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Old Sep 9, 2002, 6:06 PM   #1
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Default minimum megapixel

i saw some demo prints of poster sized images that were printed on inkjet printers and they look great and i was wondering, what is the minimum required megapixel needed in order to print poster size images? I have a 3 megapixel camera and know that this size isn't big enough.
could somebody tell me?
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Old Sep 9, 2002, 6:22 PM   #2
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A lot depends on both the camera and subject. It's possible to print poster sized images from Canon D30 captures (3 megapixels) providing the subject was amenable to interpolation at this resolution. What that means, in essence, is that the image was a macro or closeup such as a head and shoulders portrait which allowed the 3 megapixels to be vested in a relatively small geographical area. It takes a certain pixel density to capture detail. If the subject is small, the detail super fine or if both conditions exist or the subject if far away (basketball team shot - eyes, nose, etc.) then 3 megapixels will probably not be sufficient to properly define the boundaries of the detail.

If the detail in the image isn't defined well, when the interpolation algorithm "sees" it, it will not know what to make of it. But it will assuredly make "something," and that something will probably not resemble the features in any way. It's like looking at a newspaper shot of people in the distance under magnification or an oil painting from a distance. Our human brains are perfectly happy to "imagine" detail which isn't there, but when the magnifying glass is applied, we realize that what we assumed was a particular detail is really only a few dots or "marker pixels."

The other side of the issue is digital noise. What looks fine at 5x7, or even 8x10 from a consumer grade 3 megapixel sensor, may look pretty bad when enlarged to the point that noise (which resembles film grain) overtakes the aesthetics of the image. Since professional level sensors have much better signal to noise ratios (larger and deeper photosites) there is little digital noise and images amenable to interpolation may be blown up to some incredible sizes. This can sometimes be successfully done in excess of poster size - even to 8x10 feet in some cases.

The bottom line is that there is no absolutes here. Six megapixel professional digital does indeed replace 35mm color film and transparency in most cases. Because of the low noise from the professional level sensors, images from the S2, D60, D100, DCS-760, etc., can often be enlarged well beyond the typical 16x20 limits of 35mm color film. But to do this, the original subject must have been captured close enough to allow the sophisticated interpolation algorithms to work properly.

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