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However, large prints are meant to be viewed from large distances, which makes the pixels smaller to the eye. The resolution of the human eye is the controlling factor. The claim that you need more megapixels for larger prints only counts if you're holding a 20x30 print as close as a book and trying to count the blades of grass. 6-8 megapixels is fine.
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In practice, however, viewers will move as close to a photograph as physically possible--especially with landscapes. Unlike their expectations with other visual media, viewers expect photographs to unfold more detail as they move closer...so they do, unless physically restrained. So rules about viewing distances are irrelevant.
The expectations of viewers also affects what we can "get away with" in regards to image resolution. As I mentioned, viwers expect a landscape image to reveal more and more detail as they get closer or the image is enlarged. Practically speaking, the resolution requirement of a landscape image is infinite--at the point of enlargement that viwers can detect that the image has reached the bottom of its detail, they are disappointed.
OTOH, a portrait is accepted as "sharp" as long as facial hair is sharp, and viewers seldom even want to see more detail than that--nobody but a dermatologist wants to see skin flakes and hair mites. Moreover, hair in a digital image is simple for software to interpolate because it's simply a detailess line--the software needs only to add more detailess pixels along the length of the hair.
As long as the original image resolves the facial hair, the portrait can be successfully interpolated to any size and yet stand reading-distance inspection.
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Any thoughts on lens resolution vs. pixels? Have we reached a point where pixel density exceeds resolving power?
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That's theoretically impossible, and as a practical matte we're still far from there with digital sensors. The total system resolution is mutually limited by both the sensor and the lens, the total system resoution is always less than the weaker of the two, and neither can ever (theoretically) be "maxed out." As one factor (sensor or lens) is improved, it pulls the total system resolution closer to the maximum of the weaker, but diminishing returns prevent the system from ever fully reaching the capabilities of the weaker.
We know for a proven fact that a 21 megapixel sensor does not exceed the resolving power current lenses because the 21 megapixel sensor does not yet have the resolution of thin-emulsion black and white films (such as Adox and Kodak Technical Pan) that we used with the same lenses decades ago...and those films had not "maxed out" the lenses...we got images with greater resolution.
As the sensor resolution increases, that does make lens faults more easily recognizable because the sensor becomes a lesser cause of unsharpness. For instance, a poorly resolving sensor with a poorly resolving lens will show blurred corners. When a high-resolving sensor lessens the sensor's contribution to the blur, we can deduce remaining blur is from chromatic abberations in the lens. If the sensor resolution is great enough, we may even be able to actually the double-image pattern of the chromatic abberation.
But even though we can now discern the faults of the lens, a lesser lens with a superior sensor still provides better image quality than a lesser lens with a lesser sensor, although eventually "dimishing returns" do catch up to you.