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Old Aug 29, 2010, 5:43 PM   #51
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AND DOF is a property of physical aperture (not f-number), focal length, and distance to subject.
No. Once again, the formula for calculating the Hyperfocal Distance, and therefore the DoF, uses the f-number, not the physical aperture size. See http://www.dofmaster.com/equations.html
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Old Aug 29, 2010, 5:46 PM   #52
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Its not difficult. Its called enlargement.

.3/1.3 = .23
.3/1.6 = .1875

If you blow up the crop to the same size, COC is the exact same.

This argument is getting silly.
Get cocky and get embarassed.

Replace the .3 with .03 and the calculations yeild .023 and .01875.

Isn't math fun.
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Old Aug 29, 2010, 6:01 PM   #53
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No. Once again, the formula for calculating the Hyperfocal Distance, and therefore the DoF, uses the f-number, not the physical aperture size. See http://www.dofmaster.com/equations.html
You argued that it was sensor dependent. It is not. It is lens dependent and COC varies by enlargement. It you blow a print to poster size, COC scales to the enlargement of that also.

As you seem to be having some trouble with the algebra, f=FL/A and can be substituted into the equation. Yes, it is a function of physical aperture.

The calculation you display is simply designed to make it easy to vary f-stop for easy, practical every day use. Its general rather than specific.
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Old Aug 29, 2010, 6:04 PM   #54
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Thank you.
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Old Aug 29, 2010, 6:21 PM   #55
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OK - let's re-iterate the basics. Fstop is a ratio that involves the aperture size. It just so happens that the depth of field calculations use the same parameters - so you are really both correct - aperture size (and distance from lens to focal plane) both affect DOF. The relationship between those two variables is what we call the f-stop. If you alter the aperture opening without changing that distance you change the f-stop.

Math is funny that way.
as to COC being affected by sensor size - yes it is. Now if you perform operations on the image that approximate changing the physical attributes of sensor size, focal length, distance and f-stop then you're changing the values of the equation.

Again, if you've got algabraic formulas Greg that leave circle of confusion (and their dependence on sensor size) out of it I'd like to see them. But the formulas published widely do, in fact, use all those variables. Again assuming you do not alter the resulting image in software.

So, since you are fond of chastising people for not understanding the algebra - perhaps you can show us how using the different COC values produces the same results. Or are the formulas incorrect?

Again, let's deal in specifics - if you could address the example I posted earlier where dof calculations were different simply by changing sensor size I'd appreciate it. Or is it your assertion that the math used in the DOF calculater is incorrect?
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Old Aug 29, 2010, 6:26 PM   #56
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If an APS-C Nikon D300s dSLR creates an image with a 100mm focal length and an F-Number of f/4, with a focus distance of 50 feet, the depth of field will be more than 12 feet.

If you try to create the same image with a 'Full Frame' Nikon D700, you can either increase the focal length or decrease the subject distance. If you increase the focal length to 150mm to reproduce the angle of view, but keep the F-Number and subject distance the same, the DoF is about 8 feet. If you decrease the subject distance to 33 feet 4 inches to reproduce the perspective, but keep the F-Number and focal length the same, the DoF is about 8 feet.

So, the larger image sensor gives a shallower depth of field.
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Old Aug 29, 2010, 6:33 PM   #57
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fldspringer, you shoot macro with a 4/3 system. Are you telling us that you have an even smaller DoF with the smaller sensor than someone shooting macro with a 'FF' image sensor?

Extrapolating that out just a little, wouldn't P&S digicams have infinitesimal DoFs?

Or are you saying that the smaller focal length lenses compensate for the smaller DoFs from the smaller sensors?
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Old Aug 29, 2010, 6:39 PM   #58
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So, the larger image sensor gives a shallower depth of field.
No, not really - in your example, changing the distance is what provided a shallower depth of field. See, the whole discussion comes down to this:
IF you have a larger sensor and create the same image perspective as a smaller sensor camera the resulting image will have shallower DOF. BUT, if you cannot do that - if you can't get closer to your subject and you're using the same prime lens on both cameras the shot from the larger sensor camera will actually have a larger DOF. In and of itself, larger sensors actually create more DOF. But in most practical applications the photographer adjusts focal length or distance to get the framing the same and it's that adjustment that creates shallower DOF.

It's always easier to appreciate these things when you use extreme examples. Can we all agree that the sensor size of the 1dsIII is larger than the sensor size of the Panasonic fz-35. When we use a focal length of 86mm on both cameras with a subject distance of 25 feet and f-stop of f4 we see the DOF for the fz35 is 1.02 feet while the dof on the 1ds is 6.2'. Again, math is funny that way. But for those that like to think in practical terms that shouldn't surprise people - since the FOV on the panasonic is equivelent to 480mm+. So, this is an extreme example but the math is the math.
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Old Aug 29, 2010, 7:02 PM   #59
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No, not really - in your example, changing the distance is what provided a shallower depth of field. See, the whole discussion comes down to this:
IF you have a larger sensor and create the same image perspective as a smaller sensor camera the resulting image will have shallower DOF. ...
That's what we're talking about. The OP is trying to create identical images from two cameras, a Canon T1i and a Fujifilm S1800. There is no combination of settings that can be used in either or both cameras that will achieve that goal, because the image sensor in the T1i is so large and the image sensor in the S1800 is so small. The larger image sensor creates a more shallow depth of field. Even stopping the T1i's lens to the smallest aperture his lens has, the DoF is smaller than he can get with the lens on his S1800 wide open.
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Old Aug 29, 2010, 7:08 PM   #60
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fldspringer, you shoot macro with a 4/3 system. Are you telling us that you have an even smaller DoF with the smaller sensor than someone shooting macro with a 'FF' image sensor?

Extrapolating that out just a little, wouldn't P&S digicams have infinitesimally small DoFs?

Or are you saying that the smaller focal length lenses compensate for the smaller DoFs from the smaller sensors?
Macro is a different animal. There are assumptions made it the DOF calculations that subject distance is much, much greater than the focal lengths and drive parameters to insignificance. That cannot be used in macro. Instead, it largely boils down to a formula depending on f-number and magnification. It is really quite independent of format also.

DOF= [2(coc)f] * [(M+1)/(M^2)]

so its a function of f-number and magnification. The thing to notice here is that its is largely independent of focal length.
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