What Is Aperture?
Every lens has a built-in diaphragm that opens and closes like the iris of your eye. By opening and closing it the brightness of the image coming through the lens can be adjusted.
In today’s cameras, most camera bodies communicate with their lenses to tell them what aperture the photographer requested. The lens has a little motor in it that sets the aperture when the shutter is triggered. Otherwise the aperture is wide open.
How Is Aperture Used?
Aperture is measured in f/stops. An example is f/8. The f/stop looks like a fraction and indeed, it is. The f stands for the focal length of the lens. And the fraction defines the diameter of the opening. Here’s an example:
Suppose the lens has a focal length of 50 mm. If the f/stop was f/2, then the diameter of the opening would be 50/2 or 25 mm. If the f/stop was f/4 then the diameter of the opening would be 50/4 or 12.5 mm. If the f/stop is f/22, then the diameter of the aperture is 2.27 mm.
The aperture numbers are created in stops. A change of one stop either doubles or halves the brightness of the image coming through the lens. The f/stops have very weird numbers. Here is a table of f/stops where the interval between settings is 1 stop.
|
f/stop |
Aperture diameter on 50 mm lens |
|
f/1 |
50.0 mm |
|
f/1.4 |
35.7 mm |
|
f/2 |
25.0 mm |
|
f/2.8 |
17.9 mm |
|
f/4 |
12.5 mm |
|
f/5.6 |
8.9 mm |
|
f/8 |
6.2 mm |
|
f/11 |
4.5 mm |
|
f/16 |
3.2 mm |
|
f/22 |
2.3 mm |
The speed of a lens is identified by its widest aperture. An f/4 lens is not considered fast. An f/2.8 is in the class of fast lenses. But f/2, f/1.4 and especially fast and an f/1 lens is extremely fast. And they generally, the fast lenses have a price to match.
What Else Do Apertures Do Besides Control Brightness?
Apertures have an effect on depth of field. Wide apertures have a shallower depth of field and small apertures have a deeper depth of field. This makes the aperture setting important when taking landscape photographs, especially with a near-far composition. It is important to get an aperture that will give you the depth of field you need.
But apertures also affect sharpness. When photographing at small apertures such as f/16 or f/22, the size of the aperture is so small that it actually interacts with the light passing through it. The edges of the diaphragm in particular interact with the light and cause it to scatter. This produces an overall softening effect.
Shooting wide open can also have some softness. This is not because of the aperture but of the design of the lens optics. Typically, the sharpness sweet spot of a lens is two to three stops above wide open. With an f/4 lens that would be f/8 or f/11. With a f/2.8 lens that would be f/5.6 or f/8.
Exposure Triangle
Aperture is one side of the Exposure Triangle. The other two sides are ISO and shutter speed. The sides of the exposure triangle are measured in stops or Exposure Values (EV). The light coming through the lens at an aperture of f/4 would be twice as bright as the light at f/5.6.
Join me on an exciting workshop. Click here to see what’s available.
Read more:
Tell Me More About Exposure
Tell Me More About ISO
Tell Me More About Shutter Speed
Tell Me More About the Exposure Triangle
(58)