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jstevewhite · 07-11-2009, 11:51 PM

Originally Posted by falconeye

1. Lens projecting exactly the same image: Yes, and out of context, DoF would always be zero. Optically, only one subject plane is in focus and all other parts of a subject are not sharp.

2. Therefore, reasonably sharp is the keyword here.

3. As this term needed a precise definition, an otherwise sharp point is allowed to smear out over a finite-sized area, the Circle of Confusion (CoC). And its size matches the human eye's capabilities. Typically, the Zeiss formula is used:
CoC = image diagonal / 1730

Based on the capabilities of a theoretical human eye, that is. The Zeiss formula is based on assumptions involving final magnification, visual acuity, and assumed viewing distance. This leads to the inescapable, counter-intuitive, and fractious reality that if you have better vision, you'll get less depth of field, and the converse. And such gems as: "If you don't have enough DOF, step back from the image."

So, whenever the image diagonal changes (e.g., when cropping), so does DoF. Note that pixelpeeping is cropping because your monitor is limited in size.

If you go thru all the math, you'll see that independently from sensor size, focal length and everything else, DoF only depends on three factors:
- The distance to the subject [m]
- The physical diameter of the lens' aperture in millimeter [mm]
- The Field of View (FoV) of the image, in degrees [°]

Of course, the 2 parameters, FoV and lens' aperture in millimeter can be computed from 3 variables (sensor size, focal length and f-stop). However, this obscures things and sometimes leads to endless debates

Hehe - you pointed out the pig-in-a-poke, above, then did some hand waving to turn the subjective ( "reasonably sharp" and "visual acuity" ) into precise-sounding math. The reason this subject generates so much debate is that the foundational assumptions are completely subjective. These debates raged back in the days of film. We can all see *something* is going on, but it's really tricky to actually quantify from an experiential standpoint. The math certainly means something, but it's at least one layer of abstraction away from what we're looking at, as, likely as not, you and I would see different DOF in a print at 8x10.

I'm not disagreeing with your post, I'm just pointing out that there are gotchas in there that make it impossible to remove the contentiousness from this topic.

One note: One place where "The lens projects the same image" is an accurate description of affairs is in Macro applications. 1:1 is 1:1, no matter who's leg you're pulling, and the DOF is constant across sensors, and dependent on the visual acuity of the viewer.

07-11-2009, 11:51 PM	#12
jstevewhite Senior Member Location: Midwest Gallery Photos: 0 Posts: 130	Originally Posted by falconeye 1. Lens projecting exactly the same image: Yes, and out of context, DoF would always be zero. Optically, only one subject plane is in focus and all other parts of a subject are not sharp. 2. Therefore, reasonably sharp is the keyword here. 3. As this term needed a precise definition, an otherwise sharp point is allowed to smear out over a finite-sized area, the Circle of Confusion (CoC). And its size matches the human eye's capabilities. Typically, the Zeiss formula is used: CoC = image diagonal / 1730 Based on the capabilities of a theoretical human eye, that is. The Zeiss formula is based on assumptions involving final magnification, visual acuity, and assumed viewing distance. This leads to the inescapable, counter-intuitive, and fractious reality that if you have better vision, you'll get less depth of field, and the converse. And such gems as: "If you don't have enough DOF, step back from the image." So, whenever the image diagonal changes (e.g., when cropping), so does DoF. Note that pixelpeeping is cropping because your monitor is limited in size. If you go thru all the math, you'll see that independently from sensor size, focal length and everything else, DoF only depends on three factors: - The distance to the subject [m] - The physical diameter of the lens' aperture in millimeter [mm] - The Field of View (FoV) of the image, in degrees [°] Of course, the 2 parameters, FoV and lens' aperture in millimeter can be computed from 3 variables (sensor size, focal length and f-stop). However, this obscures things and sometimes leads to endless debates Hehe - you pointed out the pig-in-a-poke, above, then did some hand waving to turn the subjective ( "reasonably sharp" and "visual acuity" ) into precise-sounding math. The reason this subject generates so much debate is that the foundational assumptions are completely subjective. These debates raged back in the days of film. We can all see something is going on, but it's really tricky to actually quantify from an experiential standpoint. The math certainly means something, but it's at least one layer of abstraction away from what we're looking at, as, likely as not, you and I would see different DOF in a print at 8x10. I'm not disagreeing with your post, I'm just pointing out that there are gotchas in there that make it impossible to remove the contentiousness from this topic. One note: One place where "The lens projects the same image" is an accurate description of affairs is in Macro applications. 1:1 is 1:1, no matter who's leg you're pulling, and the DOF is constant across sensors, and dependent on the visual acuity of the viewer.