Mike,
I looked up your quoted statement on Norman's web site. Let's disect his claim. Go to the section just above where the "claim" is made:
"The circle of confusion C at the DOF limit is based on the 0.01 inch = 0.25 mm feature in an 8x10 inch print."
Which, is of course, correct and is simply visual accuity.
"On the film, C (mm) = 0.25/(magnification for an 8x10 print)."
OK...simple arithmatic...
"For a constant angle of view, lens focal length f is proportional to the format size (cropped for an 8x10 inch image) and inversely proportional to the magnification."
Again, no problem here...
"f/C is therefore a constant, independent of the format, about 1600 for a "normal" lens."
Which is focal length over CoC, and of course, that's correct. 50mm / .033 ~= 80mm / .06, all agreed there...
And...then he shows a table, which all seems correct...and then this statement:
"Since f/C is a constant, independent of format, depth of field is constant for constant aperture opening a."
Er, right...so DOF for f11 is the DOF for f11, no matter what the film format, for the same FOV...exactly what I was saying, and the premise/concept DOF/CoC is based on.
And then...:
"And since f-stop N = f /a,"...
We've established f is focal length, and a is aperture...
> "Depth of field is constant when the f-stop is proportional to the > format size, i.e., DOF is the same for a 35mm image taken at f/11, a > 6x7 image at f/22, a 4x5 image at f/45 or an 8x10 image at f/90"
But...he makes no mention of focal length, so I am assuming he means for the same focal length! Well, that means the FOV (Field of View) is different...and that changes the entire meaning of this statement.
35mm format, 50mm lense, f11, 1M DOF = .535 6x6 format, 50mm lense, f22, 1M DOF = .597
OK, so here is the root of the misunderstanding. Of course changing the FOV will give you more/less DOF, BUT, not of the same subject area, only of the OVERALL image, and the images will not be the same, one will contain 2x the subject matter than the other. If you enlarged them to show the same image on the 8x10, then what you'd see is the DOF would be the same.
QED ;-)
Austin
I looked up your quoted statement on Norman's web site. Let's disect his claim. Go to the section just above where the "claim" is made:
"The circle of confusion C at the DOF limit is based on the 0.01 inch = 0.25 mm feature in an 8x10 inch print."
Which, is of course, correct and is simply visual accuity.
"On the film, C (mm) = 0.25/(magnification for an 8x10 print)."
OK...simple arithmatic...
"For a constant angle of view, lens focal length f is proportional to the format size (cropped for an 8x10 inch image) and inversely proportional to the magnification."
Again, no problem here...
"f/C is therefore a constant, independent of the format, about 1600 for a "normal" lens."
Which is focal length over CoC, and of course, that's correct. 50mm / .033 ~= 80mm / .06, all agreed there...
And...then he shows a table, which all seems correct...and then this statement:
"Since f/C is a constant, independent of format, depth of field is constant for constant aperture opening a."
Er, right...so DOF for f11 is the DOF for f11, no matter what the film format, for the same FOV...exactly what I was saying, and the premise/concept DOF/CoC is based on.
And then...:
"And since f-stop N = f /a,"...
We've established f is focal length, and a is aperture...
> "Depth of field is constant when the f-stop is proportional to the > format size, i.e., DOF is the same for a 35mm image taken at f/11, a > 6x7 image at f/22, a 4x5 image at f/45 or an 8x10 image at f/90"
But...he makes no mention of focal length, so I am assuming he means for the same focal length! Well, that means the FOV (Field of View) is different...and that changes the entire meaning of this statement.
35mm format, 50mm lense, f11, 1M DOF = .535 6x6 format, 50mm lense, f22, 1M DOF = .597
OK, so here is the root of the misunderstanding. Of course changing the FOV will give you more/less DOF, BUT, not of the same subject area, only of the OVERALL image, and the images will not be the same, one will contain 2x the subject matter than the other. If you enlarged them to show the same image on the 8x10, then what you'd see is the DOF would be the same.
QED ;-)
Austin