Digital negative

Started Feb 1, 2013 | Discussions thread
sherwoodpete
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Re: Digital negative
In reply to rpenmanparker, Feb 2, 2013

rpenmanparker wrote:

This idea reminds me of a perpetual motion machine. You can't fool thermodynamics, and you can't fool sensors either. Negative film clips highlights (which look like shadows on the film) just the same as positive film or sensors do.

Not exactly. Slide (transparency) film clips highlights. The image in the highlights contain no detail whatsoever, because the film is totally transparent. With negative film that's what happens in the shadows. The deepest shadow on a film negative is totally transparent. But the highlights of a negative don't experience clipping. Instead, the response of the film gradually rolls off and additional exposure has less and less effect.

But the key word is clip. In a digital image that is directly analogous to clipping in a digital audio signal.

Clipping of an audio signal. Top, pure sine wave. Bottom, clipped signal

The diagram shows what happens to a digital audio signal when clipping occurs. The result is harsh distortion.

Analogue systems, such as magnetic audio tape, or or photographic film, tend to deal with an excessively large signal in a more gradual way. Too much signal is still a problem, but the results tend to be more gradually rounded off, rather than going instantly from pure and clean, to harsh distortion.

The whole question is do you have too much light in the highlights and/or too little light in th shadows. If either of those are the case, it doesn't matter whether you record the light as bright or dark, the loss of detail will still be the same.

Not exactly. In one case the signal itself is strong, but is clipped (information is simply discarded). In the other, the signal is weak and gradually gets lost against background noise (not enough information).

Regards,
Peter

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