Why do some DSLRs offer ISO settings below 100?
Asked 11/17/2019
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Many cameras have a minimum ISO of 100, but some DSLRs or mirrorless models offer lower settings such as ISO 64 or even expanded values below that. Does a lower ISO mean the sensor is more sensitive, and what practical effect does it have? Is there any real lower limit to ISO values in cameras?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
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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:
- What exactly is "base ISO" and how do I find what is base ISO on my camera?
- Why is a low base (minimum) ISO desirable in stills cameras?
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.
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
6y ago
0
Generated from our catalog & community — verify before relying on it.
A lower ISO does not mean the camera is more sensitive — it means it is set to be less sensitive to light. To get the same image brightness at a lower ISO, you need more exposure, usually by using a slower shutter speed or a wider aperture.
Why offer ISO below 100? Mainly so you can use slower shutter speeds or wider apertures in bright light without needing an ND filter. In some cameras, a lower native/base ISO can also be associated with lower noise or slightly more dynamic range, depending on sensor design.
There isn’t a meaningful universal “lowest possible ISO.” Film has existed in very low ISO values, and digital cameras may offer different minimum values based on sensor behavior and how the maker rates ISO. Also, manufacturers do not always rate ISO identically, so one camera’s ISO 100 may not match another’s exactly.
In short: ISO below 100 is normal, it means lower sensitivity, not higher, and its main benefit is greater exposure flexibility in bright conditions.
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