Film was very different, but that was a decade ago.
Not so very different I think- 35mm film cameras often had diffraction issues too, and film curvature was often more of a sharpness culprit than not...
Since f/C is a constant, independent of format, depth of field is constant for constant aperture opening a. And since f-stop N = f /a,
Depth of field is constant when the f-stop is proportional to the format size, i.e., DOF is the same for a 35mm image taken at f/11, a 6x7 image at f/22, a 4x5 image at f/45 or an 8x10 image at f/90.
This has important consequences when the lens sharpness becomes diffraction limited beyond around f/11 for 35mm; slightly larger for large formats. (High quality lenses become diffraction-limited at larger apertures. The f-stop at which diffraction becomes dominant increases rather slowly with format size.) A lens is likely to be diffraction-limited when a large depth of field is required; the larger the format, the more it must be stopped down; hence the more likely it is to be diffraction-limited. Once a lens is diffraction-limited its resolution is inversely proportional to its f-stop. This leads to a rather surprising observation.
When a lens is stopped down so to achieve a large depth of field, and is diffraction-limited, increasing the format size does not increase image sharpness, i.e., total resolution. For example, an 8x10 image taken at f/64 will be no sharper than a 4x5 image taken at f/32.
This statement applies primarily to large formats (4x5 and above). For small formats, particularly 35mm, image sharpness is limited by film resolution. Fuji Provia 100F, one of the finest grained slide films, has resolution roughly equivalent to diffraction at f/16 ( f50 = 40 lp/mm; f20 = 70 lp/mm), but since the total system MTF is the product of the MTF of the individual components, you can see some improvement in overall sharpness for lens apertures as wide as f/8. You must choose film with care for optimum sharpness in the 35mm format. Film resolution also limits the sharpness of medium format images, but this is only noticeable on images larger than 13x19 inches the maximum for inexpensive consumer printers.
and
Film doesn't lie perfectly flat especially roll film (35mm and medium format). Sheet film is better. Film flatness is probably the least predictable of the factors that degrade image sharpness. According to Robert Monaghan, "film often buckles in 60% of 35mm SLRs tested, and virtually all medium format backs - by an average of 0.2mm (on 35mm). Yet even a 0.08 mm film bulge can reduce contrast by an astonishing 48%!" The latter number depends on the f-stop. The equation for the circle of confusion due to film bulge is (for focus near infinity: s >> f ),
Cbulge = bulge/f-stop
For a 0.08 mm bulge at f/5.6, Cbulge = 0.014mm. For a 0.2mm bulge at f/5.6, Cbulge = 0.036mm worse that the circle of confusion at the DOF limit. Pretty bad. That's why we sometimes need to stop down a little more than optimum.
To further confound you, film flatness is a function of time after winding the film. And it's different for 35mm and medium format. According to Robert Monaghan, film gets flatter if you wait up to 30 minutes after winding 35mm film, but according to both Mohaghan and Zeiss (in Camera Lens News No. 10) the bulge increases with time after winding medium format film: it's small at 5 minutes, significant at 15 minutes and maximum after 2 hours. One solid piece of useful information from Zeiss: the bulge is only half as much for 220 film as it is for 120. (That means I have to buy a new back if I go back to using my old Hasselblad; a great temptation.) The Zeiss rule of thumb is, " For best sharpness in medium format, prefer 220 type roll film and run it through the camera rather quickly." Temperature and humidity probably also affect flatness.
Oh yes, digital cameras don't suffer from film flatness problems. That's one reason why their performance is expected to exceed 35mm with only 6 to 10 megapixel sensors (multiplied by 3 when converted to RGB file formats). For much more detail on film flatness, I recommend Robert Monaghan's exhaustive discussion (with reader comments).
Ref:
http://www.normankoren.com/Tutorials/MTF6.html
