Gene L.
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Veteran Member
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Posts: 3,788
Re: Experiment ignores FOV (Field Of View)
therickman
wrote:
I'm not ignoring anything. I understand that for the same shot (identical field of view, composition, whatever...) from both cameras, you must either use a lens 1.6x longer on the 5D, or increase the distance by 1.6 times. Of course this will change DoF, bokeh, and even cause compositional inconsistencies.
Agreed, your premise is correct. Equal focal length, aperture, and distance produce the exact same DOF when cropped and printed identically. Even so, what I've seen trip people up is that they forget about FOV, so they think that the difference is the sensor, when the real difference is due to needing different focal lengths.
Duh! That's not what I'm trying to prove here.
Please don't patronize me. I am a mature adult, and while I may not have earned your respect, I certainly have not earned disrespect.
Many people have argued that the DoF and bokeh using the same lens on a full frame and 1.6x crop camera at the same aperture and the same distance from the subject will yield different DoF and different bokeh... And they're flat out wrong!
While I don't recall seeing such a post making that claim, it certainly doesn't surprise me that you have come across it more than once. There is a lot of misinformation on the Internet and this stuff takes some time and a bit of thinking to absorb the underlying principles.
Even DOFMaster's calculator shows a difference between the two at the same FL, distance and aperture. That makes no sense, and I've proven it. There's absolutely no difference. If you took a 4" X 6" printed photo from a full frame 5D, and with scissors cut the edges off to make the photo 2.5" X 3.75", that doesn't change the depth of field. This is the same thing as using a 1.6x crop sensor camera. All the crop sensor is doing is producing the central 63% of a full frame sensor.
But you would have to compare a 2.5" x 3.75" crop from the 5D's 4" x 6" photo against an uncropped 2.5" x 3.75" photo from the 50D. If you print both 5D & 50D at 4x6 and do not crop, then the 5D actually has a small amount more DOF.
This gets into a different issue that deals with the circle of confusion. In a nutshell, the circle of confusion is the max distance between converging rays of light originating from the same source. It represents the maximum out of focusness the eye will tolerate while still considering the image to be sharp. Unfortunately, (correct me on this if I am mistaken) there is no standard the circle of confusion relative to a sensor's size, and without a standard, this is a subjective matter. Nonetheless, because an APS sensor is smaller than a full-frame sensor, the image must be magnified more for an equal print size. Because of this, one must use a smaller circle of confusion when calculating DOF because the image will be blown up larger and out of focusness will be magnified in the same fashion.
Now, bear in mind that the difference from the smaller COF for a crop sensor is a rather small difference and is almost negligible. The real issue is that for an equal FOV, one must use a smaller focal length, and that has a relatively large impact on DOF. This doesn't negate the original premise from the OP, but it does hopefully shed some light on things.
For the record, "focusness" is my own made up term.
-Gene L.
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