the shutter speed by one stop or increase the ISO to double the amount of light falling onto the sensor and expose the image correctly. If you lower the ISO (reduce sensitivity and reduce grain/noise) from 800 to 400, then you have to reduce the shutter speed by one stop e.g. 1/30 becomes 1/15th increase the aperture (make the hole bigger) by one stop e.g. f8 to f5.6 to get the same amount of light onto the sensor/film. This may prove to be a problem because the aperture might be fully open and the low shutter speed will cause blurring of the subject. Unless the effect is to have a blurred image, like the one above, then it is better to have a noisy image than a blurred one, so bump the ISO back up again until you can handhold the camera. Another way of getting more light onto the subject is by using a strobe and these will be covered in another article.
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