Archived from groups: alt.comp.periphs.dcameras,rec.photo.digital,rec.photo.equipment.35mm,rec.photo.film+labs,rec.photo.darkroom (
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[Hm, way too many groups, but I can't decide which ones to remove.]
Kibo informs me that Tom Phillips <nospam777@aol.com> stated that:
>pk wrote:
>>
>> Whats wrong with you? You are making this too complicated. And did you
>> really need to cross post this to half a dozen newsgroups?
>
>I do agree this is should not have been crossposted.
>
>> Digital film (senor) sensitivity are equivalent to film.
>
>Incorrect. CCD or CMOS sensors are not equal to film; one is
>not equivalent to then other. There are differenent physics
>and imaging properties that apply to any giving situation.
Yes, but the end result is quite similar for photographic purposes. In
the context of lattitude/dynamic-range (ie; the OP's problem), the only
important difference is that an electronic image sensor has a brick-wall
cut off at the highlight side of the scale, whereas most people are used
to shooting colour negative film, which is somewhat more forgiving of
highlights. OTOH, a lot of that is more to do with it being reversal
film, because people shooting positive colour film (ie; slide film) also
have to watch out for sharply cut-off highlights as, (although, IIRC,
not quite as sharply as an image sensor).
But leaving aside all the technical details, the fact is that a piece of
film & an image sensor are both devices for trapping photons & keeping
track of how /many/ photons they've trapped. They're both subject to the
same laws of physics in that regard, resulting in similar compromises.
>Film has the ability to _accumulate_ light and endure long
>exposures or multiple exposures that can capture detail
>in the dimmest of situations. Reciprocity failure is the
>only factor. Silicon simply can't do this and is limited by
>both exposure latitude and length of exposure.
That's incorrect. CCD & CMOS sensors accumulate electrons in a charge
well in each pixel. They can be, & often are, used for long exposures of
very dim light sources. As a bonus, their equivalent of reciprocity
failure (charge leakage) can be controlled far more easily than for
film. The tradeoff is that they pick up thermal noise, but in for really
long exposures, the sensor will be cryogenically cooled.
>> The higher
>> the ISO the more sensitive the film is to light. So the higher the ISO
>> on a digital camera the more sentive to light the digital sensor is.
>
>Digital sensors have a _nominal_ "speed" at which they
>produce the best quality image. When you alter that speed
>the image quality goes down.
Um. There's a certain amount of truth to that, but it's not quite so cut
& dried as you are implying. The 'inherent' speed (actually sensitivity)
of a CCD/CMOS sensor is a function of a number of design factors, such
as the size of the charge well & photo-sensitised area at each pixel,
how much light is lost to the CFA (if one is used), the accuracy &
signal-to-noise ration of the sense amps, resolution/sensitivity of the
A2D converter(s), whether microlenses are used, the amount of noise
considered acceptable for the sensors intended use, etc. All of those
variables determine the maximum usable sensitivity of a particular
sensor, certainly, but that's equally true of the way a particular film
emulsion is formulated as well.
> Film can be rated at a different
>speeds and still produce quality results because as development
>of film is altered, effective speed also alters. Also, a "slow"
>film can be simply be exposed for a longer time than a faster
>film, and achieve the same results. No loss in image quality.
You don't think that reciprocity failure counts as "loss of quality"? -
I certainly do!
>Film can be exposed for hours. Try that with a digital sensor.
>It simply one of the differences between these two imaging
>mediums.
Well no, it's the difference between standard film & the design goals
for digital cameras sold to the general public. There is no inherent
limitation to semiconductor image sensors that makes it impossible to
design one suitable for multi-hour exposures - there's just not a very
big market for such sensors. (That said, try Googling for information on
the kinds of image sensors used for astronomy - it's fascinating what
you can do with a CCD if you're willing to spend enough money.)
--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
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