Inside the lens is a mechanism called a diaphragm that adjusts the size of the lens opening (aperture), larger or smaller. Controlling that lens opening is the first step in controlling the exposure. The photograph on the left shows a 50 mm lens aperture set to f/2.8 (#1) and then adjusted to an aperture of f/16 (#2).
The size of the lens opening is always expressed as a fraction of the lens focal length and is called an f-stop or a focal ratio (the f = 1). When the opening in your lens is 1/2 as big as the focal length of your lens the f-stop is f/2. When your lens opening is 1/4th as big as your lens focal length the f-stop is f/4. If you have a zoom lens on your camera and it is set to 100 mm and your f-stop is set to f/4 the lens opening is 25 mm wide, 1/4th the focal length. If you then zoom to 50 mm and your f-stop is f/4 the lens opening is 12.5 mm wide, 1/4th the focal length.
Here is where it gets really interesting: the 50 mm with the lens open 12.5 mm (f/4) lets in the same amount of light as when the lens is set to 100 mm and the lens is open 25 mm (also f/4).
The most used part of the standard f-stop scale is: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32.
For those mathematically inclined each step in the scale is a power of the square root of 2 (√2) or about 1.414.
Each of the steps listed above are a full stop. Remember I mentioned that most DSLR cameras are adjustable in 1/3 stop increments so you'll see other numbers between these in your camera viewfinder. Quite a few DSLR's will let you set up the camera so you can adjust stops in 1/2 stop increments also.
The important thing to remember about a full f-stop is that it is a unit with a factor of 2. F/2.8 lets in twice as much light as f/2. Notice that 2 times f/2 is not f/2.8. Instead f/2.8 is f/2 times √2. I know that can be confusing but here is why it works that way.
To let in twice as much light, or cut the amount of light in half, we don't have to make the diameter of the lens opening 2 times bigger or smaller we only have to make it 1.414 (√2) times bigger or 1.414 (√2)times smaller because that's how you make the area of the lens opening twice as big or small.
So, to solidify the concept of stops. f/1.4 to f/2 is 1 stop and 1/2 as much light. F/1.4 to f/2.8 is 2 stops and 1/4 as much light. F/1.4 to f/4 is 3 stops and 1/8 as much light. You can see that a larger f-number means less light because the lens opening is getting smaller. Some may want to think of it as more dark but we'll see in later installments that that is really not true once the other 2 exposure factors, shutter speed and ISO settings, are also considered. Go the other way, f/4 to f/1.4 and you have 8 times more light because the lens opening got larger.
There is another visual effect controlled by the lens opening and that is the depth of field.
Depth of field is that portion of an image in front of and behind the point of focus that is sharply focused. The total distance of that range can be fractions of an inch to infinity. Depth of field is determined by the f-stop, the focal length of the lens and the subject to camera distance. We will only explore the effects of the lens f-stop here.
The diagram at the left shows an aperture (#4) completely open and an aperture stopped down part way (also #4). Notice the spade is associated with the green dotting lines (#1), the heart with the blue dotted lines (#2) and the club with the red dotted lines (#3). The image sensor is #5.
You can see in the fully open aperture portion of the diagram the heart is the only portion of the image sharply focused at the image sensor. The spade has focused in front of the sensor and the club behind the sensor so both are blurry at the sensor. In terms of depth of field (DOF) the image the DOF is very shallow.
In the example with stopped down aperture all 3 image elements focus sharply at the image sensor. The DOF is much deeper than with the aperture wide open.
Portrait photographers want their subject to be the only image element sharply focused and tend to make most of their images with shallow DOF by using large lens openings (small f-numbers). Landscape photographers want virtually everything in an image sharply focused and make most of their images with very deep DOF by using small lens openings (large f-numbers).
We now know how the size of the lens opening controls the amount of light passing through the lens and how the lens opening also effects the depth of the sharply focused portion of our images.
The next camera control that effects exposure is the Shutter speed so it's on to the next installment.
See you then,
All images courtesy of Wikimedia Commons.