Doug Kerr
Well-known member
In the matter of depth of field we ask this question:
For a given aperture and focus distance, over what range of object distances will the object be imaged with not over some degree of blurring from misfocus (which degree of blurring we choose based on our needs)?
We usually use as the metric of "degree of blurring" the diameter of the circle of confusion for an object at the distance of interest: the circular blur figure created from a point on the object in the case of misfocus.
To adopt a limit on the degree of blurring, we state a "circle of confusion diameter limit" (COCDL); the diameter of the circle of confusion at the chosen degree of blurring.
But how do we select a COCDL? Ideally, we would choose it taking into account how the image will be viewed (at what magnification compared to the image on the focal plane, and from what distance), together with what the viewer's expectations of "un-blurred-ness" will be.
But to provide a universal answer for "general use", especially in the film era, COCDL values were recommended based on:
• Some arbitrary (if perhaps reasonable) assumptions as to the size of the viewed image compared to the image on the focal plane, and the distance from which It will be viewed.
• The concept that the "acceptable" misfocus is that which can be noticed by the human eye, based on its resolution.
In modern times, there has been a trend to move away from this concept - which has a potentially great disconnect from the actual viewing situation - in favor of adopting as the "maximum acceptable blurring" that blurring which would noticeably degrade the resolution of the system compared to what it would be in a situation of perfect focus.
But defining this is problematical as well, in part because the "resolution" of a camera system does not even have a precise, objective definition.
Because of that conundrum, when we feel we need a quantifiable, objective metric for "resolution", we tend to base it on the MTF of the system. We may arbitrarily (there's that word again) choose the spatial frequency at which the MTF is 50%. This is arbitrary but fully reproducible.
I have earlier suggested that, in order to quantify the impact of diffraction on the resolution of the system, we might wish to choose as our criterion for "resolution noticeably degraded by diffraction" the situation in which the MTF reflecting the effect of diffraction is 50% at spatial frequency where the basic MTF is 50%.
Now we note that for any degree of blurring, the effect of the blurring can be reflected by an MTF (just as is true for the effect of diffraction).
So perhaps we should follow the same philosophy as for diffraction with regard to choosing a COCDL to be used to reckon the depth of field of a certain camera setup: we would choose as the limit the COCD for which the MTF reflecting the blurring has the value 50% at the frequency we consider as representing the resolution of the system (for perfect focus).
Typically that resolution is on the order of 75% of the Nyquist frequency (sn) dictated by the sensor sensel pitch (sn = 1/2p, where p is the sensel pitch).
If we go through all the math involved, we find that the "blur" MTF has the value of 50% at this "system resolution" frequency when the diameter of the circle of confusion is approximately twice the sensel pitch.
That is, we would choose the COCDL as 2p.
Best regards,
Doug
For a given aperture and focus distance, over what range of object distances will the object be imaged with not over some degree of blurring from misfocus (which degree of blurring we choose based on our needs)?
We usually use as the metric of "degree of blurring" the diameter of the circle of confusion for an object at the distance of interest: the circular blur figure created from a point on the object in the case of misfocus.
To adopt a limit on the degree of blurring, we state a "circle of confusion diameter limit" (COCDL); the diameter of the circle of confusion at the chosen degree of blurring.
But how do we select a COCDL? Ideally, we would choose it taking into account how the image will be viewed (at what magnification compared to the image on the focal plane, and from what distance), together with what the viewer's expectations of "un-blurred-ness" will be.
But to provide a universal answer for "general use", especially in the film era, COCDL values were recommended based on:
• Some arbitrary (if perhaps reasonable) assumptions as to the size of the viewed image compared to the image on the focal plane, and the distance from which It will be viewed.
• The concept that the "acceptable" misfocus is that which can be noticed by the human eye, based on its resolution.
In modern times, there has been a trend to move away from this concept - which has a potentially great disconnect from the actual viewing situation - in favor of adopting as the "maximum acceptable blurring" that blurring which would noticeably degrade the resolution of the system compared to what it would be in a situation of perfect focus.
But defining this is problematical as well, in part because the "resolution" of a camera system does not even have a precise, objective definition.
Because of that conundrum, when we feel we need a quantifiable, objective metric for "resolution", we tend to base it on the MTF of the system. We may arbitrarily (there's that word again) choose the spatial frequency at which the MTF is 50%. This is arbitrary but fully reproducible.
I have earlier suggested that, in order to quantify the impact of diffraction on the resolution of the system, we might wish to choose as our criterion for "resolution noticeably degraded by diffraction" the situation in which the MTF reflecting the effect of diffraction is 50% at spatial frequency where the basic MTF is 50%.
Now we note that for any degree of blurring, the effect of the blurring can be reflected by an MTF (just as is true for the effect of diffraction).
So perhaps we should follow the same philosophy as for diffraction with regard to choosing a COCDL to be used to reckon the depth of field of a certain camera setup: we would choose as the limit the COCD for which the MTF reflecting the blurring has the value 50% at the frequency we consider as representing the resolution of the system (for perfect focus).
Typically that resolution is on the order of 75% of the Nyquist frequency (sn) dictated by the sensor sensel pitch (sn = 1/2p, where p is the sensel pitch).
If we go through all the math involved, we find that the "blur" MTF has the value of 50% at this "system resolution" frequency when the diameter of the circle of confusion is approximately twice the sensel pitch.
That is, we would choose the COCDL as 2p.
Best regards,
Doug