Sensor Size vs. Sensitivity

[mfeldt]

Good day everyone,

  across all the threads here you encounter some kind of mantra that keeps getting repeated over and over, and that is that bigger sensors do exhibit better low-light performance because tezhy are more sensitive.

  My professional background being astronomy and the design of quite advanced instruments for large telescopes,  I keep wondering why everyone keeps taking this mantra for granted. For us, where capturing every single photon if possible, sensor size never really is an issue.

  The physical size of a pixel determines primarily ts full-well capacity, i.e. how many electrons it can take before one needs to apply a reset. Via the number of bits that you can use to store this maximum value, and of course the inevitable read-out noise, this is connected to the dynamic range you can achieve.  The quantum efficiency, i.e. the ability to turn arriving photons into photoelectrons, is not connected to the size of a pixel. 

  What may appear connected is the "light gathering area", but in fact this is true only in a limited sense, as it depends on the optical design of the camera. In astronomy, at least in what we call the high-angular resolution part of it, we tend to try to achieve diffraction limited sampling. In this case, a pixel samples a solid angle that corresponds to about half the resolution limit of the optics, given in turn by the diameter of the pupil.  If the pixels of the detector become bigger (imposed e.g. by the manufacturer), we adapt the f/ratio to keep the sampling of a point image constant, and thus each pixel will still keep receiving the same amount of photons per second as was the case with smaller pixels.  Of course you could refrain from adapting the f-ratio, whereupon that amount would increase and more signal per amount of time provided to each pixel. however, your fewer pixels would be available for sampling a single "point" (smallest structure the optics can produce), up to the point where you get single pixels representing e.g. the image of a star - something you would clearly not want!

  So I keep wondering whether there is maybe a secret difference between astronomical instruments and photographic cameras that makes pixel size (and via the number of pixels thus sensor size) play a role - or whether the whole discussion is maybe on the wrong subject. Could it be, that in fact it''s more that bigger sensor are usually in bigger cameras that carry bigger lenses with bigger apertures but use the same number of pixels per image area, so that they are more sensitive because of the larger aperture? Or is there some hidden influence of electronics that make smaller sensors have more readnoise, thus simply requiring more photoelectrons to overcome it, adapting the digital sampling and leading to "darker" images?

  Looking forward to answers...

 

 

[richg101]

if megapixel count is the same and sensor technology is also the same then a larger sensor will always outdo a smaller one for low light.  But in recent years as lens options for aps-c and m4/3 have offered a speed advantage and mega pixel counts on full frame sensors has increased beyond what most full frame capable lenses are able to resolve it is now the smaller sensors that have the advantage as far as speed is concerned.  not to mention speed booster from metabones.  a 1" sensor'd bmpcc and a 35mm f1.4 on the 0.58x speed booster will compete with the a7s and a fast 50mm out and out low light performance and the bmpcc sensor is old technology.    

 

canon's new fullhd sensor capable of doing 1million asa is a exception to the rule.  but 1920x1080 stills are not enough for astro work I expect

[Nikkor]

15 minutes ago, mfeldt said:

So I keep wondering whether there is maybe a secret difference between astronomical instruments and photographic cameras that makes pixel size (and via the number of pixels thus sensor size) play a role - or whether the whole discussion is maybe on the wrong subject. Could it be, that in fact it''s more that bigger sensor are usually in bigger cameras that carry bigger lenses with bigger apertures but use the same number of pixels per image area, so that they are more sensitive because of the larger aperture? Or is there some hidden influence of electronics that make smaller sensors have more readnoise, thus simply requiring more photoelectrons to overcome it, adapting the digital sampling and leading to "darker" images?

Exactly. And the larger sensors are also better when you aren't fighting for photons, bright scenes or tripod, because of larger full well capacity.

There is also one thing, the gaps between pixels may reflect photons.

[Cinegain]

@mfeldt: yeah, that would (roughly) be true, but now you're assuming there's 'the same number of pixels per image area', which means every individual pixel would be of equal size (in microns).

However, with an increase in sensor dimensions, in most cases the pixel density actually decreases (even if you compare the 20.2MP APS-C Canon 7DmkII to the 50.6MP FF Canon 5DS R), giving each individual pixel more surface, which increases its light gathering capability, turning it more sensitive and efficient... solar panel logic dictates. That's why the A7SII, a fullframe camera with low megapixel count and 70.6 µm² of surface per pixel is so damn good in low light conditions. And especially with video you can't just take more time to gather more light per frame.

[mfeldt]

1 hour ago, araucaria said:

Exactly. And the larger sensors are also better when you aren't fighting for photons, bright scenes or tripod, because of larger full well capacity.

There is also one thing, the gaps between pixels may reflect photons.

That's good point, actually!  I do not know the filling factor of photographic sensors - anyone has information on that?