Banding (pattern noise in deep shadows) is a mildly contentious issue for Nikon D7100 users. For the most part, it's only visible... and faintly so... in deep shadows, but it does becomes apparent if files are aggressively manipulated in post processing. Exposed properly, the D7100 produces clean image files that clearly have more detail than the D7000, but those files aren't as malleable as those from the older camera. The Pentax K-3 is an interesting alternative to the D7100 in that it closely matches the Nikon in terms of sensor specs, but is apparently a Sony design rather than a Toshiba design. This has given reason for some people to take a second look at the K-3, especially those looking for more post-processing wiggle room. The following is a quick visual comparison of the read/dark current noise of the Canon 70D, Nikon D7100 and Pentax K-3.
Dec 3, 2013 Update: Now includes the Nikon D5300.
These samples are out of camera JPEG crops from the three different cameras, exposed at ISO 12,800 at 1/125s with the lens cap on in order to capture the appearance of the read and dark current noise, which originate from the sensor itself, and to eliminate the appearance of shot noise, which is a "statistical" function of the amount of light hitting the sensor. Because these are JPEG's, this isn't meant to be a definitive analysis of these three cameras, but rather, something of a starting point in exploring the overall behaviour of their sensors.
The odd man out in this example is the Canon, which uses an off-sensor analogue to digital (ADC) converter design. This has benefits for video and astrophotography, but does make for higher levels of low ISO image noise and reduced low ISO dynamic range. Out of all three cameras, the Canon has the "purest" black when taken out of the camera under these circumstances.
|Canon EOS 70: ISO 12800, 1/125s|
|Canon 70D: +5 EV|
Certainly, there is a large amount of grain in the 70D, but texture of the noise looks natural without being blotchy... and again, rather film-like in appearance.
Unlike the 70D, the D7100 isn't produce as pure a "black" as the Canon despite being exposure in complete darkness. There's a bit of a magenta cast, and the begins of banding can be made out. it's not quite full-on banding under these circumstances, but there is a regularity to the noise that causes the eye to perceive straight lines out of the randomness.
|Nikon D7100: ISO 12800, 1/125s|
|Nikon D7100: +5 EV|
Pushed further, the pattern noise becomes more apparent, and the horizontal bands in the pattern noise are visually unavoidable. Since this is the equivalent of a theoretical "Hi.6" setting on the Nikon, this amount of exposure pushing would not be practiced in any real-world shooting. When shooting at ISO 12,800, the image noise from the D7100 looks like this:
Compared to the 70D, the noise from the Nikon at this level has a tighter grain, but a somewhat "harder" and more synthetic quality to it. The image is not directly comparable to the one above (one is in-focus, the other is out), however, the qualitative difference in appearance is visible in the broad areas of the pictures.
Strange as it may seem, the Nikon D5300 does not produce the same pattern in noise grain as the D7100. The pattern noise is not quite as orderly, and is a bit more random. The overall "flow" of the grain seems to take up a wider area of image than the tightly knit grain in the D7100. In real-world shooting, this is suggestive of less banding in the deep shadows. This also seems to suggest that the D5300 may in fact have struck a better balance between rending fine detail and file malleability than the D7100.
|Nikon D5300: ISO 12800, 1/125s|
|Nikon D5300: +5 EV|
What's strange is that Nikon has now produced four different 24mp APS-C sensor cameras, the D3200, the D5200, the D7100, and now the D5300.... and in that time they have used three different families of sensors (respectively, Nikon design, Toshiba, and Sony.)
The Pentax produces a black that is somewhere between the Canon and Nikon when exposed in complete darkness. There is a speckling pattern that neither camera exhibits.
|Pentax K-3: ISO 12800, 1/125s|
|Pentax K-3: +5 EV|
When pushed, the K-3 sensor shows a mottled texture that is closer to the Canon 70D than it is with the D7100. This is visual confirmation that the D7100 and the K-3 are using different sensors (in-camera JPEG processing won't change the pattern of the noise in this way.) When you view a real life image exposed at ISO 12,800, you can expect similar results to:
Again, the noise has a mottled pattern that is something of a hybrid of the texture of the Canon and the Nikon.
For most people, these differences will be academic, as the three cameras are competent and competitive when used at reasonable ISO levels with appropriate amounts of image processing. Even at the extremes, image noise isn't just a matter of quantity, but also of quality... and thus something that is subject to personal taste. However, the differences in features set and handling are more important than overall image quality when choosing between these three cameras, as they are quite distinct in terms of their "personality" and feature set.