I do not undserstand why manual focus would work with film, and be a problem with digital. Isn't depth of field at a given f stop with a given lens independent of what the light hits? (film or sensor). I have a serious interest in this issue of MF digital manual focus. I have 4 beautiful manual focus Pentax 645 lenses, and I had fully intended getting the new 645D as soon as it is available in the US. Am I going to find the lenses are unuseable from a practical point of view? Pentax says all their legacy lenses, both AF & MF will work. Any help/advise?
Thanks in advance
Until you have actually experienced this for yourself, it is very hard to believe. And only those who have shot film for years on a particular camera/lens combination and then put a digital back on the same camera will have encountered it. But the fact is that your effective and usable depth of field appears to simply evaporate into thin air with a MF DB. And your ability to focus accurately seems to vanish along with it.
As you and the OP point out, DOF is DOF. The laws of physics are immutable and apply equally to whatever emulsion or medium the light from the lens falls on at the point of the focal plane. However, there seem to be other factors at work. All of us here, as photographers, have a highly developed sensitivity to focus and sharpness in a print. Much more so than non-photographers. In fact, rather similar to musicians who develop a refined sensitivity to pitch and can instantly tell when one of a group of instruments is out of tune.
There is actually no such thing as DOF. There is only one plane of focus in any photograph where the subject is perfectly sharp, a plane which has no physical depth. Everything else is out of focus to a greater or lesser degree. What happens is that the grain of film masks this. All film has some grain, and it covers the whole of the image, unlike digital noise. If you can imagine printing a photograph through ever coarser litho screens, then what you will observe is that the apparent depth of field increases because your ability to judge sharpness is progressively impaired by the screen. The sharp and soft parts become more equal, as resolution is diminished by breaking the image down into coarser chunks of information. (For an extreme example of this, walk up close to a big billboard poster. Can you tell any longer which parts of the image are in or out of focus? Or where the DOF begins and ends?)
Film grain works in the same way as the litho screen. We don't really see it most of the time (except perhaps in skies) but it is all pervading, and has the effect of expanding the area of apparent sharpness, which is what we call DOF. With a digital back of say 39 MP on a MF camera, grain suddenly ceases to exist for all practical purposes at low ISO. There is the granularity of the sensor photosites, of course, but the demosaic algorithms are designed to process this out and interpolation ensures that we never see it.
With (effectively) no grain, we can see for the first time exactly where true focus is. Without the screening effect of grain, the sharp parts of the image appear sharper and the soft parts appear softer. It is rather like going from ISO 400 film on MF to ISO 25 on a 10x8 view camera. And we find, to our horror and dismay, that what was good enough in terms of focus and DOF for film falls far short on a high-end digital back.