Does lower resolution result in better signal-to-noise ratio, given the same sensor size?

Question

With the release of the Canon EOS R5/R6, there have been many saying that they prefer the 20MP R6 camera over the 45MP R5 camera, due to its lower price but also because of "better low-light performance."

In camera comparison sites, I've also seen the sentiment that bigger pixels (lower resolution) = better low light performance. However, is this a myth?

Here is my reasoning for this. Let's say that there are two full-frame sensors, one 20MP sensor and a 80MP sensor. According to many people, the 80MP sensor would be terrible for low-light photography since the pixels are smaller. However, each 2x2 block in the 80MP sensor could be mapped, and in post averaged to a 1x1 pixel, effectively making the image into a 20MP image. This could also be done in-camera as an option. Would this image have just the same amount of usable information as the image from the 20MP sensor?

In other terms, since both sensors are 35mm sensors (i.e. equal area), the amount of light falling onto the sensor is the same. So, at the end of the day, there will be no difference between a 20MP image and a 20MP image that has been downsampled from 80MP, right? So their low-light performance would be the same. You even get an advantage from having the option to get 80MP photos if you wanted.

Why do people say that a lower-resolution camera has better low-light performance, when it should theoretically be the same as one with higher resolution (all other factors equal)?

Note: this question is about sensors with the same size but with different pixel density. Hence, it is different from this question asking about different sensor sizes. I am aware that a full-frame camera will have better low-light signal due to its bigger area than an APS-C sensor, but what about the similarly sized sensors?

Answer 1

It depends.

Assuming both sensors have the same linear dimensions:

If you are viewing the images from both sensors at the same display size, then the low light performance of both will be similar, assuming they use the same generation of technology. There are other advantages unrelated to low light S/N performance that make using a higher resolution sensor and then downsizing the result slightly better for reproducing fine detail when images (and video) are shot under better light.

If you are viewing the images from each sensor at 100% magnification (1 image pixel = 1 screen pixel), then the image from the higher resolution sensor is being enlarged more and will have poorer low light performance, all other things being equal (which they never are).

There are also some scenarios with very small, very bright specular highlights, such as is the case with astrophotography, where the better performance of the sensor with larger photosites can be due to the smaller photosites (a/k/a sensels or pixel wells) on the higher resolution sensor having a lower full well capacity than the larger photosites of the lower resolution sensor. If the scene contains bright specular points, a larger sensel with only one specular point illuminating all of its surface will allow a brighter exposure before full saturation than a smaller sensel with the same specular highlight illuminating it.

Answer 2

Sensors have antialiasing filters that block higher frequency image content in order to avoid Moiré patterns. Averaging pixels will also average (and thus reduce) noise but is comparatively bad as a low-pass filter and thus will not work as well for suppressing Moiré patterns as an optical antialiasing filter made to size would. While you can try using different interpolation functions than a mere average, while they work better for reducing high frequency content, they work worse for noise suppression.

Also averaging light on a larger pixel means that the averaging of noise happens right on the pixel area. That makes it quite less likely that noise will make a single pixel exceed its dynamic range than averaging in the digital stage would. And digitisation noise different from the optical quantum noise will not decrease along with pixel size: there is a cost of having to deal with smaller sites.

Smaller pixel sites also tend to be more susceptible to charge leakage in the form of "hot pixels", outliers that tend to self-saturate under longer exposure times without actual optical excitement. Once a pixel saturates, it is no longer useful for averaging purposes. If the underlying defects mess with a larger charge well, their effect is smaller and will take longer to saturate.

Answer 3

The SNR begins with/originates from the scene generating it... if there is nothing in the camera's signal chain that reduces that SNR, then all sensors receive the same SNR when used with the same lens and w/ the same Ap/SS settings.

The reduced pixel level SNR associated with a higher resolution sensor is simply because the light/scene generated SNR is divided between more photosites. To some small degree it may also be due to a slightly lower fill factor (micro-lens gaps), but these days that is really negligible.

Conversely, smaller photosites perform better in lower light as they have a lower gain (fill) requirement. This why many modern sensors are incorporating dual gain photosites which have a smaller capacitor (photo diode) with a lower capacity (FWC) for low light situations, and a second capacitor (second gain stage) for brighter situations.

What really matters is light/SNR per image area. And larger sensors recieve more light with a higher SNR when the same image is recorded with the same SS/Ap settings (i.e. from closer, or with a longer lens w/ a larger entrance pupil).