Archived from groups: alt.comp.periphs.dcameras (
More info?)
"Ed Mullikin" <edmull2@cox.net> writes:
> I've got a Sony DSC-F828 8 meg camera. I'd like to know just what
> happens when I press the shutter button. Everyone knows what
> happens with a film camera. The books that I've read/bought seem to
> start with how to take the shot and how to treat the image to get
> the best results.
> How does the camera perform the film equivalents of aperture,
The aperature in a digicam works exactly in the same way as the
aperture in an AE film camera. The size of a mechnical pupil is
adjusted by a motor to let in the right amount of light through.
> ISO,
ISO used to be the measure of a films sensitivity to light.
The defintion of ISO for film described how much "fogging"
the film should have for a given amount of light at a given
ISO-value.
The sensor in a digital camera doesn't "fog" like film when
hit by light, so the original definition of ISO does not
make sense for digicams.
Instead, the ISO-values given for a digicam is calibrated so that
digicams behave like film cameras with respect to ISO. I.e.: at
a given ISO value, and given the same scene and lighting conditions
- a film camera and a digital camera will use the same aperture and
the same shutter speed to acheive the correct exposure.
For example, when shooting an outdoor scene on a bright sunny day,
setting the aperture to f/16 and the shutter speed equal to the
ISO value will usually give the right exposure. This rule (known
as «sunny sixteen») is equaly valid for digicams as it is for film.
While film come with a fixed ISO value, digicams let you change
the ISO-value - so you can switch to higher ISO when shooting
in low light. This increase in ISO is done by /amplifying/ the
signal from the sensor inside the camera. Amplification has the
un-desired side-effect that noise (showing up in the image as
speckles of discolouring) is amplyfied along with signal.
> shutter speed, etc?
Quality digital cameras have mechanical focal plane shutters just like
their film counterparts. This is the case with all DSLRs, and some
compacts (like the Canon Powershot G5). In that case, the shutter
speed works just like in a film camera (i.e. it is determined by
the size of the slit between the two curtains travelling across
the focal plane).
Some compacts, in particular cheaper models, does not have a mechnical
shutter. Instead they use an electronic shutter. With an electronic
shutter, there are no moving parts. An electronic signal inside the
camera controls when the sensor should start gathering light and when
it should stop, for the purposes of taking a single image. The
time difference between start and stop determines the shutter speed.
> How does the camera take a 5 megapixel shot when it's an 8 meg?
By taking an 8 Mpx shot and then using software to downsample it
to 5 Mpx before storing it on the memory card.
> Any books on the subject?
Not that I know of.
--
- gisle hannemyr [ gisle{at}hannemyr.no - http://folk.uio.no/gisle/ ]
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«To live outside the law, you must be honest.» (Bob Dylan)
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