Re: Busting the lack of common sense
Cal Dawson
wrote:
I did this years ago and all it brought was grief.... Nobody wants to see the truth they all want to change something distance, FOV etc then they say the test will be fair or unfair dependent upon whether your results are what they want....
Cal Dawson continued:
Try explaining that to some. they say move the FF closer then the results will be real (Real skewed that is) or you have to use an 85mm and a 50mm so the comparison is the same (NOT). There is a large group here that go through life with their blinders on....
I'm afraid I'm missing your point(s). When comparing DOFs between systems, we need to spell out the conditions. In my opinion, the most natural conditions are the same perspective (subject-camera distance), same framing, and same display dimensions (this is what DOF calclulators presume). In this situation, FF needs to use 1.6x the focal length for the same framing and 1.6x the f-ratio for the same aperture. So, under these condtions, the images will have the same DOF. Do you disagree?
Another situation, that which the OP presented, is different. In this case, he used the same perspective (subject-camera distance) but
different
framing. This is an unusual way to compare the DOF between systems. However, what he demonstrated was that, under these conditions, if we use the same focal length and f-ratio (and thus same aperture), but then
crop
the more widely framed FF image to the same framing as the more tightly framed 1.6x image, and display the two at the same dimensions, then the resulting DOFs will also be the same.
However, as no one I know of has used a FF camera in that manner, except when focal length limited for extreme telephoto, or needed greater apparent magnification in macro, then this is an odd way to discuss the DOF between systems.
From
http://www.josephjamesphotography.com/equivalence/#dof:
We can now make the following generalizations about the DOF of images on different formats for non-macro situations (when the subject distance is "large" compared to the focal length), keeping in mind that aperture = focal length / f-ratio:
1) For the same perspective, framing, f-ratio, and display size, larger sensor systems will yield a more shallow DOF than smaller sensors.
2) For the same perspective, framing, aperture, and display size, all systems have the same DOF.
3) For the same focal length, framing, aperture (same focal length and aperture also means same f-ratio), and display size, all systems have the same DOF (but different perspectives).
4) For the same perspective and focal length, larger sensor systems will have a wider framing. If the same f-ratio is used, then both systems will also have the same aperture. As a result, if the image from the larger sensor system is displayed at a larger size in proportion to the sensor sizes, or the image from the larger sensor system is cropped to the same framing as the image from the smaller sensor system and displayed at the same size, then the two images will have the same DOF.