- Jan 2, 2006
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I was curious as to what causes DOF.
Why does DOF increase when you stop down your aperture? Why does DOF decrease when you opening up your aperture? What are the physics behind this?
Here's a primer:
http://www.minoxlab.com/Don_Krehbiel/mpl/dkdof.htm
The size of the circle of confusion defines how out of focus something is when it is projected on the film plane.
After having asked around various photography forums, I can tell you this: photographers are not physicists.
FBB: What causes DOF? Why does increasing aperture decrease the DOF and why does stopping down increase it? What are the actual optical physics behind it?
A: *links to the above link which gives a formula for DOF. The formula uses aperture as a variable but doesn't explain why it's there*
FBB: It doesn't explain why.
B: *gives a link, says the same thing as the formula, only in words*
FBB: It doesn't explain why.
C: *gives a link that says "circle of confusion is proportional to the amount of light that can pass through the lens tube"*
FBB: This is effectively a correlation, not an explanation. It's like saying the number of car accidents is proportional to the amount of cars on the road. It does not answer WHY there are car accidents. It doesn't explain why.
D: *gives me Wikipedia article that says the same thing as A + B*
FBB: It doesn't explain why.
E: "A 50mm f/1.4 lens stopped down to f/8 is like a 50mm constant f/8 aperture lens. This reduces the lens diameter, which reduces the circles of confusion.*
FBB: Wha... wha?
FBB: *sits down and stares at a rudimentary lens diagram and sketches light paths*
I think I've figured it out.
***** Hypothesis *****
(reference the link below)
Basically the CoC is smaller when stopped down because stopping down results in a smaller angle between the intersecting light paths right in front of the film plane, which results in a narrower circle of confusion that falls on the film plane.
Some properties as a result of this:
1. Changing the diameter of the aperture changes this angle.
2. Moving the subject towards or away from the focus plane does not change this angle.
3. Moving the subject towards or away from the focus plane DOES change the location of the intersection of the light paths directly in front of the film plane, resulting in changes in the CoC size, and thus, the sharpness of the image.
http://fuzzybabybunny.smugmug....photos/181575794-O.jpg
Why does DOF increase when you stop down your aperture? Why does DOF decrease when you opening up your aperture? What are the physics behind this?
Here's a primer:
http://www.minoxlab.com/Don_Krehbiel/mpl/dkdof.htm
The size of the circle of confusion defines how out of focus something is when it is projected on the film plane.
After having asked around various photography forums, I can tell you this: photographers are not physicists.
FBB: What causes DOF? Why does increasing aperture decrease the DOF and why does stopping down increase it? What are the actual optical physics behind it?
A: *links to the above link which gives a formula for DOF. The formula uses aperture as a variable but doesn't explain why it's there*
FBB: It doesn't explain why.
B: *gives a link, says the same thing as the formula, only in words*
FBB: It doesn't explain why.
C: *gives a link that says "circle of confusion is proportional to the amount of light that can pass through the lens tube"*
FBB: This is effectively a correlation, not an explanation. It's like saying the number of car accidents is proportional to the amount of cars on the road. It does not answer WHY there are car accidents. It doesn't explain why.
D: *gives me Wikipedia article that says the same thing as A + B*
FBB: It doesn't explain why.
E: "A 50mm f/1.4 lens stopped down to f/8 is like a 50mm constant f/8 aperture lens. This reduces the lens diameter, which reduces the circles of confusion.*
FBB: Wha... wha?
FBB: *sits down and stares at a rudimentary lens diagram and sketches light paths*
I think I've figured it out.
***** Hypothesis *****
(reference the link below)
Basically the CoC is smaller when stopped down because stopping down results in a smaller angle between the intersecting light paths right in front of the film plane, which results in a narrower circle of confusion that falls on the film plane.
Some properties as a result of this:
1. Changing the diameter of the aperture changes this angle.
2. Moving the subject towards or away from the focus plane does not change this angle.
3. Moving the subject towards or away from the focus plane DOES change the location of the intersection of the light paths directly in front of the film plane, resulting in changes in the CoC size, and thus, the sharpness of the image.
http://fuzzybabybunny.smugmug....photos/181575794-O.jpg
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