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The resulting brightness has nothing to do with the lens size. The important factors are the focal ratio and the sensor size. Obviously the sensor should have a

The resulting brightness has nothing to do with the lens size. The important factors are the focal ratio and the sensor size. Obviously the sensor should have an appropriate size to receive all the focused light otherwise it's just wasted.

scythe 10y ago
C'mon HN this guy is right, you can't apply Gauss's Law with a sphere when you have photons from every angle. However I'm not sure whether the total number of photons collected is the right thing to be measuring. Aren't there serious aberrations at high magnifications in practice? And aren't there, y'know, glowing objects in the night sky? Stars I believe? The moon? I don't know how many photons there are but a single green photon carries a miniscule 376 zeptojoules and I don't think my eye responds to anything below the picojoule range. Counting photons and looking for the information limit seems a little extreme in this, er, light. I wonder whether you could use this technology for medical imaging if you shield the camera from any light that isn't being transmitted through the body. The possibility of recovering color information is exciting.
banaanbanaan 10y ago
Surely 'nothing' is an exaggeration. In the limit of the lens size going to zero, the brightness also goes to zero. (Ah, you're also scaling other things in your mind I guess, so then this is not true.) If you keep the setup such that all the light that comes through the lens will get to the sensor, and keep the sensor the same size, a larger lens would mean a higher brightness, right? Maybe in practice you're limited by other factors, but 'nothing' can't be right. Especially when we're talking about theoretical limits. Edit: I think what you're saying is that the lens doesn't matter for the brightness if you scale the focal length and sensor size in such a way that the brightness stays the same. Which is obviously true, but if you leave out the last part, it isn't.
- tigershark 10y ago
  A lens with a smaller focal length will produce a projected image smaller than the same lens with a longer focal length. A lens with a focal length of 1m and an aperture of 50 cm will produce a projected image that will be 4 times bigger than a lens with a focal length of 50 cm and 25 cm of aperture. So even though the amount of light captured in the first case is 4 times, the effective brightness is exactly the same because it is spread in an area 4 times bigger compared to the second lens. For this reason the only way to increase the brightness is to reduce the focal ratio, simply increasing the aperture maintaining the same focal ratio won't help. Obviously if you have an aperture of size 0 then all the light is wasted because you can't have a sensor with size zero. In an empirical way you can see this concept in the real world with the abysmally small lenses in the smartphones that have the same brightness of the much bigger lenses in SLR cameras or in refractor telescopes.

scythe 10y ago

C'mon HN this guy is right, you can't apply Gauss's Law with a sphere when you have photons from every angle. However I'm not sure whether the total number of photons collected is the right thing to be measuring. Aren't there serious aberrations at high magnifications in practice? And aren't there, y'know, glowing objects in the night sky? Stars I believe? The moon? I don't know how many photons there are but a single green photon carries a miniscule 376 zeptojoules and I don't think my eye responds to anything below the picojoule range. Counting photons and looking for the information limit seems a little extreme in this, er, light. I wonder whether you could use this technology for medical imaging if you shield the camera from any light that isn't being transmitted through the body. The possibility of recovering color information is exciting.

banaanbanaan 10y ago

Surely 'nothing' is an exaggeration. In the limit of the lens size going to zero, the brightness also goes to zero. (Ah, you're also scaling other things in your mind I guess, so then this is not true.) If you keep the setup such that all the light that comes through the lens will get to the sensor, and keep the sensor the same size, a larger lens would mean a higher brightness, right? Maybe in practice you're limited by other factors, but 'nothing' can't be right. Especially when we're talking about theoretical limits. Edit: I think what you're saying is that the lens doesn't matter for the brightness if you scale the focal length and sensor size in such a way that the brightness stays the same. Which is obviously true, but if you leave out the last part, it isn't.

tigershark 10y ago
A lens with a smaller focal length will produce a projected image smaller than the same lens with a longer focal length. A lens with a focal length of 1m and an aperture of 50 cm will produce a projected image that will be 4 times bigger than a lens with a focal length of 50 cm and 25 cm of aperture. So even though the amount of light captured in the first case is 4 times, the effective brightness is exactly the same because it is spread in an area 4 times bigger compared to the second lens. For this reason the only way to increase the brightness is to reduce the focal ratio, simply increasing the aperture maintaining the same focal ratio won't help. Obviously if you have an aperture of size 0 then all the light is wasted because you can't have a sensor with size zero. In an empirical way you can see this concept in the real world with the abysmally small lenses in the smartphones that have the same brightness of the much bigger lenses in SLR cameras or in refractor telescopes.