I am not referring to running the focus to adjust for film curvature, but focusing up and down through the emulsion itself. This is possible to some extent, and can yield different things into sharp focus. If you haven't done this, you truly don't know what I am talking about.
That has more to do with the film not being perfectly flat than focusing on different layers in the film emulsion. It you're talking about color film, only certain colors would be in focus, as the different layers of the emulsion respond to different wavelengths of light. If focusing on a specific layer in the emulsion was a relevant factor, then you'd have a negative where objects of one color would be more sharply focused than objects of different colors. I've not shot 4x5, but I did scan film before going digital, and film flatness always had a far more significant impact on the scan than focusing on different layers in the film emulsion.
Now I understand as you describe it, that the digital sensor is very 2 dimension like and has no depth of any siginificance. The sensor plane does not move, I understand this. Up until now, we hopefully are in some kind of understanding. You have stated, if I understand you, that the sensor would not recognize anything immediately in front of or behind its position, this too I agree on at this point, and had from the beginning. From the sensors' perspective this information doesn't exist and is why you consider it irrelevant. But from an optical perspective, there would be slightly a different image available in any of these other positions.
We're in agreement so far.
Using film, that has more of a thickness to its emulsion, you would also capture some of this detail at different positions within the emulsion. While this is not a huge distance, it is measurable. By using this thickness in film emulsion, it is possible to increase YOUR DOF, or my depth of sharp focus, to show this sharpness in a print.
Here's where you're going off base. Here's why: Assuming what you say is true (which I doubt, due to the color-specificity of the emulsion layers involved) about a meaningful focus differential being achievable
within the thickness of the film emulsion, your ability to extract that information optically is going to be hampered by the fact that the light from your scan or enlargement has to pass through the entire thickness of the film, not just the part that is "in focus".
To illustrate, let's conduct this mental experiment: let's load a camera with three layers of film, each of which has a perfectly clear backing, and is 1/3 the thickness of normal film. Assume perfect film flatness, and that keeping the 3 layers of film in perfectly correct registry after development is possible. Expose this "sandwich" so that focus is centered on the middle layer of film. Now you have three individual exposures with a very slight focus bracketing. At this point, you're saying that these three sheets of film have some intrinsic collective property that results in a better-quality image than any one sheet of film by itself. And that is the fatal flaw in your reasoning. Here's why:
Even if you're focusing precisely on the middle sheet of film during exposure and enlargement, the top and bottom sheets of film will still be contributing an out-of-focusness to the enlargement; they are two layers of film that were out-of-focus during exposure and are now out-of-focus again during the enlargement. That means that the projected image will be a combination of of the sharply-focused center film sheet and the double out-of-focus top and bottom sheets, with the double-out-of-focus sheets dominating the final result by a ratio of 2:1 over the contribution of the properly focused film sheet. And if you focused on the top or bottom sheet of fim during exposure and enlargement instead of the middle sheet, the situation gets even worse; instead of having 2 slightly out-of-focus sheets of film, you now have one slightly out-of-focus sheet of film and one
really out-of-focus sheet of film, and you still only have one properly-focused sheet. As long as you're enlarging or scanning all three sheets of film together, there's no way to avoid this, and there's no way it can possibly improve overall image quality in any meaningful way. On the other hand, if you remove and discard the top and bottom sheets of film, that
will make a significant improvement in overall image quality, as long as you manage to properly focus on the single sheet of film.
So the final print image is sharper then it would be if you used a wider aperture with less of your DOF, or have you assumed that you only use a smaller aperture to adjust for film flatness? By stopping down an enlarging lense both things really occur, not just the correction for film flatness. I can see how one would assume that since the film flatness is probably several times of magnitude larger, it would be easy to ignore the emulsion thickness issue. Do you understand what I am saying here, or am I "wrong" on this point as well, ?
I understand exactly what you are saying, I just don't buy any of it, for the reasons I've explained above in the thought experiment, as well as the following:
1. The film base is much thicker than the emulsion layer(s) for any given film stock, (95 microns/base vs 25 microns/emulsion for Agfa RSX 50 slide film in 120 format, as an example) so I don't believe the emulsion thickness is significant enough to be relevant to depth of focus or overall image quality in real-world imaging, especially given that
2. Film flatness is a problem during exposure, enlarging, and scanning, and is of much greater magnitude than overall film thickness or emulsion thickness.
3. If your premise regarding "focusing within the emulsion" had any validity, then enlarged images would render objects of one color markedly sharper than objects of another color, and one could "tune" a scan or darkroom print for best sharpness for objects of one color or another with a simple focus adjustment. And one would also have to factor in the order of color emulsion layers within the film stock being used when deciding how to focus for a given scene with objects of assorted colors.
Given that, I see no reason to conclude anything but that the thicker imaging layer of film vs a digital sensor is either irrelevant (because it's not thick enough to make a meaningful difference in focus and is completely overwhelmed by film flatness issues anyway) or is a distinct
disadvantage for all the reasons I outlined in my thought experiment above. I believe that the focus effect you're seeing is primarily caused by a lack of film flatness, not "focusing within the emulsion", and that far from contributing to image quality in any meaningful way, it detracts from it.
To make a fairly bad analogy, you need to stop down some more with digital because because with digital you're only using the "middle sheet of film". You can get better resolution and clarity as a result, but you need to be more precise with your focusing and stringent with your definition of "in focus" (AKA a smaller circle of confusion) to maximize what the sensor is capable of.