Re: [APML] Fw: images

[Chuck Vaughn <aa6g@aa6g.org>]

Carlos,

> Perhaps not, but I do if we are talking of 16bit and 24bit (3 
> channels)... and I'm sure I can see the steps in a gray 6bits 
> gradient. As I said, we need at least a little over 7bits to see a 
> continuom

I agree but this doesn't have anything to do with how many bits it 
takes to represent a typical astrophoto. :-)

> >> For all practical ways a photograph of a continuom gray gradient
> >> indeed looks continuos... and the human eye needs a little more of
> >> 7bits to see that.
> >That's probably true in viewing the real world but not a monitor or a
> >print.
> ? :-S
> I don't understand what you are saying...

I'm saying the human eye can see a much greater brightness range than a 
monitor or print can display. I think we agree here.

> >Perhaps, but compare my 10 bit image example to any astrophoto.
> Ok. Here we are talking about diferent matters. Astrophotos make use 
> of other zones in the dinamical range, and usually the information we 
> want (don't think on the stars, they cover the whole Drange) is 
> located in very few levels.

Ahhh!! You need to look at my raw image. I posted it because it has 
plenty of data right down at the bottom where noise would be found.

>  I recognice that indeed if we count how many levels we use from the 
> original scan it could be 2^6 per channel (in a 16bits file, scanned 
> at 10bits). Anyway, it doens't mean that film has a bit deph of 6bits.

For some reason I can't make myself clear. I'll try again. I never said 
(or at least meant) film in general can't represent more than 6 bits. I 
said astrophotos on film can't.

> I prefer to talk about bit deph just as a matter of precition... 
> Imagine that we have nice curves controls in the scanning process, and 
> that we can scan the original and obtain the directly "final" result. 
> How many grays do we have? Think that the colors are represented in 
> the 0 - 1 range, with the 16 bits as precition. I think that we'll 
> have more than 64 grays. Perhaps not 2^16, becouse the original data 
> isn't stored strictically as a continuom in the film, but indeed more 
> than 64.

I think you're overestimating how many gray levels are necessary for a 
nice looking image. I'd be really happy if ever one of my TP images had 
64 noise-free gray levels.

> >> PS: Photoshop is a nasty program... as Vicent said, it destroys the
> >> data. I don't trust the ways it works. Just make a gaussian blur,
> >> you'll see steps in the gradients.
> >I tried it. I made a continuous gradient from black to white and
> >performed a guassian blur. No steps here. It is very smooth at any
> >pixel setting of the blur.
> Use large radii to blur a astronomical image, just as if you were 
> creating an AV mask with that procedure.

Now I see it but only with the guassian blur, not with a continuous 
gradient. Those steps are only one level. You should not be able to see 
that. I doubt it's a PS problem. You must be right that it is a monitor 
problem. I wish I could try it on a digital video LCD display.

Chuck

_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo