Originally Posted by
bglick
> I have said this before. If the detail is not on the film you are not going to be able to pull it out, no matter the quality of the scanner or the resolution of the scan.
Yep, this is the point often overlooked and is the basis of all the confusion of how much information is in a 4x5 scan, (in terms of MP's). This also relates to the scanned film vs. digital capture comparisons. Consider this.....
A 4x5 Provia capture of a landscape image, at f22 = a max. recorded resolution of 32 lp/mm assuming 1500/f and 60 lp/mm for Provia at a given contrast that is practical. Lets assume at f22, the lens IS apt. diffraction limited. As we know, often you must stop down futher for DOF....of course, if you shoot an infinity shot, you can open up a stop or two, based on the lens MTF data... (most LF lenses only perform well at f16+, but some of the newer modern designs can get to f11, (such as the SSXL's) Anyway, so f22 is nice middle of the road f stop for 4x5. We also know that there is other issues, such as film alignment, depth of focus at film plane, etc. All slightly degrade image quality, but lets assume there is no losses here, since f22 provides decent leeway for a decent camera set up.
Now, assume you had a scanner that was 100% eff and could extract every bit of that 32 lp/mm from the film. 4x5 film is 90x120mm.... extract the math, at 64 dots per mm, you have a 44MP worth of clean, captured data on the film. Of course, to extract this data, you are forced to oversample by a factor of 3 - 6, you can end up with 150MP to 300MP file....but that is only because its second generation image, again, Nyquist (1/R) is the cause of these large file sizes. They key is, there is only 44MP of recorded data in the entire 4x5 film.
Now, lets look at a 60MP digital back capture.... lets use a hypothetical 40x50mm sensor size, which it would take a 7000 x 8750 pixels = 61 MP. This equates to 88 pp/mm (pixel pairs per mm) To get the same DOF in the smaller format at the same size final print, you can reduce the f stop by the enlargement factor (and also the lens fl, but thats not relevant here) , which is 2.3x. (2.3 x 40 = 90 mm)... f22/2.3= f9.6. Of course, this assumes same composure.
Now run the math....
9.6 f stop
157 lp/mm aerial rez of lens (1500/f stop)
56 lp/mm 1/R
2.3 format factor to match 4x5 final size
25 lp/mm at 4x5 size
This demonstrates how a typical 4x5 color image shot at f22, scanned with perfect efficiency will match a 60MP digital capture on a smaller digital format. So why do scanned files have 6x more data? It's due to second generation oversampling requirement, which is Nyquist, regardless of what Phil once said. It's can't be more obvious.
Now you can see why those touting a 4x5 film capture is equal to 150 - 300MP digital capture is simply NOT comprehending all the issues at play. Many have stated, they believe what they see. Well, this explains why Mike R's Luminous Landscape test, of scanned 4x5 film vs. 39MP capture were very close, with the film just barely edging out the digital capture. this example above, suggests he is right. And even if his eyesight is poor as suggested, it's the same set of eyes being laid on both images. Also, he had several other people come to the same conclusion.
But then you have variables...such as Velvia will record more resolution than Provia, so the 4x5 film capture will win by maybe 15 - 20%......if you go to Tmax, the 4x5 capture will destroy the digital capture, probably by 40%+. But, as a more fair comparison, to match the DR of the MF digital capture, one must go to color neg film, now the 60MP digital capture will exceed scanned 4x5 film probably by 15%.
Anyway, the numbers are close, except B&W films like TMAX. Hopefully this will shed some light on how many MP are actually in scanned 4x5 film......vs how massive the scanned files are.
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