This is boring !!
I know, and I am sorry but there is more. Now that you know what a 'stop' is you may realise that to change or control exposure you can alter either one and get the same effect. You may even have worked out that you can have loads of combinations of aperture and shutter speeds that will amount to the same exposure.
Here is a wee example:
Your light meter tells you to set your camera to f-8 at 1/125th of a second. You decide that you want to change it. You will find out why you might want to change it later.
You could reduce the aperture by one stop to f-11 (Stop down or close down). Now your film is receiving half as much light as it requires (underexposure). To compensate for this you select a slower shutter speed of 1/60th of a second so it now stays open twice as long as before and passes twice as much light as before.
Or.
You could increase the aperture by one stop to f-5.6 (Open up). Now your film is receiving twice as much light as it requires (overexposure ). To compensate for this you increase your shutter speed to 1/250th of a second so it now stays open for half as long as before and passes half as much light as before.
| f-32 | 1/8th of a second |
| f-22 | 1/15th of a second |
| f-16 | 1/30th of a second |
| f-11 | 1/60th of a second |
| f-8 | 1/125th of a second |
| f-5.6 | 1/250th of a second |
| f-4 | 1/500th of a second |
| f-2.8 | 1/1000th of a second |
| f-2 | 1/2000th of a second |
You could work your way through the whole range of aperture and shutter speeds as in the table on the right:
When you combine a shutter speed and an aperture you get an 'Exposure Value (EV)'. The table shows a range of shutter and aperture combinations which will all result in the same exposure value. If an aperture of f-8 at 1/125th of a second produces a perfectly exposed photograph then any of the other combinations will do the same.
Film speed
Here is something else just to confuse you.
In order for your lightmeter to come up with a suitable combination of aperture size and shutter speed it needs to know how sensitive to light a particular film is. A film's sensitivity is known as its ' speed' and is expressed as an ASA/ISO number. The higher the number the more sensitive it is and consequently the less light it needs to form an image. The lower the number the less sensitive it is and the more light it will require. Sensitive films are said to be 'fast' and will have a speed of 400 ASA/ISO or above. Films with low sensitivity are said to be 'slow' and will have a speed of less than 100 ASA/ISO. General purpose films suitable for everyday use fall into the 100-400 ASA/ISO range with 100-200 being the most popular.
Like shutter speeds and aperture sizes, film speeds follow a standard sequence.
Wouldn't you know it ! Film speed goes up in steps just like shutters and apertures. Each one is twice as sensitive as the next. I know you have worked this out already but the difference between one film speed and the next is a 'stop'.
As far as exposure goes all you really need to know about film is its speed. It is very important that you set the correct film speed on your light meter before you start. Most modern cameras read the film speed from a magnetic strip on the film cassette and set the meter accordingly (DX coding). Otherwise you will have to set it yourself using whatever method your camera/meter is equipped with.