May 15
Camera Settings – the relationship between shutter, aperture, and ISO
I know this topic is probably quite familiar for many of you out there, but I am sure there are many like my friend who recently told me that she always needs to read more on this topic because she still does not understand it fully.
To start off simply, there is a relationship between shutter speeds, aperture settings, and ISO settings (or film speed as it used to be known).
To begin with, the ISO setting indicates how sensitive to light the digital chip (or film) is that records your image in the back of your camera. The lower the number, the less sensitive the chip is, and so the more light will be required to achieve a proper exposure. For example, an ISO setting of 100 will require more light than a setting of 800.
The second important point to note about ISO is the sequence of settings. The main settings stand at 50, 100, 200, 400, 800, 1600 (please note that the sequence may extend higher or lower in either direction depending upon your specific camera model). Notice that each number is the double of the one preceding it or halving of the number following. Additionally, each number represents a doubling of the sensitivity of the chip and therefore a halving of the amount of light required to make an optimum exposure (make sure you understand this point). i.e. ISO 200 is double the sensitivity of ISO 100, and because ISO 200 is twice as sensitive, you only require half the amount of light.
Next, shutter speeds. Shutter speeds are measurements of the amount of time your camera shutter stays open (and therefore admitting light to the chip). Shutter speeds are represented by a series of numbers with the main settings along the lines of 2, 4, 8, 15, 30, 60, 125, 250, 500, etc.. Each one actually represents a fractional number, so when you see on your camera the number 30, it actually means 1/30 and is measured in seconds, i.e. one thirtieth of a second. Therefore as the above sequence increases it is actually referring to the shutter staying open for less and less time, 60 meaning one sixtieth of a second, 250 meaning one two hundred and fiftieth of a second, etc. When a shutter measurement is talking about full seconds, it is usually represented by 1”, 2”, etc, or is sometimes printed in a different colour on your camera dials.
Please also note that the sequence of numbers is once again a double (in some cases approximate, i.e 8 to 15 and 60 to 125) of the number before or halving of the number following. This means that each increase in the setting (i.e. 30 to 60) will let in half the amount of light, as the shutter will open for half the amount of time than the previous setting.
So, putting it all together; for a given ISO setting, and under particular lighting conditions, we need a certain amount of light to make a correct exposure. Any of the combination of settings below would let exactly the same amount of light through to the chip.
|
ISO 100 |
|||||
|
Shutter |
1/8 |
1/15 |
1/30 |
1/60 |
1/125 |
|
Aperture |
f11 |
f8 |
f5.6 |
f4 |
f2.8 |
When we increase the sensitivity of the chip (i.e. we change the ISO setting) and take another photo under the same lighting conditions, look at what happens to the combination of shutter / aperture settings.
|
ISO 200 |
|||||
|
Shutter |
1/15 |
1/30 |
1/60 |
1/125 |
1/250 |
|
Aperture |
f11 |
f8 |
f5.6 |
f4 |
f2.8 |
N.B. remember that 1/15 will often be shown as 15 on your camera, 1/30 as 30, etc.
A quick note on choosing which combination of settings to use.
The aperture setting determines how much of your scene will be in focus. A wide aperture settings (f2.8 – f4) will mean the subject you are focusing on will be sharp and everything in front and behind will start getting fuzzy which is good for portraits for example, whereas narrow aperture settings (say f11, f16, higher) will get more foreground and background to the focusing point being sharp, which is good for landscape.
With shutter speed, you do not want the shutter speed set to slow as you will begin to get camera shake, which is when the camera moves whilst the shutter is open and you get a blurry picture. If this is happening you will need to increase your ISO setting, or change the aperture setting as in the above tables to compensate, which will consequently increase the shutter speed.
I hope this helps somewhat to clarify the mystery of aperture / shutter / ISO. Good luck with your image making.
Benjamin Townsend
Photographer
http://www.btphotography.com.au
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