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If you scale up everything, the aperture stays constant, correct. But since the larger sensor can work with much more light, it therefore has a much higher S/N

by Gring 14y ago

If you scale up everything, the aperture stays constant, correct. But since the larger sensor can work with much more light, it therefore has a much higher S/N-ratio. If you switch that camera to a higher aperture, you get less light, leading to a lower ratio again. So the lower S/N-ratio of the smaller sensor could be estimated as a higher aperture. Whether the numbers calculated by wisty are correct, I don't know, though.

arrrg 14y ago
Please don’t do that. That’s just confusing the issue and highly misleading. The f-number is a property of the lens and nothing else. The S/N ratio of the sensor is independent of that. It’s correct: smaller pixels tend to have a lower S/N-ratio, which mostly means that you can’t turn up the gain as much as with sensors that have larger pixels (you can’t increase the ISO by as much) and you will see more noise at the same gain. But that’s no law of nature. It’s possible to have a crappy large sensor and an excellent small sensor. It’s the sensor that matters here – and there are many more variables involved there. Sensors with pixels a certain size don’t have a certain S/N-ratio only depending on pixel size. And any exact mathematical formula (like f-number scaling with crop factor) doesn’t make sense there, not at all.
- wisty 14y ago
  Depth of field does scale perfectly with crop factor, and that's a big thing. f-number is a ratio of apature to length. A 400mm f/5.6 will have a 71mm apature. An iPhone lens does not (despite having a better f-number). The light is spread out more, so you'll a higher ISO to take fast photos, but let's say ISO is proportional to sensor size. A larger sensor captures more photons, so it can have a higher ISO. That is a law of nature. As you say, ISO is neither deterministic nor linear, but I'm more confortable considering ISO to scale with crop size than pretending it's constant. If we are ignoring ISO altogether that's fine, but then there's no point talking about the amount of light hitting the sensor. That leaves diffraction, but I don't really understand diffraction.

by Gring 14y ago

arrrg 14y ago

Please don’t do that. That’s just confusing the issue and highly misleading. The f-number is a property of the lens and nothing else. The S/N ratio of the sensor is independent of that. It’s correct: smaller pixels tend to have a lower S/N-ratio, which mostly means that you can’t turn up the gain as much as with sensors that have larger pixels (you can’t increase the ISO by as much) and you will see more noise at the same gain. But that’s no law of nature. It’s possible to have a crappy large sensor and an excellent small sensor. It’s the sensor that matters here – and there are many more variables involved there. Sensors with pixels a certain size don’t have a certain S/N-ratio only depending on pixel size. And any exact mathematical formula (like f-number scaling with crop factor) doesn’t make sense there, not at all.

wisty 14y ago
Depth of field does scale perfectly with crop factor, and that's a big thing. f-number is a ratio of apature to length. A 400mm f/5.6 will have a 71mm apature. An iPhone lens does not (despite having a better f-number). The light is spread out more, so you'll a higher ISO to take fast photos, but let's say ISO is proportional to sensor size. A larger sensor captures more photons, so it can have a higher ISO. That is a law of nature. As you say, ISO is neither deterministic nor linear, but I'm more confortable considering ISO to scale with crop size than pretending it's constant. If we are ignoring ISO altogether that's fine, but then there's no point talking about the amount of light hitting the sensor. That leaves diffraction, but I don't really understand diffraction.