Michael Fryd
Forum Pro
In another thread, a beginner photographer was having trouble understanding why aperture affects depth of field. A few people had tried to explain it, but he was having trouble with the math.
I thought I would put up a few diagrams, which hopefully will make things a little clearer without any need for math.
.
To keep things simple. Let's assume we are taking a photo of three points, which are at various distances from the lens. A purple point, a blue point, and a yellow point.
Light from each point spreads out in all directions, however, we only care about those rays of light that reach the lens. Let's look at a diagram showing some of those rays.
Notice that when the rays hit the lens, they change direction. This is the purpose of a lens.
In this example, the lens is focused on the blue point. Notice that all of the rays from the blue point converge at a single point on the sensor.
The yellow point is further away. Note that all the rays from the yellow point converge in front of the sensor, and then start to diverge. By the time they reach the sensor, they spread out. Thus they are spread out when they hit the sensor. If the yellow point had been further from the lens, the amount of spread would have been larger.
The purple point is closer to the sensor. Those rays hit the sensor before they have a chance to converge. They are also spread out over a small area of the sensor. If the blue point had been even closer to the sensor, the amount of spread would have been larger.
.
The yellow and blue points are not in perfect focus. Their images get spread out over a small circle on the sensor. The size of this circle (the amount of blurriness) is called the "Circle of Confusion", which is abbreviated as "CoC".
The human eye is not a microscope, nor is it perfect. If the CoC is small enough, the human eye won't notice the blurriness, and the corresponding point will look like it is in focus.
Remember, the CoC gets bigger as we move away from the focus distance. There is a range of distances around the focus distance where the CoC is small enough that everything looks to be in focus. This range is the depth of field.
.
Smaller apertures tend to increase the depth of field range. With a smaller aperture we get a "deeper" depth of field.
Let's look to see how the aperture reduces the size of the CoC.
In the above diagram we have added an aperture. This is a hole that blocks some of the light. Notice that rays of light that were blocked are towards the edges of the lens. If you look carefully at the diagram, you will see that with the outer rays blocked, the spread of the remaining rays is smaller in size. A smaller spread is a smaller CoC. A smaller CoC means the the point looks sharper.
If the purple point was previously just outside the depth of field, the CoC reduction from the smaller aperture may be enough to bring it within the depth of field.
Thus a smaller aperture yields a deeper depth of field, as the smaller aperture blocks the outer rays, reducing the blurriness.
I thought I would put up a few diagrams, which hopefully will make things a little clearer without any need for math.
.
To keep things simple. Let's assume we are taking a photo of three points, which are at various distances from the lens. A purple point, a blue point, and a yellow point.
Light from each point spreads out in all directions, however, we only care about those rays of light that reach the lens. Let's look at a diagram showing some of those rays.
Notice that when the rays hit the lens, they change direction. This is the purpose of a lens.
In this example, the lens is focused on the blue point. Notice that all of the rays from the blue point converge at a single point on the sensor.
The yellow point is further away. Note that all the rays from the yellow point converge in front of the sensor, and then start to diverge. By the time they reach the sensor, they spread out. Thus they are spread out when they hit the sensor. If the yellow point had been further from the lens, the amount of spread would have been larger.
The purple point is closer to the sensor. Those rays hit the sensor before they have a chance to converge. They are also spread out over a small area of the sensor. If the blue point had been even closer to the sensor, the amount of spread would have been larger.
.
The yellow and blue points are not in perfect focus. Their images get spread out over a small circle on the sensor. The size of this circle (the amount of blurriness) is called the "Circle of Confusion", which is abbreviated as "CoC".
The human eye is not a microscope, nor is it perfect. If the CoC is small enough, the human eye won't notice the blurriness, and the corresponding point will look like it is in focus.
Remember, the CoC gets bigger as we move away from the focus distance. There is a range of distances around the focus distance where the CoC is small enough that everything looks to be in focus. This range is the depth of field.
.
Smaller apertures tend to increase the depth of field range. With a smaller aperture we get a "deeper" depth of field.
Let's look to see how the aperture reduces the size of the CoC.
In the above diagram we have added an aperture. This is a hole that blocks some of the light. Notice that rays of light that were blocked are towards the edges of the lens. If you look carefully at the diagram, you will see that with the outer rays blocked, the spread of the remaining rays is smaller in size. A smaller spread is a smaller CoC. A smaller CoC means the the point looks sharper.
If the purple point was previously just outside the depth of field, the CoC reduction from the smaller aperture may be enough to bring it within the depth of field.
Thus a smaller aperture yields a deeper depth of field, as the smaller aperture blocks the outer rays, reducing the blurriness.
