Re: [APML] Equivalent ISO of an ST10E CCD

[Jerry Lodriguss <jml@astropix.com>]

Loke,

One second to eight seconds is three stops.  ISO 1600 to ISO 400 is two stops.

I don't get your example.

If you example is true, it's like saying I can underexpose the gray card by 
two stops on film, and that makes the film speed two stops faster, but then 
the middle gray is down in the toe of the film where it's noisier, and not 
middle gray anymore.

The gray card should have one level only, so how does the "light from the 
gray card now fills the 4-8 bottom bits of the well"?

Are you really telling me that NO ONE has ever hooked an ST-10E up to a 
camera lens and made exposures of a gray card in sunshine?

Jerry


At 10:15 PM 4/5/2002 -0800, you wrote:
>Jerry,
>
>I have already answered your main question in a separate response but
>here goes with your gray card example.
>
>Expose the gray card for 1 sec. You get all the information on the gray
>card but electrons fill only the bottom of the CCD well. Lets say the
>bottom 1-5 bits of dynamic range. You are approximately 1 bit above the
>noise level. This is ASA 1600.
>
>Expose the gray card for 8 secs. Because of the linearity of the CCD,
>now the well is filled 8X more. Or log2(8) is 3 bits higher. Then the
>light from the gray card now fills the 4-8 bottom bits of the well
>(there are 12 bits). Now the lowest light intensity of the gray card is
>sitting 4 bits above the noise level as such the SNR is higher but the
>speed is slower, maybe more like ASA 400.
>
>The example is simplistic and the quantitative numbers used only makes
>sense relative to each other. I am assuming noise is constant with time.
>I reality it is not but is very close to constant especially for 1-8
>secs.
>
>Loke
>
>
>Jerry Lodriguss wrote:
>
> >
> > If I hook up an ST10E to a camera lens and take an exposure of a gray card
> > in the sunshine, what exposure would I need in terms of exposure length at
> > a given f/stop or correctly record the gray card?
> >
> > Simple, see? <G>
> >
> > Jerry
> > Photoshop for Astrophotography Book:
> > http://www.astropix.com/APBOOK/0_PROMO/PROMO.HTM
> >
> > Astrophotography, Tips and Techniques
> > for Digital Enhancement in Photoshop:
> > http://www.astropix.com
> >
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Photoshop for Astrophotography Book:
http://www.astropix.com/APBOOK/0_PROMO/PROMO.HTM

Astrophotography, Tips and Techniques
for Digital Enhancement in Photoshop:
http://www.astropix.com

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