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I've seen generalizations that full-frame sensors tend to exhibit better low-light performance and dynamic range than a cropped-sensor specifically because they tend to use larger photosites that gather more light per unit.

I'm not sure if there's ever been a full-frame and a cropped sensor built on essentially the same manufacturing process (ie, same pixel density, micro-lens technology, etc.), so this is probably somewhat theoretical, but given a full-frame sensor and a cropped sensor both built with equivalent technology, would the per-pixel IQ of both sensors be equivalent?

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It's not particularly theoretical; full-frame cameras often offer a crop mode (Nikons default to DX crop mode when a DX lens is attached). Per pixel, the image is the same whether shot cropped or cropped to the same dimensions in post processing. That said, the image quality for the "same" picture (cropped versus full-frame at the same output size) is not the same, since the same per-pixel noise values represent 2.25 times more of the final image's total data. There may be every bit as much noise per pixel in a full-frame image, but the total error (difference from the "ideal" values in the image or in any object represented in the image, assuming the same composition/framing) will come closer to approximating zero simply because there are more of them.

(A decent-enough, though still inaccurate, rough analogy would be inkjet or process printing. Almost none of the actual individual values in an inkjet print is correct, but the higher the printer DPI for the same image pixel dimensions and PPI the more the resulting print looks like a continuous-tone image without altering the accuracy of any of the individual dots.)

With a smaller sensor (assuming the same desired output size), you either need to use the same number of smaller pixels, each with a greater individual error, or fewer pixels, the error from each of which is going to be more visible in the final image.

  • I'm particularly interested in same size & makeup of pixels, so there would necessarily be fewer of them in a cropped-sensor. – D. Lambert Mar 6 '14 at 19:04
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    And, as I pointed out, that means that each individual noise quantum will contribute more to the full image. The Nikon D800 is a great DX camera in DX mode, but it's only a DX camera in DX mode. – user2719 Mar 6 '14 at 19:33
  • Additionally, to produce an image viewed at the same size the cropped image must be enlarged more than the full frame image. If the pixel densities are the same on (both) the sensor(s), then the cropped image's pixels are magnified more than the full frame image and so are any errors. The noise contributed by an individual pixel from the cropped image contributes a higher percentage of the final result than the noise contributed by an individual pixel from the full frame image. – Michael C Mar 6 '14 at 23:33
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The crop sensor is just like cropping the center out of a full frame sensor of the same pixel density (you can see this by comparing the Nikon D800 to the Nikon D5100. They have nearly identical pixel density.) If you mount both cameras on the same lens at the same target you will see the "effective" zoom increase of the APS-C sensor when scaled to the same size. But if you match pixels one to one you will see they are pretty much the same. (the D5100 actually has slightly higher pixel density.)

The noise comes from the sensor heating up. The larger sensors allow heat to be spread out better making it take longer for hot pixels to show up. Higher pixel density of smaller sensors causes them to heat up... this makes it a balancing act.)

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