The Exposure Triangle
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 100. Let’s 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? Let’s 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 moving 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.
This table is true when the ISO is set at 100 in a given lighting condition. Should you wish to change your shutter speed and retain your Aperture then select a different ISO as your fixed element of your exposure triangle.
Each change in ISO (100 to 200, 200 to 400. 400 to 800 and so on) will double the exposure.
|200 ISO||400 ISO|
|Shutter Speed||Shutter Speed||Aperture|
Your camera programmes work on the Exposure triangle principle.
The sports photographer would select a shutter speed to stop the action e.g. 1/1000 at f8 on a dull day using 400 ISO. The Landscape photographer using a tripod may use 1/60 at F22 (200 ISO). How do we get to such figures as F11 or F5.6 ?
If a Lens is 100mm long and has an aperture of 12.3 mm then the minimum aperture is f8. 100 divided by 12.3
How the metering works in a DSLR.
Until the advent of the SLR exposure was obtained from a hand held exposure meter.
There are various ways of using a hand held meter but for many the use of an inverdome over the light sensing cell proved to be almost fool proof as it would read the light falling onto the subject rather the light reflecting off of the subject as is with the metering within your camera.
Depending on the subjects colour different amounts of light will be reflected, for instance; a snow scene will reflect up to four times the light that is falling onto it (2stops).
A black cat in a shadow would prove to be difficult or a person with their back to a bright light source. By selecting the correct metering mode on your DSLR you may overcome some of these extremes. Spot metering may be used to photograph a person against the light source, Average metering for the landscape (keeping the sky in the top third) or Matrix for people or sport as this type of metering has many more metering points than other types of metering.
If by experience you find that a given situation requires more or less than the indicated exposure then use the exposure compensation control on your camera, as this will keep the over or under exposure to a constant.
By far the most important aid to correct exposure is the Histogram chart on your camera which will show you the amount of exposure against a scale of pixels with range from black to white and often will indicate highlight over exposure. Again the histogram will depend upon the reflective nature of the subject so be aware of the likely amounts of white and black (Red, Green and Blue) pixels.
In most cases you will get reasonable exposures from the automatic setting on your camera,
however when you come to process your image you may find that you need to make a lot of
adjustments. Should this be the case then think have you been using the right exposure mode or compensation?
The Hand Held Rule; Always use a speed that relates to the length of lens.
50mm lens = 1/60, 100mm Lens = 1/125, 300mm = 1/500 and so on. Use the nearest speed to the length of lens or better.