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Camera Lens Aperture

The aperture diaphragm is a clever device that acts rather like a valve, allowing more or less light through as desired. It appears similar to the iris of the human eye, with a hole that is able to be altered. They mostly consist of a series of "leaves" that close when the photographer moves the control but the earliest types were strips of metal with holes punched in them, called Waterhouse stops. A camera would be issued with a set of Waterhouse stops of various sizes - the required stop would be slipped through a slot in the side of the lens barrel. This is the origin of the expression "stopping down", which means to reduce the size of the aperture. A development of this was a circular disc, again with holes punched in it, but this time built into the camera. The required stop was simply rotated into position. Both these early forms of aperture control are incredibly cheap to make they enjoyed considerable longevity, surviving in box cameras into the 1960s. However the majority of cameras with aperture control use the iris diaphragm type, the design is hardly unchanged since it was first introduced.

Aperture leaves This Pentacon made diaphragm consists of 19 leaves but most have far fewer and the hole formed is often less than circular. Early diaphragms tended to use more leaves and consequently produced neater looking holes, but photographically there is no advantage. The physical size of the hole is relative to the focal length and also the position of the diaphragm along the lens barrel. The physical size of the hole for a 500mm telephoto will be considerably larger than the equivalent stop with a 50mm focal length lens. The 'f' number is in fact the denominator of a fraction of the focal length, so that the hole formed by the aperture leaves at f/11 for a 50mm lens will be given by 50/11 = 4.5mm diameter, assuming the aperture leaves are fitted at the objective lens. In practice this is never the case, so the size has to be compensated for, depending how far back along the barrel it is.

The diaphragm may be placed in front, behind or sandwiched between lens elements, the latter is desirable as it prevents a form of image distortion called barrel distortion.

The purpose of the aperture is to regulate exposure, so what aperture should you use? It's fairly obvious that the brighter the conditions, the smaller the aperture can be for any given exposure time. This is important to remember as exposure time and the size of the exposure are inextricably related when it comes to the total amount of light that reaches the film. Prior to the existence of light meters, photographers learnt to judge the exposure by eye. The starting point for acquiring this skill, is the f/16 rule. Put simply, it states that the exposure on a bright sunny day around mid day is the reciprocal of the film speed at aperture f/16. 'Reciprocal' is just a mathematician's word for "One divided by". so assuming you have a film of 200ASA and it's bright and sunny, you set the camera to f/16 at 1/200th sec. But wait! This assumes the subject matter is quite bright, if the colours are fairly absorbent, rich deep green fields maybe, you need to allow for this, so open up one stop to compensate. After a while it becomes really good fun to see how close you can guess. Modern colour print films have a considerable margin of error, which helps. Efke black and white film has a similarly wide latitude. Below is a rough exposure table for use with vintage cameras.

 Shutter Speed

 Dazzling Bright Sun

 Sunny

 Bright but Cloudy

 Cloudy

 Dull

 @1/Film ASA

 f/16

 f/11

f/8

 f/5.6

 f/4

If you want to use a different shutter speed to the film reciprocal, you need to adjust the aperture accordingly, if you halve the shutter speed you need to open up one stop and vice versa should you wish to double the speed. So imagine the day is sunny and you have a film of 100ASA. Assuming the subject matter is reasonably reflective, a shutter speed of 1/100th sec would be used in conjunction with an aperture of f/11. However should you require to use a faster speed of 1/200th sec, you would open the aperture one stop to f/8.

Apart from the role of regulating the amount of light entering the camera, the aperture has an important effect upon focus in terms of the depth of field.

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