MAubrey
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flash wrote:
There's zero DoF advantages. You can always find a much faster lens in 24x36. f1.2 lenses are a dime a dozen in 24x36 and 0.95 is in several systems. If you must have blurry backgrounds then 24x36 is the only choice. that's where all the R&D money went.
Just to follow up on this—for the others. I know Gordon knows all this stuff.
With f/2.8 being so common, basically every 645 lens ends up being equivalent in DoF to f/1.7 on 24x36. Lenses like the Mamiya 80mm f/1.9 and Contax 645 80mm f/2 were novelties—beautiful lenses for—where that faster aperture only appeared at one focal length, where today getting an f/1.2 equivalent for 35mm can be had from 28mm to 85mm.
Things improve only a little at larger formats. If you can drop the cash, Hasselblad's 2000/200 series had f/2.8's at 50mm (~25mm f/1.4) and 150mm (~75mm f/1.4) and an incredible f/2 at 110mm (55mm f/1). All incredible lenses, but the 50mm and 110mm are massive.
I haven't shot the Pentax 67, but their later 67 lenses step things up a bit. They also have a set of 75mm f/2.8 (35mm f/1.35), 90mm f/2.8 (43mm f/1.35), 105mm f/2.4 (50mm f/1.15), and 165mm f/2.8 (80mm f/1.35).
The caveat to all of this is that these equivalencies are all assuming comparisons of the diagonal. But of course, you shoot 6x6 partially because you want square. And if you want square on 24x36, you need to crop to 24x24. And now the diagonal is different. Now the Zeiss 110mm f/2 on 6x6 actually gives the same combination of AoV, DoF, and framing as a 47mm f/0.86. So, at least now, we're talking about something more interesting—but something that's really hard to nail focus with film and a split prism...
Diagonal Crop factors:
645: 0.62
6x6: 0.53
6x7: 0.48
6x9: 0.43
Cropped to equivalent ratio:
645: 0.58
6x6: 0.43
6x7: 0.43
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