[Leica] RE: Analog v. Digital

[jcb at visualimpressions.com (JCB)]

At 01:21 PM 7/14/2004, Douglas Herr wrote:


>Purely out of a desire to be a pain in the rear, I must ask if film 
>capture is also electronic.  What happens to a silver halide crystal when 
>a photon hits it - are electrons displaced to form the latent image?

In much abridged detail:

An electron in a silver halide molecule is bumped up one valence level when 
hit by photons. There it becomes capable of roaming throughout the "grain" 
where this molecule resides. This gives an electron deficit in that 
particular silver halide molecule. Basically, one of its electrons ran 
away. When immersed in a developer, electrons move in from the developer 
and replace the missing electron in the silver ion converting it to a 
silver atom. The roaming electron is used up in another chemical 
combination. The result is that all silver halide molecules that were 
struck by photons (of sufficient power) will develop into a silver atom. It 
takes about four atoms to be a visible (under a microscope) speck.

The average grain size in a fine grain emulsion is one cubic micron. There 
are 20 billion silver halide molecules in each 1 cu micron grain. This 
gives a very wide dynamic range for each grain. Grains are randomly 
dispersed throughout the emulsion.

Each pixel of a good digital sensor is around five square microns. You get 
one pixel of information with a dynamic range of 256. If you want more 
dynamic range, you need pixels of nine or twelve microns each.

Images on film are stunning in the raw. Like Velvia, Kodachrome, etc. Raw 
digital images look like crap. It takes very powerful firmware (in the 
camera interpolators) and software (after the camera) to manufacture a good 
image from the CCD/CMOS A/D converter's output. You never see this. You 
cannot get raw A/D output. All digital images are processed to take care of 
out of balance pixels, missing pixels, etc. The raw output delivered to a 
memory device has been processed, at length, by the camera firmware.

There is no current flow in analog images. No batteries, photo transistors, 
capacitors, A/D converters, etc, required. Just some film, a dark box, good 
lens, f/stop, and shutter. Swish the exposed film around in some chemistry 
when done, and voila! Stunning images like Doug Herr's. Assuming you are 
Doug Herr!

:-)

JB