Archived posting to the Leica Users Group, 2001/02/08
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> > DOF is basically a limit of resolution. Whether it is lense resolution,
> > film resolution, paper resolution or the resolution of the eye.
> Specifically, the eye, as it relates to the viewed image.
Agreed.
> Also, the films of today are much better at resolving fine detail than the
> films of 30 years ago -- yet our perception of DOF has not changed much,
> which would run counter to your argument.
How does that counter my argument? In fact, a number of people have said that because of new APO/ASPH etc. lenses, and new films etc, that DOF should be re-thought. That does bolster my argument.
> > To determine the DOF of the film, simply view the film and see where the
> > grid runs together.
> Ah -- and therein lies the crux, n'est pas? "see where the grid runs
> together". Which requires perception.
Er, in reality, no. You are going to argue that everything requires perception or it doesn't exist or something like that. It is simply a matter of the impact the 'area of confusion' has on your result. That does not negate the ability to measure it, it only adds error to your measurement. I believe the 'area of confusion' will be negligible to the overall measurement that can be achieved. BTW, all measurements have some error inherent in them, since every measurement is relative.
> Which requires something to be
> viewed at a particular magnification and physical distance from the eye.
No. It converges, whether YOU view it or not.
> Which leads us right back into CoCs, which leads us directly to the issue
> that DOF is subjective and cannot exist independently of the observer.
Technically, everything is perception, and that appears to be your only argument, which is a completely circular argument It is the impact of that perception that is critical.
> Note, however, that depth-of-field is NOT the same as resolving power of
> the film.
Agreed, but they can inter-relate.
><snip> Their answers will change, although nothing execpt magnification has.
Of course, that has never been disputed, and it isn't relevent to what I have been saying.
> Example two: Take exactly the same negative and calculate the DOF using the
> established formulas. Let's assume it was taken with a 35mm camera and
> therefore we use a CoC size of 0.033mm. Note what the near and far ranges
> of the DOF are. Now, substitue a CoC size of 0.020mm in the formulas. You
> will find that you now have a completely different DOF range (narrower) yet
> absolutely nothing else has changed. We can measure every damn thing we
> choose about the negative, the print, the camera, the focal length,
> aperture, film resolution, MTF resolving power, distance to subject,
> lighting conditions, or anything else and they will be identical for the
> two cases: yet the DOF will be different.
No, the DOF will not be different. Only the calculated DOF will be different, since you changed one of the values of the equation. The ability of the same person to 'resolve' parts of the same image ('measure' actually) from the same distance will not change because you changed a variable in your calculations.
> If DOF is an objective property, how do you explain the two examples above?
I don't know what else to explain to try to get you to get it. I don't know what your background is, but I can only guess it isn't in engineering or science. That's not meant rudely at all, BTW.
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