Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Essential principles

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8 décembre, 2011 Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISOCopyright David Keast/Association Fils et Cordes

Essential principles chapter 1

• Aperture, Shutter timing and ISO (sensitivity) setting

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Aperture, Shutter timing and ISO (sensitivity) setting

• Your camera records light. – Too much light and the image is too light– Not enough light and it is too dark.

• To let more light from your subject into the camera you need either : – the hole must be open for longer (shutter

timing) – or a larger hole (aperture)

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Automatic exposure control

• Your camera's automatic exposure control works this out for you and adjusts the shutter timing and aperture to give the correct exposure– BUT…. there is more than one way to

achieve the correct exposure and the results have some important differences.

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Look at this pair of images

• They were taken at the same time, same camera, same light. Why are they different ?

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In this image the movement of the water is 'frozen' by a fast shutter timing

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Here the water is allowed to blur,

much as your eye sees it

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Both are correct exposures

• You can choose, it is part of the creative process – but you have to tell the camera what to do – if you use fully automatic exposure control,

the camera decides for you.

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Another pair of images

• taken at the same time, same camera, same light - why are these different ?

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Here the background is sharp

• the impression is of a room with a bottle in the foreground

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Here the background is out of focus

• the main impression is of the bottle in the foreground

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Both are correct exposures.

• You can choose, but once again – only if you tell the camera what to do,

otherwise it decides for you.– To do this, you need to take control of the

settings for aperture, shutter timing and ISO, the three elements of a correct exposure :

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Aperture

• Aperture (the size of the hole) is specified by a series of values written as f/4, f/11 etc. There is a complicated technical

reason for this, but you can ignore it

• what you need to know is :– Bigger ‘f ’ number = smaller aperture = less

light

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Aperture

• Bigger ‘f ’ number = smaller aperture = less light

– Each change of 'f' setting, say from f/4 to f/5.6, or from f/8 to f/11 reduces the light by half

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Shutter Timing

• This is often called shutter speed – very confusing because speed is distance

per unit time, eg km/hr.

– The settings are actually times eg 1/100s or 1/1000s for which the shutter remains open.

• Most modern cameras have a huge range of setting from say 10s to 1/2000s

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Shutter Timing

• What you need to know is :

Longer time = more light – (obvious really ! )

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ISO (or sensitivity)

• The final leg of the triangle is ISO, the measure of sensitivity of the camera's sensor.

– Higher ISO number = higher sensitivity = less light needed for a correct exposure

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A choice

• Now you can already make a choice of how to achieve the correct exposure :

– Bigger ‘f ’ number (smaller hole, less light) + longer shutter time (more light)Is the same as :

– Smaller ‘f ’ number (bigger hole, more light) + shorter shutter time (less light)

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Try it.

• Set the camera to Av (Aperture priority), set f/5.6, see what shutter timing is proposed by the automatic metering for, say, the wall opposite. Now set to f/16 and try again – write down the values, notice the difference.

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What can I do with this ?

• You can control the style of picture your camera takes, not just the subject.

• 2 important rules – A fast shutter (short timing) will freeze the action of

moving subjects– Larger apertures have smaller depth of field

(backgrounds will be more likely to be out of focus)

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Using ISO

• Higher sensitivity settings produce noise that tends to blur the image, use the lowest ISO that will work in the shot you are taking

– lower sensitivity needs more light - that will mean longer exposures or bigger apertures

– Slow shutter speeds tend to produce blurred images due to camera shake (use a tripod).

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How do I achieve the correct exposure with the auto turned off ?• Mode semi auto !

Most cameras have two semi-automatic modes– aperture priority (called Av on Canon cameras

and A on Nikon) – shutter priority (called Tv on Canon cameras and

S on Nikon).

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Av aperture priority mode

• you select the aperture you want to use – the automatic exposure control measures

the light and sets the shutter timing required for a correct exposure.

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Tv shutter priority mode

• you select the shutter timing you want to use – the automatic exposure control measures

the light and sets the aperture required for a correct exposure.

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Possible problems

• It is, of couse possible to select a value that cannot be achieved with the light available, eg. a fast shutter timing in poor light. – Each make and model of camera reacts differently

in these situations. Some override the settings, some refuse to take the shot, some flash a warning sign - you will have to read the handbook and learn how your own camera behaves.

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Possible problems

• Sometimes (quite often actually), the automatic exposure control is not clever enough to work out the correct settings. How to predict when this will happen and how to do better is covered in a future session (ch3).

Documents

Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Essential principles