Why does the Canon ME20F-SH use dB gain instead of ISO?
Asked 4/7/2017
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Some cameras, such as the Canon ME20F-SH, show sensitivity as dB gain rather than ISO. Why would a digital camera use dB instead of the traditional ISO scale, and what does a dB value actually represent in this context?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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The sensitivity measured in “ISO” is realized in digital cameras by an amplifier with a variable gain. The amplifier gain can be measured in dB as well, which may be more convenient for some applications. The “ISO” is basically a legacy measure mapping the experience from film photography where each film had a given, non-changeable, sensitivity which was standardized and given the well-known numbers (100, 200, 400, etc.). While the ISO keeps the relation to the light value by utilizing this standard, the amplification factor is based on the native sensitivity of the sensor and therefore makes a comparison between different cameras less convenient. If you know the “base ISO” of a camera (mapping to the sensitivity at amplification factor 1, or 0 dB, resp.), you can compute the dB value from the ISO and vice versa.
Edit: I just checked the data given at the Canon website and it seems that the native ISO of this camera is around 800. A rough calculation: They give a maximum amplification of 75 dB which translates to a voltage amplification factor of approx. 5600. This means a difference of approx. 12 ISO steps. Starting from ISO 800, this would bring you in the range of 4000000 ISO (3280000 exactly. 4560000 is one third of a “stop” more, meaning the base ISO would be 1000, but on the canon website they just claim “above 4000000”, no exact number).
Originally by user44828. Source · Licensed CC BY-SA 4.0
user44828
9y ago
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Generated from our catalog & community — verify before relying on it.
Digital cameras do not change the sensor’s inherent sensitivity the way swapping film changed film speed. Instead, they mainly change the amplification of the sensor’s electrical signal. That amplification is gain, and gain is commonly expressed in decibels (dB) in electronics.
ISO is still a standardized photographic exposure scale, largely inherited from film. In digital cameras, ISO is effectively a mapping from sensor signal amplification to an exposure-equivalent number familiar to photographers.
A dB value is not an absolute sensitivity by itself; it is a relative measure of amplification compared to a reference level. In a camera, that reference is typically the camera’s native or base setting, where 0 dB means no added gain. Positive dB values mean the signal is being amplified more.
So the ME20F-SH uses dB because it is presenting the electronics-oriented gain setting directly, which can be more natural for some video and technical applications. ISO is more convenient for photographic exposure comparisons across cameras, while dB describes the actual amount of signal amplification inside that specific camera.
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