Re: [APML] Fw: images

[Chuck Vaughn <aa6g@aa6g.org>]

Rick,

> You are "confusing" the innate recording bit level of film with its 
> useful
> range needed for recording what you are interested in.
>
> The bit level of film, refers to its ability to record.

You are confusing bit depth with dynamic range. The two have absolutely 
nothing to do with each other.

> What you are
> showing here is how many bits of detail there are in the "desired" 
> part of
> the image.

I'm not sure what you mean by "desired."

>   Now its true, that with clever settings of the scanner, you
> probably could have gotten this detail out of the negative with only 
> an 8
> bit scanner, but to say that film can record detail to only 4-5 bits 
> is not
> correct.

I'll concede this applies to astrophotos and maybe not entirely to 
regular daylight shots on color film. It may be a little better in the 
latter case.

> There is "unwanted" (for your application) detail in the part of the 
> scan
> that you threw away when you contrast stretched.

What does "unwanted" mean? I don't throw any of the image.

> A quantitative measure of
> films bit recording capability would be to measure a gray scale from 
> dmin
> to dmax, and do a similar kind of test.

How many steps do you propose to be in the grayscale?

> Try your test again, but set your scanner so that dmin is < full scale
> counts, and dmax is > 0 counts.  Limit the number of bits in the scan, 
> and
> see how many bits you need.  I suspect it's 11-13.

It seems like what you're saying here is that dynamic range implies 
some bit depth. It does not. One bit can represent Dmax and Dmin, 
1=white, 0= black.

I'm not sure I can say this correctly but I'll take a stab at it. What 
you want to determine is the number of bits it takes to just resolve 
noise in the strongest unsaturated signal the media can record. This 
would be the bit depth required to record the strongest signal to the 
noise.

> Also don't forget that 16 bit files of CCD record linear counts.  The 
> 8 bit
> files of photoshop use a gamma of 2.2, so it can store a large dynamic
> range.

The implication of what you said is that the S/N is different depending 
on whether the data is stored as linear or log. Of course this is not 
correct. The only difference between linear and log is that the 
absolute amplitudes are compressed, i.e. the steps are smaller. The S/N 
remains unchanged and so does the bit depth required to represent that 
S/N.

Chuck

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