Re: [Leica] Technical Question

["Michael Chmilar" <chmilar@mminternet.com>]

Afterswift@aol.com said:

> Does a digital sensor array have a constant -- like film -- or varying 
> sensitivity?
> For example, a sensitivity that would change to reflect various ISO settings.

Here is my understanding of it. If I am wrong, I am sure someone
will come along and correct my mistakes.

Think of the sensor as having three stages:

1)  A photosensor that is struck by light, and generates an
    electrical signal which gets stronger as the light gets
    brighter. The photosensor has a constant sensitivity.

    (Some cameras may have a "stage 0", where some sort of
    mechanical filter is placed in front of the photosensor
    to attenuate the light that strikes the sensor.)

2)  A "gain" stage, where the signal is amplified. The amount
    of gain can be varied. This is where you simulate the
    ISO of film. To increase the ISO value, you increase the
    amount of gain applied to the signal.

3)  A "sample" stage, that reads the signal after gain is
    applied, and outputs the "digital" value for that
    signal strength. This stage has constant sensitivity.

For compactness, the three stages are manufactured together
as one package: the digital sensor that goes into a camera.

Ideally, you want to choose a gain (or ISO) that amplifies your
"brightest" signal in stage 1 to the "largest" number in stage 3,
while your "darkest" signal maps to the "smallest" number. Then,
you have captured all of the information available in the scene.

(Practically, this can be difficult.)

The "quality" of a sensor depends on how cleanly its photosensor
generates values (especially with weak stimulation), how cleanly
it applies gain (especially when the gain is high), and how
accurately it reads the final signal to generate the digital
value.

Film has some analogies:

When you "push" film, you are stimulating the emulsion weakly.
To compensate, you develop longer (or stronger), which is
equivalent to increasing the gain on the film.

If you think about it, the digital sensor is like having one
film emulsion (the photosensor), and using push/pull development
(gain) to change its ISO.

later,
Mike

- --
To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html