11-22-2018, 12:14 PM
(11-19-2018, 10:25 AM)obican Wrote:(11-18-2018, 02:58 PM)you2 Wrote:(11-18-2018, 12:29 PM)Klaus Wrote: Well, it is just a 50mm lens so you have to get fairly close for a real shallow DoF.
And yes, it seems darn sharp - at least at 30mp (which is not a lot these days)
There is a zeiss paper that talks about this aspect of lens design. For a given focal length (50, 80, ...) the lens design can play a significant factor in the total blur in foreground/background. The net summary is not all xxmm lens are created equal in this aspect. However, it can be tricky to design a lens to meet different objects (clean bokeh, high bokeh, high resolution). The paper didn't really go into that aspect it just explained in detail why different lenses of the same focal length had different amount of blurring.
DOF only depends on two things, Magnification and Aperture. So, lens design apart from these two factors shouldn't create any difference in DOF. Total background/foreground blur is something different but then even then only depends on the Magnification and Aperture. You can't change sensor size, lens design, amount of glass, number of blades or anything else and expect a difference in the amount of DOF / background blur without touching those two variables.
well, maybe it relates to the entrance pupil. While for most applications aperture is a "good enough" approximation, DOF is actually dependent on the entrance pupil (which is usually taken to be the same as the aperture) - at least that's how I understand the info one finds. But entrance pupil and aperture does not have to be the same, and depending on the lens design, this can be quite different. Hence it should be possible to design lenses with the same focal length and aperture, but difference in DOF. I think a factor of 2 is usually the maximal difference one can see (mostly in macro lenses?). So for real world experience, it's not a big deal, so DOF calculators just use aperture, as that's what's easily readable from the lens.
Anyway, the experts can probably explain that better than me.