Looks pretty good to me even if it is only 8-10 lp/mm. Ever since
reading Bruce Fraser's book on sharpening I've learned to evaluate the
look of a printed image on screen by showing it at 25-40% of actual
pixel size. The (rule of thumb) percentage is determined by taking the
pixels per inch of your monitor (93 for mine) divided by the print
resolution. For a 300 dpi print you'd get 93/300 = 31%. At 240 dpi
you'd get 93/240 = 39%.
Taking one of the 100% pixel crops (the GE sign) and displaying it at
39% (simulating 240 dpi) shows fairly evident grain. But displaying at
31% (simulating 300 dpi) makes the grain nearly invisible. On an actual
print it probably would be invisible. Fraser's full sharpening rule of
thumb for ink jets was to display the image at the recommended
percentage and then sharpen until the image looked slightly "crunchy".
The reason is that an ink jet print will be slightly softer after the
ink spreads and is partly absorbed into the paper. I think an ink jet
print would probably do something similar with the small amount of
residual grain.
I found it very interesting that, despite the theoretically low
resolution that the real limit in the image appears to be film grain.
Incidentally, I like the shot. It reminds me of my old TOPE image of
the Wurlitzer (organ) building in North Tonowanda, NY (just a bit south
of Niagara Falls. <http://www.tope.nl/tope_show_entry.php?event=11&pic=19>
By my calculations you should have an image that's 9449 pixels square
after scanning a 6x6 neg at 4000 ppi. That should give you a 31.5"
square print at 300 dpi. But the math seems awry here and I don't
understand why. If the image resolution was really only 10 line
pairs/mm that's equal to 20 pixels per mm. The film is 60mm wide so the
actual image resolution should only be 1200 pixels total or more like
500 ppi. I'm wondering what happens to this image if you scan it at,
say, 1000-1200 ppi or just enough to avoid aliasing. Does it look just
as good?
Chuck Norcutt
Chris Crawford wrote:
> A couple weeks ago I mentioned my first time using a Vivitar 2X
> teleconverter with my Hasselblad and the 150mm f4 CF-Sonnar lens to get an
> effective 300mm f8 lens. I shot it at f22, for an effective aperture of f45.
> At that aperture, as we noted, even the best lenses are limited by
> diffraction to a resolution of less than 10 l/mm (Someone, I think it was
> Chuck Norcutt, said the actual number was only 8!). The image looks pretty
> good nonetheless! Here's a scan of it, and a couple of 100% crops.
>
> http://www.chriscrawfordphoto.com/family-snapshots/vivitar-hassy2x/
>
> What do you guys think? I paid very little for the 2x, I think it was $35
> from eBay. I wonder if the closest Hasselblad lens to this combo, the 250mm
> f5.6 CF-Sonnar would be any better given that I had to use such a small
> aperture for the depth of field needed to keep the whole sign and the
> foreground building facade sharp.
>
>
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