Saturday, May 4, 2013



After many years of constant sensor iteration, users have gotten used to the idea that newer sensors, and therefore "better" sensors, have less noise and more dynamic range than older sensors. So it may come as a bit of surprise that the more expensive D7100 is in fact, noisier.... if every so slightly... than the cheaper Nikon D5200. It would probably be a further surprise that it's fundamentally the same Toshiba image sensor in both cameras, with the exception of the removal of the optical low pass filter (OLPF) in the D7100.

Aha, that must be the culprit.

Well, yes and no. To understand why, we have to come back to the idea of what noise is. However, to avoid repeating myself, I'm going to refer to an older post about image noise here. In summary, we can break image noise down into three major categories:

  • Dark current and amp noise: Produced by the inherent electrical properties of the sensor, and usually only relevant for long exposures
  • Read noise: Created by moving the data off the sensor. This is relatively constant for every shot, and is extremely low in modern Nikon DSLR sensors
  • Shot noise: This pertains to the quantum nature of light itself, meaning that light doesn't arrive at the sensor in a constant stream, but in a barrage of "packets". 

It's the last type of noise that we are going to focus on, because it is this type of noise that predominates at higher ISO's. AS I described previously, we tend to talk about high ISO noise as if it's caused by the amplification that the camera is performing in boosting dim light up to usable exposure levels, but the noise is inherent to the light itself, not the amplification. The key is that increasing the ISO is really about using an exposure time that is shorter than what you would ordinarily could have, but as the exposure times get shorter, the quantum "packet" nature of light becomes more visible. In other words, it's the (small) amount of light itself that "creates" the noise, though create is the wrong word, as the noise is inherent to the quality of the light itself.

For a fascinating look at the quantum nature of like, check out this following video from Derek Muller's Veritasium channel:

So high ISO produces noisier images: not an earth-shattering revelation. However, on a traditional sensor that does have the OLPF, the situation is as if noise reduction is being applied physically by the filter, as opposed to digitally in the camera's image processor or in a post-processing software program. The way that it does is is by scattering a small percentage of the photons while allowing the majority to go straight through. In other words, most of the light will pass through the filter into the photosite below, but some of it will be scattered into adjacent photosites. Thus, a small amount of controlled blur is created, which is really what noise reduction is at it's most basic level.

Hence, if you view images of the D5200 and the D7100 side by side, after a given rise in ISO, the D5200 will actually look cleaner. At what point? I still haven't decided, because the difference can be subtle depending on what subject matter you are shooting, but above ISO 3200 is what I'm comfortable saying for now. It is probably visible at ISO 1600, but a confounding issue is that thegrain of the noise is subtly different between the two cameras as well. Rather than recreate the wheel, I'm going to refer to sample images from DPReview's write up here: However, the important thing is that most people would feel comfortable using both cameras to ISO3200 without much concern, and because of that, there isn't much meaningful practical difference in my mind. ISO 6400 is really only for emergencies, and above that, it's a salvage operation at best. So that begs the question: If removing the OLPF creates more noise, then is it worth it? This comes back to the subjective side of photography. It's not always about the amount of noise in an image, but the quality of it as well. The extra amount of noise that we are talking about is offset by the added contrast that removing the filter provides. If you want to be perfectly fair about it, apply the noise reduction a bit more aggressively on the D7100 image and then you would approximate what the the D5200 is doing.

So yes, there is more noise with the D7100, but there is more signal as well. In other words, the D7100 is for the most part better, but the way that its images need to be handled is slightly different than with the D5200. However, it's a subtle difference at best because of the sheer amount of detail that a 24mp camera can capture. Looking back, my impression is that there was more of a tangible difference between the D80 and the D200, which was repeated with the D90 and the D300. To that end, the removal of the AA filter in the D7100 is a nice to have, but not a game changer.

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