Sunday, August 22, 2010

Conttrol your 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 f-numbers. You can calculate an f-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 f-numbers follows a standard sequence with each f-number being half as bright, passing half as much light, as the previous one. A typical aperture range may look like this:

f 1.4; f 2; f 2.8; f 4; f5.6; f 8; f 11; f 16; f 22; f 32

There are smaller and larger f-numbers but the actual numbers used are always the same and will maintain a constant value over different lens focal lengths. This just means that f-8, for instance , will always pass the same amount of light no matter what camera or lens you may be using. Similarly, f-16 will pass half as much light as f-11 and f-4 will pass twice as much as f-5.6. The difference in value between one full f-number and the next is known as a 'stop'. If you change aperture from f-8 to f-5.6 you will give your film one stop more exposure.
The smaller the f-number is then the larger the aperture is and the more light it will pass. The f-number is also used as a guide to the light gathering abilities of a lens. Lenses with large maximum apertures ( small f-number ) are described as being 'fast'.
Generally the aperture will always be held open at its maximum irrespective of whatever value you may have set it to and will not actually close down until the moment of exposure. The main reason for this is to produce the brightest image possible onto the focusing screen. To see the aperture in operation you will have to remove the lens, unless you have a preview control, and look through the lens while turning the aperture control ring.

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. A typical shutter speed range may look like this;

1sec; 1/2sec; 1/4sec; 1/8th; 1/ 15th; 1/30th; 1/60th; 1/125th;
1/250th; 1/500th; 1/1000th; 1/2000th

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.

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.
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-321/8th of a second
f-221/15th of a second
f-161/30th of a second
f-111/60th of a second
f-81/125th of a second
f-5.61/250th of a second
f-41/500th of a second
f-2.81/1000th of a second
f-21/2000th 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 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.

25; 50; 100; 200; 400; 800; 1600; 3200

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.

Over and Under Exposure. ( Briefly)
Giving your film more exposure than necessary will result in overexposure. Pictures will be pale or light with poor washed out colours.
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 realised 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.

    Coming up next.
    In the next couple of bits we will look at the unbelievably interesting use of shutter speeds and apertures. There will be a few pictures or illustrations so it should be a bit less of a drag than this bit was. Also, just to make sure you are paying attention I am going to set you a little test. Not too hard, just enough to make sure you know roughly what is going on with the stuff we have just done.

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