Re: [APML] Fw: images

[Chuck Vaughn <aa6g@aa6g.org>]

Jason,

> I'm not sure how useful it is to talk about bit depth with an analog 
> media, since the amount of exposure is not recorded in digitized steps 
> (or histogram bins) in the silver halide grain.  Are you saying 5-6 
> bits of information after conversion to digital?

Yes. The noise-free range can be represented by 5-6 bits of data.

> Do your numbers account for scanner noise?

Scanner noise is not really an issue unless you're close to the Dmax 
the scanner can handle. I have trouble with scanner noise on Kodachrome 
slides and dense TP negatives.

>  6 bits =  64 levels per channel, which is hard to believe for 
> something like a color photograph.

64 levels per channel would give you 262,144 colors. You can't see the 
difference between that and 16.7 million on a monitor or a print. Try 
it for yourself in PS.

I have noticed that monitors don't have enough dynamic range to see 
anymore than about 128 gray levels. I could see no difference at all 
between 128 and 256 gray levels.

>   It is awfully similar to what practical tests have shown for S/N of 
> film:  A S/N ratio of 4-5 if the element is to be detected against its 
> surround, 8-10 if it is to be recognized.

What units is this in? Probably something I never heard of. :-)

>   I don't know if it's correct to talk about film directly using 
> digital nomenclature...maybe for a scanned image of a film negative, 
> though.  I think this requires some clarification, because even after 
> dealing with this stuff at work I must admit you have me confused.
>
> On the other hand, there is an expression giving the information 
> capacity of film..that is the bits per unit area that can be stored on 
> a photographic layer.  Altman and Zweig (Phot. Sci. Eng. 7:173, 1963) 
> reported levels up to 160x10^6 bits/cm^2 for one set of fairly 
> stringent conditions.

We may have some apples and oranges comparisons going on here. Remember 
I'm talking about astrophotos. On color film the images are mostly very 
thin so that reduces the S/N. They'd be just as thin on TP if we didn't 
develop it in D19 to really enhance the contrast. Again that reduces 
the S/N. TP developed in Technidol is another story.

All I'm saying is that the signal levels we typical deal with on film 
astrophotos can be represented by 5-6 bits when converted to digital.

>  Naturally, an actual film would have a gaussian-type distribution of 
> actual # of grains at a given "bit depth" (due to grain size, etc), 
> but it probably is a good estimate.

I played around with this a little in PS, starting with a 60% gray 
image. I added various amounts guassian noise. This is just an 
observation, but 2 times the standard deviation converted to bits seems 
to give an estimate as to how many bits are noise.

Chuck

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