Doug Kerr
Well-known member
A recent flurry of palaver on another site made me think it might be time to give my periodic set of lectures on the topic of depth of field.
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What is depth of field, anyway?
The distance called depth of field is the answer to this question:
How can we quantify "that degree" of blurring?
As an actual optical phenomenon, the effect of the blurring is that a point on a subject will not form, in the image on the sensor, a point, but rather a circular "blur figure" (a circle of confusion). A measure of the degree of blurring is the diameter of that blur figure (circle of confusion diameter). If we want to quantify blurring "not over a certain degree", one way is to express that as a limit on the allowable diameter of the blur figure (circle of confusion diameter limit which I call COCDL), on the sensor.
Without our adopting a COCDL, there is no such thing as a depth of field. The COCDL is not calculable from any parameters of the camera setup - it is our own limit, just as we might adopt a limit for how large an error a surveyor can make before we deem him "unqualified".
Choosing the COCDL
Now, on what basis might we adopt a COCDL? In traditional photographic work, a common premise was that the COCDL should be the diameter of a blur figure that would make the blur just clearly visible to the human eye. So a common basis was to make the COCDL correspond to perhaps twice the human eye resolution.
So wait a minute - the COCDL is defined on the image as generated on the sensor, and we don't look at the image there. In traditional photography, for small format cameras, we always look at a print made form the negative. The print size might be variable multiple of the actual image size, and we might view it from different distances.
To take that confusion out of the picture, we think in terms of an arbitrary print size, seen at an arbitrary distance.
It turns out the twice the angular spacing the eye can resolve at the arbitrary distance we assume for viewing is about 1/1400 of the diagonal size of that arbitrary print size.
Then, chasing this back to the sensor, it suggests we should choose a COCDL that is 1/1400 of the diagonal size of the image on the sensor.
And depth of field figures quoted for a certain camera setup without benefit of further explanation are often based on that choice of COCDL, or another fraction but defined on the same basis.
Another approach
There are some that say we should choose the COCDL so that the depth of field will be determined as the range of distances over which subjects range will be imaged with no perceptible degradation of sharpness due to imperfect focus. That means choose the COCDL as perhaps, twice the pixel pitch (on the sensor) of the camera.
Which if these is "right"? There is no "right". This whole concept is based on our choosing what we will consider "perceptible" blurring. And how we want to define that that may well depend on the use that is to be made of our image.
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In the next section, I will discuss that burning question, "how does sensor size affect depth of field."
[continued]
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What is depth of field, anyway?
The distance called depth of field is the answer to this question:
With a camera with a lens of a certain focal length, the camera focused at a certain distance, and shooting at a certain f-number, over what range of distances can we have subjects whose images will be blurred (owing to their not being at the distance at which the camera is focused) not over some "degree" (which we must establish).
How can we quantify "that degree" of blurring?
As an actual optical phenomenon, the effect of the blurring is that a point on a subject will not form, in the image on the sensor, a point, but rather a circular "blur figure" (a circle of confusion). A measure of the degree of blurring is the diameter of that blur figure (circle of confusion diameter). If we want to quantify blurring "not over a certain degree", one way is to express that as a limit on the allowable diameter of the blur figure (circle of confusion diameter limit which I call COCDL), on the sensor.
In this field of discussion, it is sadly common to call the circle of confusion diameter limit the "circle of confusion", thus making it impossible to speak of the actual circle of confusion or the circle of confusion diameter (as we must to explain many things).
Without our adopting a COCDL, there is no such thing as a depth of field. The COCDL is not calculable from any parameters of the camera setup - it is our own limit, just as we might adopt a limit for how large an error a surveyor can make before we deem him "unqualified".
Choosing the COCDL
Now, on what basis might we adopt a COCDL? In traditional photographic work, a common premise was that the COCDL should be the diameter of a blur figure that would make the blur just clearly visible to the human eye. So a common basis was to make the COCDL correspond to perhaps twice the human eye resolution.
This is in fact defined in angular terms. If the smallest object we could "resolve" at a distance of 10 feet was 0.1. inch in diameter, then the smallest object we could "resolve" at a distance of 100 feet would be 1.0 inch in diameter
So wait a minute - the COCDL is defined on the image as generated on the sensor, and we don't look at the image there. In traditional photography, for small format cameras, we always look at a print made form the negative. The print size might be variable multiple of the actual image size, and we might view it from different distances.
To take that confusion out of the picture, we think in terms of an arbitrary print size, seen at an arbitrary distance.
It turns out the twice the angular spacing the eye can resolve at the arbitrary distance we assume for viewing is about 1/1400 of the diagonal size of that arbitrary print size.
Then, chasing this back to the sensor, it suggests we should choose a COCDL that is 1/1400 of the diagonal size of the image on the sensor.
And depth of field figures quoted for a certain camera setup without benefit of further explanation are often based on that choice of COCDL, or another fraction but defined on the same basis.
Another approach
There are some that say we should choose the COCDL so that the depth of field will be determined as the range of distances over which subjects range will be imaged with no perceptible degradation of sharpness due to imperfect focus. That means choose the COCDL as perhaps, twice the pixel pitch (on the sensor) of the camera.
Which if these is "right"? There is no "right". This whole concept is based on our choosing what we will consider "perceptible" blurring. And how we want to define that that may well depend on the use that is to be made of our image.
"Is that sledge hammer big enough.? "For what?"
************
In the next section, I will discuss that burning question, "how does sensor size affect depth of field."
[continued]