Shutter Speed and Aperture
When we looked at shooting modes I concluded that:
If the photographer wishes to take control there are only 3 shooting modes of interest.
Semi Automatic
Aperture Value (Av) Priority
Shutter (Time) Value (Tv) Priority, or on special occasions
Fully Manual
Manual
Which one you use depends on the scene and your photographic intent. We will leave fully manual for now; that is for special situations once you have more experience,
So that we can understand these semi automatic settings let’s go back over the Aperture Triangle.
The Exposure Triangle
The exposure triangle (from Digital Camera World)
Shutter Speed
The shutter “speed” is the length of time that the camera shutter is open and exposing the sensor to light. The shutter speed is usually set using a control dial and/or by an electronic touch screen. The available settings will run from very small fractions of a second (say 1/8000) to several seconds (? 30 seconds). A long exposure is often referred to as slow and a short exposure as fast.
Clearly the longer the exposure the more likelihood there is that a subject will move significantly during the exposure. Fast exposures are used to freeze movement; slow exposures are used for static subjects or to capture movement blur.
See this reference for more detail.
Aperture
The aperture setting is a measure of the physical size of the aperture which allows light to reach the camera sensor. The aperture is also usually set using a rotary dial and/or an electronic touch control. The available settings depend upon the design of your lens and will run as a sequence of (f) numbers from 2.1, 4.5, 5, 5.6, 6.3 …….. 22. The sequence is strange because it is linked to the area of the lens aperture, each step represents a halving or doubling of the aperture.
The smaller the number, the bigger the aperture. The aperture gets smaller as the number increases. A large aperture (small fNo) gives a small depth of field, a large aperture (small no) gives a shallow depth of field.
How Does This Relate to Taking Photographs?
Our cameras do most of the exposure triangle “heavy lifting” for us, whether we are in full or semi auto modes. In fully auto shooting modes the camera takes all the decisions and sets appropriate values for each of the 3 sides of the triangle. Those values are likely to be average values that suit most average photos. But we are trying to create distinctive, not average, photos. We want to have “artistic control”. (Pretentious, qui moi?)
That creative control starts with our analysis of the scene and our decision about which side of the exposure triangle needs to take precedent.
The sides of the triangle
Shutter Speed
Shutter speed controls the amount of motion or camera movement blur.
Is there subject movement we wish to freeze? e.g
Birds in flight
Passing cars
Runners etc
Running water
Is there movement we wish to blur:
Including movement blur gives an impression of speed
Is there movement that would spoil the image, e.g
Movement of trees and branches in the foreground.
If you are holding the camera in hand you need a fast enough shutter to avoid movement blur.
Aperture
Aperture controls the depth of field in the image.
Do you want a blurred background?
Do you want sharp foreground to distant sbject
ISO
ISO controls the amount of “digital noise” in the image which is roughly equivalent to grain. For now let’s leave ISO on Auto while we get used to the other 2 sides.
Take the Picture
Analyse the scene and decide how you want the photograph to look:
If freezing or showing movement is key use Tv mode (time value)
High Speed / Short time to freeze motion
how high you can go depends on the brightness of the scene
Low speed/long time to blur motion
how low you can go depends on the speed of the object or the risk of camera movement.
If Depth of Field is key use Av Mode (aperture value)
Large aperture / high FNo for high DoF
Small Aperture / low FNo for low DoF