On Jun 25, 2006, at 2:26 PM, Bob W wrote:
> I've been doing some calculations of print sizes and megapixels, and
> found something I don't understand.
>
> If we assume the correct viewing distance for a print hanging on the
> wall is about 90cm, and we accept that the maximum size of the
> diagonal of the print should be half the viewing distance, then for
> the 4:3rds system the print should be 36x27cm, giving a diagonal of
> 45cm. This fits comfortably on A3 paper (29.7x42.0cm, about 11x16" in
> American).
>
> Printers generally seem to print at about 300 dots per inch, which is
> 118 dots per cm, as near as makes no difference.
>
> So for the printed area we need (27x118)x(36x118) = 13,534,128 pixels.
>
> Yet I'm sure I read about people making high quality 20x16" prints
> from 6 - 10 megapixel cameras.
I've printed several hundred large prints (11x14 to 13x19 inch) from
6Mpixel digital capture, hung them in a gallery next to large prints
from film and higher Mpixel capture digital. Given that the original
exposures were sharp and the image processing work done well, they
are indistinguishable. Two things I've noticed:
- Digital capture to digital print does not get less sharp, unlike
optical enlargement of a film image. As the image output size
increases, apparent perceptual sharpness seems to increase until the
point at which pixelation becomes apparent to the naked eye at normal
viewing distance.
- Due to the lack of emulsion defects and grain, properly exposed
digital photographs are 'cleaner' than film images and can be printed
at 40-50% lower output resolution with equal quality results. In
general, a 200ppi print made from a well exposed, properly focused
and skillfully rendered digital capture looks at least as good as a
300 ppi print made from a similar quality negative, scanned and
printed digitally. From a current Pentax DSLR, that 200 ppi print is
10x15 inch printed area without interpolation. I've printed
successfully at 150ppi output for low-frequency photographs without
too much difficulty too, although I wouldn't use such a low
resolution for a high-detail oriented, wide angle photograph. I've
never gotten away with that low an output resolution setting for a
scanned film image successfully: they fall apart with grain and other
problems.
BTW: This is the reason that my Canon 10D's standard JPEG output at
full resolution is set for a 180ppi output resolution. That
effectively what Canon considers to be adequate resolution for the
largest sensible print size of 13x19 inch with a 6Mpixel 3:2 format
DSLR.
Upsampling adds no data but it allows edge contrast enhancement
("digital sharpening" or Unsharp Mask) with greater ease and tends to
print more smoothly. I usually upsample in the RAW converter if I
know that I am going to be making very large prints, but it's not
always necessary.
For your E500, with 2448 x 3264 pixel images, 200 ppi would net a
12.2 x 16.3 inch printed area. For a 16x20 print with normal 1"
borders, you'd output at about 175 ppi for 13.6x18.1 inch image area
without up-sampling.
It works. I have not yet received a single comment that the A3 and
larger prints I've hung or shown from the Pentax were poor in
sharpness, tonality and quality even when viewed close up by people
who do know what they're looking at. :-)
So why want higher Mpixel resolution cameras? Simple: more cropping
options, more detailing on my preferred wide-angle, close up, high
frequency photographs. If I can get more pixels and the same noise
levels as the DS, I'm happy.
Godfrey
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