Loris,
I said that with a scan of 5000 spi I could get a image that would print at 36X44" at 360 spi. A more realistic figure would have bee 300 dpi because I assume that my lenses will resolves 80 lines per mm or more, not 100 lines per mm or more. So it is true that the 155mp pixel count that you get from a scan of a 6X7 cm scan at 5000 is not all true resolution, but I am sure that about 4000 spi of it is. But I could ask the same question about the 24 mp Sony. How much of the 24 mp is true resolution?
That figure is quite realistic for Mamiya 7 lenses and a fine grain B&W film like Acros or Tmax-100, assuming use the camera on a tripod and an aperture that is not diffraction limited. In fact, from actual resolution tests I know that all of my lenses, except possible one, will resolve more than 80 lines per mm in the center. Certainly I expect less at the corners, but that is true of all formats so I think it best to stick to center resolution when comparing formats, otherwise the issue becomes a lens issue rather than format issue.
Obviously I am describing output in terms of size that is at the extreme end of what is possible with 6X7 cm format, but it nevertheless quite realistic with top technique in exposing, developing, scanning, post-processing and printing. I was at an exhibition about a year ago of 30X40" color prints made from Imacon scans of 6X7 cm Mamiya 7 negatives, mostly of architetural subjects that had a lot of fine detail. One of the guys looking at the prints was a professional printer at a high end studio. He remarked, "8X10" format?
I would agree that for magnifications of over 10X one is generally better off to switch to a larger format. But that trade-off, which is advantageous in terms of pure image quality, has limitations in terms of choice of subject as well as other logistical problems.
In any event the point I am making is that it is quite possible to get to print size of 36X44" with a 6X7 cm negative from Mamiya 7 and still have real resolution (i.e. no rezzing up) equal to or beyond the threshold of human resolution of 5 lines per mm. This assumes that you start with 80 lines per mm in the original negative, which is not only feasible, but likely, with good technique,and that you get a high quality scan drum or dedicated film scanner.
With a 24mp camera you reach the 5 lines per mm threshold at 14X22", if my calculations are right. Noise level will be low so you can rez up a lot with the digital file, but one can also enhance a high resolution file from a film scan quite a bit as well.
And of course, your comments about diffraction limitations apply to all formats, maybe more so to digital than film because of the very small sensor points. With APS sensors diffraction limitation can easily be at f/8 or less. With 6X7 cm MF I tend to use mostly wide angle lenses (50mm and 65mm) that give great depth of field at f/11, which by my calculations is diffraction limited at over 100 lines per mm.
That said, I won't argue with the premise that a 24mp digital camera would meet nearly all of my needs since 14X22" is about as large as I normally print, and a DSLR is of course much more versatile than a MF film camera.
Happy Holidays,
And BTW, Santa Wife left me as my Christmas present a 15 mp Canon 50D, instead of the Sony A-900 or the new 21mp Canon 5d which I listed as other choices. Maybe you know the song, "you don't always get what you want, but sometimes if you try enough you get what you need!"
Sandy
Dear Sandy,
[Curiosity] The calculations I made tells me that you need to resolve something like 117 l/mm (lines per mm ~= 58 lp/mm) on film to be able to print a sharp image at size. Can you really resolve that much with your Mamiya lenses? I'm not talking about the center here; what about the borders and corners? BTW, calculations assumed that you're viewing the print from 57" distance. (= Diagonal size of the print.)
[Actual / important point] Another calculation says that you have to use aperture values not smaller than F11 or else you'll become diffraction limited (to reach the resolution figure above). That is not an aperture value with lots of DoF considering the film format and most importantly the CoC figure implied above. (= 1 / 117 = 0.0085mm)
What I'm saying here is that I feel that you have to use a more realistic (or more relaxed, if you like) criteria to be able to make a realistic comparison -> I mean I just don't see how 36x45" is possible (sharp = sharp) from 6x7; that's 15x enlargement!??? To me, more than 10x enlargement (especially when considering a full analogue workflow) is a sign for the fact that you need to switch to a larger format.
Choosing a more appropiate / realistic criteria and comparing film / digital photography based on that would be better / more productive IMHO...
What are your thoughts?
Merry Christmas to all BTW!!!