Exposure is based on three elements. shutter speed, aperture size and the ISO rating of the film being used (or sensor sensitivity). Assuming we are using an ISO rated film or sensor of 200. Lets say our light meter reading is 1/125 @ F11. This will give us the correct exposure. 1/250 @ F 8 will give us exactly the same exposure. Which exposure setting we use depends on other creative considerations. Such as whether we want to freeze the action or show movement. Do we want a shallow depth of field or have everything in focus? Lets say we want to freeze an action shot with no movement visible at all. We would need to select a shutter speed of 1/500 or faster. Using the exposure example given above.( 1/125 @ F11), we would need to open the aperture up to F5.6 if we wanted to use a shutter speed of 1/500. This would balance the exposure. By speeding the shutter speed from 1/125 to 1/500 we are giving the exposure 2 full stops of less exposure. To ensure the same amount of light is passing though onto the film or sensor, we need to open or increase the aperture to allow more light through. Opening the aperture by 2 stops from F11 is F5.6. see the table below. All these setting give us the same exposure.
Exposure is controlled by three camera controls which are:
If you think of the shutter as a tap on a water pipe, it controls the flow of water by opening and shutting at different speeds. The faster the tap is opened and shut the less water goes through it, the slower it is opened and closed the more water passes through it. So the shutter controls the amount of light passing through to the film/sensor. The faster the shutter is set (usually in fractions of seconds ½, ¼, 1/8. 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000) the less light passing through, this helps to freeze action, a slower shutter speed gives the subject more time to move and results in more blurry images. Each setting on your camera lets through ½ or double the amount of light than the previous setting so as we move through the sequence of settings 1/15, allows twice as much light as 1/30 (of a second) and half as much as 1/8 (of a second) and so it is with each of them.
Using our water pipe metiphor, then the aperture is like the pipe diameter, a wider pipe allows more volume of water through it, a narrower pipe allow less water through it. The aperture is actually located in the lens and closes down the diameter of the lens to make the diameter that the light passes through, smaller and smaller as we go down in what is described as F-stops. The settings range usually from f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32 (these are fractions of the maximum diameter of the lens) so f2 is a biggest diameter, and f32 is the smallest in the examples given. There are wider apertures available than f2, but these are only available on the most expensive professional lenses. As with the shutter settings moving from one setting to the next halves or doubles the amount of light of the previous setting. I will speak more in depth about this in a separate post
ISO refers to the sensitivity of the sensor (digital cameras) or film usually going from 50, 100, 200, 400, 800, 1600, 3200. So 50 ISO is the least sensitive setting but provides the best quality image, and 3200 ISO is the most sensitive setting but provides the poorest quality image. Again going from one setting to the next doubles and halves the amount of light captured in the camera. ISO is usually the last resort change only when you are not able to make the necessary adjustments with either of the other two settings (shutter speed or ISO)
All three come into play to provide the right exposure for any given photo. The right exposure depends on the situation you are photographing, and what you want to achieve from the photograph. But assuming we want to move through the setting to achieve the same exposure for a particular setup this can be done by balancing one control with the other, see the table below to see how this works. We are using a starting exposure setting of 1/250, f11, 400 ISO highlighted. All setting provide the same exposure.
As we adjust the shutter speed to either let more light through or less, we need to balance the exposure with the aperture. If we let more light in via the shutter setting, we need to move the aperture to let the equivalent less light through (assuming we leave the iso unchanged).
Changing the ISO
As a professional photographer working in UK, the biggest problem we come across is being in situations were there is not enough light in a situation, say we are working in a dark church during a wedding, and need to get a shot of the action as the bride and groom are enjoying the ceremony. We may have the camera on a tripod but we cant really risk shooting an action shot much slower than 1/30 because the bride and groom might be moving and a slower shutter speed would result in blurring of the subjects. We might have the aperture open to its maximum to say f2.8, and we still might not have enough light hitting the sensor to get a bright enough exposure. In this instance we can use the ISO to help us out, if we move from 400 ISO to 800 ISO we have suddenly given ourselves double the amount of light we had previously, and if we move to 1600 we have double the amount again. This might be just enough to get a bright enough shot to add to the wedding album. Of course we can always use flash but usually this is not an option in a church during the ceremony.
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