linse said:
Don't look at the 35mm equivalent focal length bullshit. Same true focal length, aperture and subject distance means same depth of field. However, the smaller sensor will be more cropped. If you alter the subject distance to get equivalent field of view, then the smaller sensor will give you higher depth of field.
I think there is some misconception here.
In calculating DOF the final output size of the image has to be included in the calcuation. Most DOF calcuation formulas assume a standard print size, a standard viewing distance thus a certain circle of confusion (based on resolution of human vision), so it appears that it is only affected by focal length and aperture setting.
While the circle of confusion (COC) on the image sensor remains the same regardless of sensor size as long as the focal length and aperture is the same, the magnification required to produce the final output image if the same size will be different for different sized sensors.
Calculating DOF based based on the COC applied to the sensor surface alone is meaningless because we do not view the final image at the size that is the same as the physical size of the sensor.
To properly calculate DOF one has to first determine the final image size and the viewing distance. Then based on the resolving power of the human eye the circle of confusion on the final image is determined. After that the COC (on final image) is translated to the COC on the sensor, based on the size ratio between the final image size and the sensor size. From there, the sensor COC is used in the formula that takes in also the focal length, aperture and focusing distance to dermine the DOF.
DOF refers to the range of subject distacne for which objects will be rendered sufficiently sharp in the final output image.
One good analogy is to imagine viewing an image at different magnification on screen. Imagine you have an image of 2 objects that has slightly diferrent distance from the lens. When the image is taken the focus is set on one of the subjects, so the other is slightly out of focus.
If you view the image at a small magnification both subjects appears sharp. If you blow up your image more and more then the subject that was not on the plane of focus will start to appear more and more blur. The DOF has decreased as the magnification is increased, on the same image.