This is a post intended for my BA students but others may find it useful too.
This is controlled by 3 different though inter-related methods:
2. Shutter speed
This is the speed or sensitivity of the camera’s sensor & is selected by the photographer. Formally it was the sensitivity of the film that you chose to load into the camera.
At the time of writing (Sep 2012) most Nikon DSLR’s use 200 ISO as their base position whereas Canon’s begin at 100 ISO (though higher spec models have a greater range).
The lower the ISO value that is selected the less the sensitivity to light that the sensor will have and the less the electronic noise/interference that will result.
As a general rule you choose a low ISO when you have a lot of light to work with and you increase the ISO to higher values as lighting conditions become darker.
The progression of ISO values you can expect to find on a DSLR are:
100 200 400 800 1600 3200 6400 +
Each time the ISO value doubles or halves this is referred to as making an adjustment that is equivalent to 1 f-stop (more on this later).
On film the common ISO progressions are:
50 100 200 400 800 1600 3200 6400 +
This is the time for which the shutter is open and the sensor or film is exposed to light.
Shutter speeds are referred to as being either slow (when the sutter is open for a longer time) and fast (when the shutter is open for a short time).
You select the speed to suit what you are wanting to achieve as well as for the conditions under which you are working.
As a general rule:
Slow speeds may result in camera shake so think about using a tripod.
Fast speeds for action photography where a clear, concise image is needed.
The progression is:
< 15sec 8 sec 4 sec 2 sec 1sec 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000 1/2000 >
There will also be a ‘B’ (bulb) setting. This allows you to open the shutter and it will remain open until you release it.
Some cameras &/or lenses have a ‘T’ (time) setting where you press once to open and again to close the shutter.
Both ‘B’ and ‘T’ as well as speeds slower than 1/8 sec should be used in conjunction with a cable release.
This is the third (and sometimes most confusing) method for controlling exposure.
This refers to the diameter of the aperture formed by the iris diaphragm, which is a series mechanical blades built into the camera lens.
The diameter of this aperture is expressed as an f-number (f-no).
There is a set progression of f-no’s & each time you change this number you increase or decrease the amount of exposure by 1 f-stop. Thus the photographic convention of expressing exposure in f-stops. This is the inverse square law in action.
The size of the lens aperture influences both overall exposure AND the amount of subject that is in ‘acceptably’ sharp focus – referred to as ‘Depth of Field’.
As photographers we need to consider how much depth of field we require in order to be able to achieve a given result. This in turn will influence the f-no we will want to select.
When the lens is fully open to it’s widest aperture there will be very little depth of field, when stopped down to a narrow (or small) aperture you will achieve greater depth of field.
Yet more confusion comes about because a wide aperture has a low numerical value (f1.4 or 2.8) whereas a small aperture has a high numeric value (f22).
Older, non auto-fucus lenses, will have their range of f-no’s inscribed on the barrel and sometimes also the intermediate positions which may be half of third f-stops. Thus you will hear people talk in terms of fractions of a stop when considering exposure.
The standard f-no progression is:
< f1.4 f2 f2.8 f4 f5.6 f8 f11 f16 f22 f32 f45 f64 f90 >
Beware though when using a DSLR. These cameras express the intervening fractional values as f-stops & this causes added confusion. On your cameras you will see f-no’s such as f9, f14, etc. But you will not see these marked on a lens barrel or a lightmeter.
In fact modern light/flahmeters will read a full f-stop followed by fractional units in tenths. So it just got more confusing!
An example of a lightmeter reading: 1/125 sec f5.6 9
This translates as a shutter speed of 1/125th second with an aperture that is f5.6 & 9/10.
Your camera doesn’t work in tenths (yet!) so you have to be prepared to compromise & rounding up or down to the nearest full f-stop which, in this case, is f8 or the third of an f-stop interval if that is closest.
In actual fact a 1/10th of an f-stop is not going to make a huge difference to your exposure & if necessary can be adjusted in post.
Use these notes as a general guide, read around the subject for yourself & experiment so that this becomes familiar.
Diprose, G & Robbins, J (2012) Photography: The New Basics, Thames & Hudson
A first class read, thorough, easy to follow
Adams, A (1981) The Camera, New York Graphics Society
Old school and very technical, the best there is on the subject