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The lowest ISO value in most cameras is 100. But some have less than 100. Why is that so? Does that mean these are more sensitive? And how low can the value be?

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There's nothing magical about the value 100. In fact, early film had ISO values in the double and even single digits. Theoretically, there is no lowest value — well, zero, because it goes to fractions rather than negative numbers.

Lower ISO means less sensitive. It means that shutter speed needs to be slightly slower or aperture slightly wider to get the equivalent final brightness in your resulting image. Depending to sensor design the image may have less noise or more dynamic range than from a comparable camera at ISO 100, but the main practical benefit is that slower shutter speeds or wider apertures can be used in brighter lighting without a neutral density filter.

See:

On some cameras, settings below 100 (or some other base) are "extended ISO" — that is, adjusted in software rather than actually a lower sensitivity in the sensor. See:

In this case, the use is purely for more exposure flexibility and there is not usually a noise benefit.

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An ISO value in digital cameras establishes what amount of light on the sensor will lead to full brightness in the JPEG, and it adjusts the metering (and consequently exposure parameters the camera is responsible for) to match that reality.

It does not define the granularity with which the sensor will convert exposure to brightness values, nor the amount of overexposure the sensor can deal with before clipping.

In both regards, small sensor cameras (typical 1/2.3" these days) are driven to their limits, with their puny pixel size making already base ISO levels somewhat prone to noise and with little reserve for overexposure even if you do raw processing at most ISO values.

On the flipside, there are DSLR sensors these days (like Sony's Exmor sensors) that do not actually change sensor sensitivity when changing ISO values but that have enough dynamic range in their conversions and receptors to cater for all ISO values digitally. For those sensors, ISO just impacts the metering and the subsequent scaling to the JPEG scale.

Choosing a lower ISO value then means metering for more exposure and scaling down the results digitally, making the camera more prone to overexposure. More exposure results in a higher signal-to-noise ratio, though.

On cameras that are not similarly "ISO invariant", the setting of the ISO value influences analog amplification before the results are converted to digital. On those cameras, "extended" lower ISO range indicates ISO values where the camera no longer adjusts analog amplification (so an ISO invariant sensor technically only has an extended lower ISO range and no regular range). The "extended" higher ISO range on either sensor type typically indicates ranges where pixels are "binned" and the spatial resolution is reduced in order to offer fewer but less noisy pixels.

The availability of lower ISO values (that formally do not even differ from using ND filters) usually implies that the camera has reasonable reserves against overexposure which you can use for a smoother digitisation (that can particularly improve the noise levels in shadows) if you can afford longer exposure times or wider apertures.

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    No digital imaging sensor actually changes sensitivity. They only change the amount of amplification applied to the analog signals coming from the sensor before it is converted to digital information. – Michael C Nov 17 at 19:30
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There are a few reasons why ISO numbers may be different.

The first is the inherent sensitivity/reactivity of the sensor (photodiodes). With a lower ISO being less reactive.

The second is that different models/makers may use different ISO standards; which result in a different ISO rating. I.e. most compact cameras use the SOS standard (Standard Output Sensitivity) while most DSLRs use the REI standard (Recommended Exposure Index)... but I'm sure there are exceptions to both.

And the third is that the REI standard isn't actually very standardized. With this method many camera makers/models overstate the ISO sensitivity. I.e. they may state ISO 64 (SOS) as ISO 100 (REI) in order to provide some protection against the harsh highlight clipping characteristics. While others may state the ISO more accurately.

There's actually a few other standards that could be applied, but SOS/REI are the norms with digital.

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