Mike W. wrote: > A paradox of the first order, unless..... the margins of error > are so small as to be unnoticable. No, perish the thought that > this could all be marketing speak!
They're insignificant. Depth of focus can extend several millimeters. There is probably a lot more slop in many areas of camera design and function than you imagine. For instance, viewfinders are seldom 100% because it's very difficult to built to the tolerances needed to insure that what you see is precisely what ends up on the film--a 92% finder is convenient for the manufacturer in more than one way, not only because it allows them to make prisms and mirrors smaller, but also because it masks slop in alignment. Autofocus mechanisms can actually be made such that focus would change with the beating of your heart. But this would require exquisite precision in the rest of the mechanism, and it would be frustrating, because you could never lock on to a subject. So the manufacturers choose a tolerance--generally pretty loose tolerances, since, ironically, the looser the tolerance, the more precise it _feels_--because then it locks on to a focus distance and stays there. And if manual focus users are smirking here, don't, because manual focus is just as bad. When you really study camera focus, it becomes obvious that there is almost always focus error built into the camera (that is, when it appears to be in perfect focus, it isn't) _and_ it's almost impossible to achieve truly exact focus visually. Of course, this typically matters little, just as it does with AF. since the slop is masked by depth of field. Even a rangefinder like the M6 is not immune to these problems--although the focus mechanism is more precise than an SLR, meaning repeatable, it's not terribly accurate. With demanding tolerances, a rangefinder would need to be calibrated for each separate lens and for only one focus distance. If a specific 90mm lens is carefully calibrated for a ten-foot focus distance, for instance, then it will be very precise at that distance, but chances are that it will still show error at three feet and at 35 feet. And change lenses and all bets are off. As far as the film is concerned, the surface of the film is actually wavey--it's not perfectly flat even with a vacuum back. The ripples or whatever you want to call them are many times the depth of the emulsion. But to make matters worse, no lens is precisely "flat field," although some process lenses come close. But most camera lenses cast a "plane" of focus that isn't a plane. That's why experimenters have to compromise on the point at which lenses are at point of best focus. If you focus in the center of the field, the moderate Hrel may be out of the plane and the corners into it again; so do you split the distance and achieve the best compromise, or do you focus one area of the field perfectly and say to hell with the rest of it? So anyway, not need to worry about whether the depth of individual emulsion layers fall within the tolerance of the apochromatic correction--such miniscule tolerances are completely overwhelmed by the slop inherent in the entire system. --Mike - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
