From: Roger N. Clark (change username to rnclark) on
John Sheehy wrote:
> "Roger N. Clark (change username to rnclark)" <username(a)qwest.net> wrote
> in news:4601FEBA.4080801(a)qwest.net:
>
>
>> Well, lets look at this another way. Go to:
>> http://www.clarkvision.com/imagedetail/dynamicrange2
>>
>> 4 bits is DN = 16 in the 0 to 4092 range. In 16-bit
>> data file, that would be 16*16 = 256.
>>
>> Now go to Figure 7 and draw a vertical line at 256 on the
>> horizontal axis. Now note all the data below that line that
>> you cut off. Now go to Figure 8b and draw a vertical line
>> at 4 stops, and note all the data you cut off. Now go to
>> Figure 9D and draw the vertical line at 256 and
>> note all the data you cut off. (Note too how noisy the
>> 8-bit jpeg data are.)
>
> You can't just divide by 16, to drop 4 LSBs. 0 through 15 become 0. You
> have to add 8 first, and then divide by 16 (integer division), then
> multiply by 16, and subtract the 8, to get something similar to what you
> would get if the ADC were actually doing the quantization.

Fair enough, I'll redo the test.

Here is the full set of images:

See figure 9 at:
http://www.clarkvision.com/photoinfo/night.and.low.light.photography

Here is the original raw data converted linearly in IP, scaled by 128:
http://www.clarkvision.com/photoinfo/night.and.low.light.photography/nightscene_linear_JZ3F7340_times128-876px.jpg

Now here is the same data with the bottom 4 bits truncated:
http://www.clarkvision.com/photoinfo/night.and.low.light.photography/nightscene_linear_JZ3F7340-lose-4bits_times128-876px.jpg

Now here is the same data with the bottom 4 bits truncated, doing nearest integer
using your formula. While subjectively it looks a little better, it has still
lost a lot of image detail compared to the full 12-bits:
http://www.clarkvision.com/photoinfo/night.and.low.light.photography/nightscene_linear_JZ3F7340-lose-4bits-nearestint_times128-876px.jpg

You lose quite a bit in my opinion.
It would be a disaster in astrophotography.

Roger

The ADC is
> working with analog noise that dithers the results; you lose that
> benefit" when you quantize data that is already quantized. You won't
> notice the offset when the full range of DNs is high, but for one where a
> small range of DN is used for full scene DR, it is essential. I am
> amazed that you didn't stop and say to yourself, "I must have done
> something wrong" when you saw your quantized image go dark. That's what
> I said to myself, the first time I did it. I looked at the histograms,
> and saw the shift, and realized that an offset is needed unless the
> offset is a very small number relative to the full range of the scene.
>
> In the case of the mkIII image at 14, 12, 11, and 10 bits in another
> post, I used PS' Levels, because it simplifies the process, by doing the
> necessary offset to keep the distribution of tones constant.
>
>
From: Paul Furman on
acl wrote:

> On Mar 25, 6:52 am, Paul Furman <p...@-edgehill.net> wrote:
>
>
>>I finally took a shot where I wished I'd turned off RAW compression on
>>my D200. It was the new moon, shot mid-day almost straight up, kinda
>>hazy at +2 EC just before blowing then darkened in PP to a black sky and
>>the remaining moon detail was pretty badly posterized. I actually got it
>>to look good with a lot of PP work so I can't easily show the problem
>>but I guess that was the cause. A rather unusual situation.
>
>
> That's interesting; I never managed to see any difference between
> compressed and uncompressed raw. Even when I tried to force it (by
> unrealistically extreme processing) I couldn't see it, even by
> subtracting the images in photoshop. Is it easy for you to post this
> somewhere?

Here's a 'bad' curves version, what I got out of the raw converter & the
original:
http://www.edgehill.net/1/?SC=go.php&DIR=Misc/moon/2007-03-22/tech
-the final is up one folder
I'll email the NEF file if you want to tinker, just remove the hyphens
from my email. In the end I did salvage it pretty well just using ACR &
8 bit photoshop.

> From what you say, it sounds like you did some heavy
> processing, did you do it in 16 bits or 8 (I mean after conversion)?
> This sort of extreme adjustment is just about the only place where I
> can see a difference between 8 and 16 bit processing (or 15 bit or
> whatever it is that photoshop actually uses).
>
> On the one hand, I find it hard to believe it's the compression, the
> gaps between the levels that are present are smaller than the
> theoretical photon noise, so basically the extra tonal resolution of
> uncompressed raw just records noise more accurately [and since you
> can't really see shot noise in reasonably high-key areas, that tells
> you it's irrelevant resolution anyway]. On the other hand, who knows?
> Maybe there is some indirect effect.
>
>
From: Roger N. Clark (change username to rnclark) on
teflon wrote:
> Are there any real photographers here?

How's this?
http://www.clarkvision.com/galleries
From: David J Taylor on
teflon wrote:
[]
> 'Dropping LSB's'? 'Quadrant ADC's'? My brain's fallen out.
>
> Are there any real photographers here?

Obviously there are, and ones who wish to have a better understanding of
the equipment used. If you are uncertain about terms, why not ask or look
them up?

David


From: John Sheehy on
"acl" <achilleaslazarides(a)yahoo.co.uk> wrote in
news:1174792863.245906.287730(a)l75g2000hse.googlegroups.com:

> On the one hand, I find it hard to believe it's the compression, the
> gaps between the levels that are present are smaller than the
> theoretical photon noise,

That still posterizes the noise and signal a little bit. You're not
likely to see it with any normal tonal curve; you really need to increase
the contrast quite a bit, and you will see it. For example, I remember
shooting in extreme fog a couple of years ago, where I used +2 EC with my
20D, at ISO 400, and raised the effective blackpoint such that the dark
parts of the Robins approached black. It brought up a bit of noise that
would not normally be seen, with any exposure compensation level, while
black was still anchored at black. Same with taking pictures of things
reflected in glass over a white background, if you try to restore black
in the processing.

> so basically the extra tonal resolution of
> uncompressed raw just records noise more accurately [and since you
> can't really see shot noise in reasonably high-key areas, that tells
> you it's irrelevant resolution anyway]. On the other hand, who knows?
> Maybe there is some indirect effect.

Recording noise better is a good thing, and the same conditions record
signal better as well (and allows the brain and algorithms to separate
them better, as well).

In this particular case, it is only likely to be seen in extreme
blackpointing, or perhaps extreme sharpening.

--

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