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Camera Exposure
We have taken a look at the creative or artistic bit of taking photographs composition if you read that bit so now we will have a look at the science bit exposure.
Unsurprisingly exposure simply means allowing light to strike your film. The tricky part is knowing how much light you need and how to control the amount of light reaching the film. The former is taken care of by a light meter usually built in to the camera and the latter is achieved by means of the aperture and shutter controls on your camera.
You control the exposure by allowing light to pass through the aperture for a given amount of time. Right now we are going to have a look at the way apertures and shutters are used to control exposure but not the other important functions they perform.
Aperture and f-numbers
The aperture is just a hole whose size can be varied to allow more or less light to pass through it. The size of apertures are expressed in numbers. You can calculate an number if you are keen or don't have much of a life by dividing the lens focal length by the diameter of the aperture. The range of numbers follows a standard sequence with each number being half as bright passing half as much light as the previous one.
Shutter and Shutter Speeds
The shutter prevents light from reaching the film until the moment of exposure when it opens for a predetermined time allowing light passing through the lens aperture to reach the film. Unlike the aperture which is always in an open position the shutter is always closed. Like the aperture shutter values or speeds follow a standard sequence with each one being half that of the next allowing half as much light to pass through.
Shutter speeds are expressed in seconds or fractions of a second. Slow shutter speeds run into seconds while fast shutter speeds will be shorter than 1/500th of a second. In normal photography shutter speeds will probably fall into the range 1/60th to 1/1000th of a second.
As you may have worked out, changing from one shutter speed to the next changes the exposure by one 'stop' in much the same way as changing the aperture.
Exposure Part 2
This is boring
I know and I am sorry but there is more. Now that you know what a stop is you may realize 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
Alternatively, you could start by changing the shutter speed then altering the aperture to compensate. The important point is that you finish up getting the same exposure.
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'. 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 light meter 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.
Exposure Part 3
Over and Under Exposure.
Giving your film more exposure than necessary will result in overexposure. Pictures will be pale or light with poor washed out colors.
Giving your film less exposure than necessary will result in under exposure. Pictures will be dark with poor detail in shadow and dark areas.
Almost done
You will have realized by know that there are three factors involved in controlling exposure Film speed. Once set you do not alter on the same roll of film.
Aperture. Which you can increase or decrease.
Shutter speed. Which you can also increase or decrease.
They in turn share another common factor, which crops up a lot in photography, the stop. Changing either of them by one full setting will always half or double the exposure the film receives.
Increasing one and decreasing the other by the same number of stops gets you the same exposure.
Putting it all together
Here is what will happen when you press the shutter release button:
The mirror will flip up out of the way.( SLR) That is the clunk you hear.
The aperture will 'stop down' to the selected value.
The shutter will open then close.
The mirror will come back down.
The film is wound on by one frame and the shutter is reset. Automatically if the camera has a winder or manually, by you, if it doesn't.
But why ?
You may be wondering why you can't just stick with one film speed, one aperture and just vary the shutter speed or one film speed, one shutter speed and vary the aperture. Well you can, in fact most cameras will allow you to do both of those things. What you need to know is how you do it and more importantly why you would want to do it.
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