Velocity Reviews > low light

# low light

Lionel
Guest
Posts: n/a

 03-25-2007
On Sat, 24 Mar 2007 14:00:08 GMT, John Sheehy <(E-Mail Removed)> wrote:

>"Roger N. Clark (change username to rnclark)" <(E-Mail Removed)> wrote
>in news:(E-Mail Removed):
>
>
>> 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.

Of course you can.

> 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.

What a complete load of crap. Have you *ever* worked with ADC's in
It sounds like you might be confusing 2 quadrant ADC's that're used
this sort of device.

--
W "Some people are alive only because it is illegal to kill them."
. | ,. w ,
\|/ \|/ Perna condita delenda est
---^----^---------------------------------------------------------------

teflon
Guest
Posts: n/a

 03-26-2007
On 25/3/07 17:00, in article http://www.velocityreviews.com/forums/(E-Mail Removed),
"Lionel" <(E-Mail Removed)> wrote:

> On Sat, 24 Mar 2007 14:00:08 GMT, John Sheehy <(E-Mail Removed)> wrote:
>
>> "Roger N. Clark (change username to rnclark)" <(E-Mail Removed)> wrote
>> in news:(E-Mail Removed):
>>
>>
>>> 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.

>
> Of course you can.
>
>> 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.

>
> What a complete load of crap. Have you *ever* worked with ADC's in
> It sounds like you might be confusing 2 quadrant ADC's that're used
> this sort of device.

Are there any real photographers here?

Paul Furman
Guest
Posts: n/a

 03-26-2007
acl wrote:

> On Mar 25, 6:52 am, Paul Furman <(E-Mail Removed)> 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...007-03-22/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.
>
>

Roger N. Clark (change username to rnclark)
Guest
Posts: n/a

 03-26-2007
teflon wrote:
> Are there any real photographers here?

How's this?
http://www.clarkvision.com/galleries

David J Taylor
Guest
Posts: n/a

 03-26-2007
teflon wrote:
[]
>
> 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

Lionel
Guest
Posts: n/a

 03-26-2007
On Mon, 26 Mar 2007 07:35:49 GMT, "David J Taylor"
<(E-Mail Removed)-this-bit.nor-this-part.co.uk> wrote:

>teflon wrote:
>[]
>>
>> 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?

And if he doesn't care about the topic, nobody's forcing him to read

--
W "Some people are alive only because it is illegal to kill them."
. | ,. w ,
\|/ \|/ Perna condita delenda est
---^----^---------------------------------------------------------------

John Sheehy
Guest
Posts: n/a

 03-26-2007
"acl" <(E-Mail Removed)> wrote in
news:(E-Mail Removed) oups.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.

--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <(E-Mail Removed)>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><

acl
Guest
Posts: n/a

 03-26-2007
On Mar 27, 2:39 am, John Sheehy <(E-Mail Removed)> wrote:
> "acl" <(E-Mail Removed)> wrote innews:(E-Mail Removed) groups.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.

Well yes, that is what I was thinking too (ie that posterising the
noise could cause problems under extreme adjustments), but didn't
actually see anything the couple of times I tried (by shooting in forg
and moving the black and white points). I also played a bit with
compressed and uncompressed raw files but could not see anything so
far. Maybe I was not extreme enough.

>
> > 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.

Yes, obviously you'd expect to see a difference under conditions that
exaggerate small differences, ie extreme tonal stretching or
sharpening (which is local tonal manipulation, after all). But I
didn't.

Well I'll try to play with Paul's example and see what happens
(unfortunately I just remembered I have an early plane to catch
tomorrow so it'll have to wait a bit).