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I am noticing that the ISO setting is quite low, usually 20 or 32 ISO when I take photos with iPhone. Why is it so low? How come a prosumer DSLR cannot get that low ISO?

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    \$\begingroup\$ Isn't it because their f-number is usually also quite low? Probably not a problem with a tiny sensor and a fixed lens...? Btw. some DSLR have a native ISO 64 option. \$\endgroup\$ Commented Oct 3, 2017 at 8:41
  • \$\begingroup\$ Yes, like Nikon cameras that have genuine 64 ISO and PhaseOne IQ180 has 32 ISO - reviews say that it's good to use it if you can get enough light because of superior signal to noise ratio but they're relatively recent. \$\endgroup\$
    – netrox
    Commented Oct 3, 2017 at 16:43

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The iPhone has a relatively wide fixed aperture. For example, on iPhone 7 Plus, it's f/1.8 for wide-angle and f/2.8 for telephoto. Since the aperture is fixed, according to exposure triangle proper exposure must be obtained only by adjusting shutter speed and sensor sensitivity. Fastest shutter speed 1/8000s is similar to what you'd see in a larger camera, so the ISO has to go lower for capturing those sunny moments.

A prosumer camera can rely on choosing a smaller aperture. Also, low-light performance is considered more important by any camera buyer - the bigger sensor found in prosumer cameras is optimized towards higher sensitivity range as it allows physical room for that (bigger sensels).

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Small sensors have tiny pixel area, resulting in low light sensitivity. It's not that smartphone creators want low ISO; on the contrary, they would love to improve their low light capabilities but physics limits what can be done with such small sensors and little light.

Now why large sensors cannot go as low as smartphones in ISO? Again, this is not because nobody wants low ISO. On the contrary, the available ISO range is often insufficient and we use ND filters to decrease the light intensity. They answer is the pixel full-well capacity. Bigger and more sensitive pixels mean less light is needed to reach the limit (even though the limit itself is bigger than in smartphones). In the end, it's always a trade-off. Increasing the full-well capacity to accept even more light is of course possible and such sensors are available for specialized purposes (science) but have their own problems - cost, complexity, readout speed, etc...

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