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Old Aug 29, 2010, 11:16 AM   #31
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It depends, here most people in the know point the less knowledgeable to dof master so they can plug in the focal lengths against body. Usually we explain that DOF is affected by focal length, aperture and distance between camera and subject, for those looking for a blurred background we also point out that getting the subject further from the background helps.

Thats very good advice Mark! And absolutely accurate. Im not trying to point out flaws, just misconception and this is not the first time. I have had this discussion MANY times, often with pros who didnt understand the concept of DOF and would go down in an all out brawl insisting that a FF sensor gives a more narrow DOF based on the sensor size alone. Its a very misunderstood subject from people who just dont know to people who either didnt understand it or were taught the wrong way. Between your take and mine this should make it an understood subject.
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Old Aug 29, 2010, 11:37 AM   #32
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Sounds good to me. The only thing that would make everything even better is to send me your 400mm f2.8 and the 200mm f2
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Old Aug 29, 2010, 12:52 PM   #33
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... Bottom line is DOF is a calculation based on focal length and aperture and has nothing to do with sensor size. ...
Not quite. The calculation of DoF depends on the focal length, aperture, and the circle of confusion, and the circle of confusion varies with the format. APS-C dSLRs have a smaller circle of confusion than 35mm SLRs and 'Full Frame' dSLRs. Therefore, they have different depths of field when everything else remains the same.

With an APS-C Nikon D300s dSLR using a lens with a focal length of 100mm and an aperture of f/4.0 photographing a subject at a distance of 50 feet, the depth of field is over 12 feet. When a 'Full Frame' D700 dSLR is used with a 150mm lens (to reproduce the angle of view), the DoF is only 8 feet for the same subject distance. If, instead, you use the same 100mm focal length, but bring the subject 1/3 closer (to preserve the proportions of the image), the DoF is 8 feet, which, given the closer distance and the proportionally larger circle of confusion, is proportional to the 12 foot DoF obtained with the D300s.
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Old Aug 29, 2010, 3:00 PM   #34
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Not quite. The calculation of DoF depends on the focal length, aperture, and the circle of confusion, and the circle of confusion varies with the format. APS-C dSLRs have a smaller circle of confusion than 35mm SLRs and 'Full Frame' dSLRs. Therefore, they have different depths of field when everything else remains the same.

With an APS-C Nikon D300s dSLR using a lens with a focal length of 100mm and an aperture of f/4.0 photographing a subject at a distance of 50 feet, the depth of field is over 12 feet. When a 'Full Frame' D700 dSLR is used with a 150mm lens (to reproduce the angle of view), the DoF is only 8 feet for the same subject distance. If, instead, you use the same 100mm focal length, but bring the subject 1/3 closer (to preserve the proportions of the image), the DoF is 8 feet, which, given the closer distance and the proportionally larger circle of confusion, is proportional to the 12 foot DoF obtained with the D300s.
Don't confuse Aperture with f-stop (AKA focal ratio).

The 100mm lens has an aperture of 25mm at f4.

the 150mm has an aperture of 37.5mm at f4.
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Old Aug 29, 2010, 3:16 PM   #35
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Aperture vs. f-stop notwhithstanding, TCAVs premise is essentially correct - sensor size affects circle of confusion which does affect depth of field.
Here's a link to the math involved. You will see that circle of confusion (which is affected by sensor size), physical focal length, f-stop, distance to subject all affect depth-of-field. People like to forget about the circle of confusion part of things. Now, arguments can be made as to which variables have the most impact on DOF but all 4 affect it. And since sensor size affects circle of confusion - it too has an impact on depth-of-field.

oops forgot the link:
Here's a link to the math:
http://www.dof.pcraft.com/dof.cgi
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Old Aug 29, 2010, 3:35 PM   #36
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so, like any multi-variable equation you can get the same result with 2 different values for the same variable - PROVIDED you change one or more of the other variables. So, you can get the same DOF with two different sensor sizes (and thus two different circle-of-confusion values) but to do so you must change at least one of the other 3 variables. If sensor size and thus circle of confusion played no part in depth-of-field you could change the value and the change would have no affect on the equation. The same argument is mistakenly made when people claim focal length has no affect on depth of field and site luminous landscape or other write-ups. But they miss the point - the point of such discussions is to demonstrate that you can achieve the same depth-of-field with different focal lenghts - but to do so you must change one of the other variables - usually distance to subject.
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Old Aug 29, 2010, 4:08 PM   #37
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Aperture vs. f-stop notwhithstanding, TCAVs premise is essentially correct - sensor size affects circle of confusion which does affect depth of field.
Here's a link to the math involved. You will see that circle of confusion (which is affected by sensor size), physical focal length, f-stop, distance to subject all affect depth-of-field. People like to forget about the circle of confusion part of things. Now, arguments can be made as to which variables have the most impact on DOF but all 4 affect it. And since sensor size affects circle of confusion - it too has an impact on depth-of-field.

oops forgot the link:
Here's a link to the math:
http://www.dof.pcraft.com/dof.cgi
DOF is a property of the lens, not the sensor.

The smaller COC values actually give shallower DOF for the same physical lens for the smaller format. Just think about it for a moment.

The shallower DOF expected for a larger format is the difference in focal length to get the same field of view.
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Old Aug 29, 2010, 4:22 PM   #38
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Greg - it's a property of BOTH. If sensor size did not affect the calculation you could change sensor size in the equations and the result would be the same. It's basic algebra. You skim past the part about needing to change distance or focal length to change field of view - thereby changing another variable. Like it or not you cannot change sensor size and keep distance, focal length and f-stop the same and keep the same depth of field. Again, it isn't a matter of theory - it's an algabraic equation. You can change 2 variables and get the same answer but you cannot change only one variable (sensor size) and get the same answer.
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Old Aug 29, 2010, 4:44 PM   #39
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Don't confuse Aperture with f-stop (AKA focal ratio).

The 100mm lens has an aperture of 25mm at f4.

the 150mm has an aperture of 37.5mm at f4.
The value used to calculate DoF is the f-number, not the aperture.
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Old Aug 29, 2010, 5:08 PM   #40
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Oh my I have stirred up a can of worms. Im going to have to go digging in my photo archive where I conclusively proved that a FF vs 1.3 and 1.6 crop at the same distance to the subject had INDENTICAL DOF shown with a yard stick. All 3 were shot with the same lens at f2.8. The only thing that changed between the them is the FOV. The comparison was between a 1Ds MKIII a 1DMKIII and a 50D. The circle of confusion is nothing more than a change in the focal plane depending on how far it is from the end of the lens to the face of the sensor and while I will agree it does factor in its not a value of the sensor but that of the camera bodies build and is miniscule.

@ Mark, you can have my 400 2.8 and 200 2.o when you pry them from my cold.........LOL

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