Sorry, I try not to think about these things when I'm taking pictures,
just like I tried not to think about silver valance states when shooting
film.
On 12/14/2010 7:51 PM, Larry Colen wrote:
On Dec 14, 2010, at 4:09 PM, steve harley wrote:
On 2010-12-13 11:53 , Larry Colen wrote:
In general, we want to boost the dynamic range of our cameras. However,
sometimes we're shooting a very flat scene, or a lens with low contrast. If we
wanted to, for example, expand six stops of dynamic range into 12, or 14, bits
of data it seems that one way of doing this *might* be to increase the ISO, but
that would also increase the noise.
Are there be any electronic ways of doing this that wouldn't change the
sensitivity, or increase the noise? I don't expect that there are ways for an
end user to do this, but could a camera manufacturer allow you to dial in the
dynamic range of your camera for a particular scene?
well any tampering of the analog signal would probably increase noise (or add
distortion of some sort), but there may be some sort of preset (or maybe
variable) gamma in the analog/digital converter that potentially could be
tweaked to do the same thing (ISO is essentially variable gamma, but it shifts
the dynamic range window rather than changing its width)
This is what I guess is happening. Please correct my errors.
If we express light in terms of Volts (which is what most a/d sensors read), a
12 bit A/D would read 1mV as 0001, and 4,095mV (4V) as full scale. At base ISO
(100), the analog circuitry will convert the raw output of the sensor at these
voltages to the values 1 to 4095.
At ISO 200, I assume that they have a gain of two on the signal off of the sensor so
that the ADC sees 0.5mV (500uV) as 1mV and 2.048V as 4.096V. Likewise up to 6400,
where 16uV ->1mV and 64mV->4.096V.
This is adjusting the gain, without adjusting the offset.
Let's say that we're shooting through a crappy lens, or a dirty window, or
we're taking a picture of a greycard on a beige sedan on a foggy day. There is
plenty of light, but there is only 5 stops of dynamic range in the scene that
we're shooting. I.e. all of the values fall between 2048 and 2080 mV at base
ISO, or ETTR 4063 to 4095mV which get represented by the values 2048-2080 (or
4063-4095). What if, instead, we could change the gain and offset so that
4.063V off the sensor read as 0V on the A/D, and 4.095V off the sensor read as
4095 at the A/D.
My point is that there is always going to be an analog stage between the sensor
and the A/D, and the reasons that make it the way to get higher ISO rather than
shooting at lower ISO and doing the math, would be just as relevant to low
contrast situations.
in most practical situations there's probably enough dynamic range in the RAW
that you might as well do it in post (just expose to the right and bring down
the shadows)
In most practical situations, you could do just fine shooting with a point and
shoot. It's the impractical situations where you need fancy gear like a DSLR
shooting with a fast prime.
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Larry Colen [email protected] sent from i4est
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