"Faiser" <> writes:
> I recently had an argument with a friend who claims that increasing
> the iso setting does in fact the exact same thing as compensating
> the exposure in post processing, but then inside the camera.
> According to him, manipulating the levels after exposure would give
> the same result. My idea is that on higher iso settings the sensor
> is more sensitive to light which is not the same thing as achieving
> this in post processing. Which of us is closer to the truth?
You, mostly.
The "ISO" setting on a digital camera is affecting the amplication of
the electrical signal (voltage) from each sensor sensel when it still
is in in analogue form.
This is not making the sensor more sensitive to light (the intrinsic
sensitivity of the sensor is indicated by its lowest ISO-rating), but
it makes sure that the dynamic range of the analogue sensor data is
distributed over the full 12 bits/channel that is the output of the
A->D proccess that takes place in the camera. If the analogue signal
had been noise free, this should in theory made high ISO as good and
sliky smooth as low ISO. Of course, the signal is not noise free, so
increasing amplification also increases noise - but if you get the
exposure right, there should be 12 bits of data/channel in your
RAW data.
By comparison, underexposing and using levels to boost gives you
data from the camera where the dynamic range is compressed into
fewer bits/channel - so there is less data to begin with. If
there was no noise, your image quality would still be degraded,
because fewer bits/channel means that there will be some level
of posterization.
(However, note that Canon only uses analogue gain to get to ISO 1600.
The "ISO 3200" provided by Canon on some models is in fact ISO 1600
underexposed one stop, and the digitally level adjusted.)
> Reason for our little argument was another C vs N discussion, where
> the iso 100 of the C is supposed to be less noisy than the iso 200
> from the N. But then the issue came up, in which the nikonian
> claims shooting in iso 200 and lowering EV a stop with Nikon capture
> would give the exact same result as the noise free iso 100 image
> from the Canon.
Huh? If I understand his workflow - this guy overexposes one stop
and then darkens in post processing? Overexposing won't reduce
noise. All he'll get is blown highlights and the same noise.
> Still I'm unsure about this, if iso in camera was really only about
> processing, wouldn't the ISO be a parameter in raw processing like
> white balance or sharpness?
True. But ISO in digital cameras isn't about (digital) processing -
and therefore it must be controlled in-camera.
> What does the iso setting really do from the electronics point of
> view?
Amplificaton of the sensor signal when it is in analogue form.
(Think of it as like the "volume control" in a sound system.)
--
- gisle hannemyr [ gisle{at}hannemyr.no -
http://folk.uio.no/gisle/ ]
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When you say you live in the real world, which one are you referring to?
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