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custom white balance/exposure questions ?

 
 
ronviers@gmail.com
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      07-14-2006
I think it only matters if you use the histogram as a gauge for setting
the exposure.

Good luck

 
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ronviers@gmail.com
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      07-15-2006

> If this helps to prevent clipping I do not know.


But what about the test? You said youself that since it was a test
heavy on one channel that it should show results clearly. You are one
tough cookie.

Thanks for the talk,
Ron

 
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Ben Brugman
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      07-15-2006

<(E-Mail Removed)> schreef in bericht
news:(E-Mail Removed) ups.com...
>
>> If this helps to prevent clipping I do not know.

>
> But what about the test? You said youself that since it was a test
> heavy on one channel that it should show results clearly. You are one
> tough cookie.
>
> Thanks for the talk,
> Ron
>


Ok, I did not completely appreciate the procedure you were using.
My tests were aimed at seeing if WB influences the Exposure or
the Raw. Other tests are at the moment not completely clear to me.

I did try to figure out how much influence the WB setting does have
on a Histogram if the histogram is derived from a jpg result.
(It's not unlikely that the camera histogram is derived from a jpg
or something were WB/contrast settings etc are included).

Also I tried to figure out in the pictures that I have on my computer
with single channel clipping if this could be prevented or reduced
by altering the WB.

I do not have a Canon, so I used histograms on the computer.
I know they are similar to the histograms on the camera, but
not exactly the same. Advantadge of the computer is, that it
is easy to play with the WB and other settings. Also can be
switched between a monochroom histogram and a histogram
between three colors. The Histogram probably does vary
between different computer programs and different camera's,
so the conclusions might not be valid for all situations.

If the WB setting does vary between tungsten, sun, and shadow,
the histogram does alter but only very slightly. Only when going to
for example 2000 K for the color temperature there is a severe
alteration in the histogram.

Depending on the situation the histogram can move to the left
and to the right. (Only a little bit, but still a difference).
Going form an average (5500 K) WB to a very low temperature
(2600 K). The histogram moved to the left. Exposure compensating
for this would make clipping worse.
This moving to the left when going to a very low Kelvin was general
with different pictures I tried.
Going from a average (5500 K) WB to a very high temperature
(12000 K), the effect was little. A littlebit of movement to the right,
but in some cases the peak less wide.
I did not realy see situations were I would apply exposure compensation
in this range.

So my carefull conclusion would be. That under normal circumstances
using the correct WB (compared to a fixed white balance of 5500) does
not help to prevent clipping.
Using an extreme WB of 2200 makes the risc of clipping larger.

The channels :
Red, fairly likely to clip the red channel.
Blue, possible, less likely, but for extreme colors blue can clip.
Green, clipping of green as a single channel is less likely.

As said my conclusions are not definitive and might be different
for different camera's.

Thanks for your time and attention.
Ben





 
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ronviers@gmail.com
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      07-15-2006

> I did try to figure out how much influence the WB setting does have
> on a Histogram if the histogram is derived from a jpg result.
> (It's not unlikely that the camera histogram is derived from a jpg
> or something were WB/contrast settings etc are included).


Likely or not, it is irrelevant. All that needed to be shown was that
the post custom wb histogram is different and that the difference is
significant - which my tests show clearly.

> I do not have a Canon, so I used histograms on the computer.
> I know they are similar to the histograms on the camera, but
> not exactly the same. Advantadge of the computer is, that it
> is easy to play with the WB and other settings. Also can be
> switched between a monochroom histogram and a histogram
> between three colors. The Histogram probably does vary
> between different computer programs and different camera's,
> so the conclusions might not be valid for all situations.


Only the camera histogram is relevant because the role of the histogram
for purposes of this discussion is to provide a guide for exposure,
i.e. size of gap between bumps and edge.


> If the WB setting does vary between tungsten, sun, and shadow,
> the histogram does alter but only very slightly. Only when going to
> for example 2000 K for the color temperature there is a severe
> alteration in the histogram.
>
> Depending on the situation the histogram can move to the left
> and to the right. (Only a little bit, but still a difference).
> Going form an average (5500 K) WB to a very low temperature
> (2600 K). The histogram moved to the left. Exposure compensating
> for this would make clipping worse.
> This moving to the left when going to a very low Kelvin was general
> with different pictures I tried.
> Going from a average (5500 K) WB to a very high temperature
> (12000 K), the effect was little. A littlebit of movement to the right,
> but in some cases the peak less wide.
> I did not realy see situations were I would apply exposure compensation
> in this range.


Custom wb always works in your favor (more accurate) because the bumps
on the right side of the histogram gets more attention.

 
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acl
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      07-16-2006

Ben Brugman wrote:
>
> Going for the risk of again misunderstanding.
>
> If nothing clips in the histogram on the camera, it is still posible that
> a single channel is clipped.
>
> Using Raw Shooter Essentials I have pictures which show no clipping
> when using the monochromic histogram. Both sides of the histogram
> display no clipping, there is quite a lot of room left on both sides.
> But examining the color histograms or the picture itself there is
> severe clipping on both sides of the histogram. There are lots op pixels
> in the picture showing 255,0,nn and lot of pixels in the histogram showing
> 255,nnn,0. So clipping while a monochromatic histogram shows no
> clipping is still possible. The exposure of the picture was correct, the WB
> setting
> of the picture was correct. The WB setting does have it's influence on
> the histogram, but not a lot, except in the most extreme wrong settings.
> Al this is done in RSE, but this doe give a similar monochromic histogram
> as my camera does.


Ben,

No you are not misunderstanding. I was thinking of my camera which has
separate channel histograms. Should have specified that. You are of
course right about the single channel histogram.
Cheers.

 
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acl
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      07-16-2006

Ben Brugman wrote:
> Ok reread your original writings. In the first reading I thought you were
> talking about little blue and green lamps. Going from there the
> remaining of the text doesn't make anything clear.
> So reread your writings again and now it makes more sense to me.
>


Little blue and green lamps? Now your post makes sense!

> No I do not make that assumption. What I do know is that it is very
> difficult
> to make things easy to understand in newsgroups. I for myself am not
> very good in making things understandable. So what I often try to do is
> trying to use steps which are clear to the average reader. This makes
> texts more verbose and sometimes I am missing the point alltogether.
>


Indeed. It is difficult to make sure that everybody is working with the
same set of background assumptions.

 
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acl
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      07-16-2006

http://www.velocityreviews.com/forums/(E-Mail Removed) wrote:
> I get lots of single channel clipping, although it may be specific to
> my camera model (Canon 300d), even though the histogram on the camera
> indicates that I should not. It could even by that my specific camera
> is the culprit. But I can correct a lot of the single channel clipping
> in the camera just by doing a custom wb. This is because I use the
> histogram as a way to gauge where to set the exposure. Probably a good
> photographer does not do that. I am not saying that I am modifying the
> raw data only how the histogram is displayed and that the resulting
> display i.e. after custom wb, is less likely to cause me to expose in a
> way that results in single channel clipping.
> I wish you were here and i could show you what I am talking about. It
> would be so easy to show you.


I think I know what the problem is: I am thinking of a camera with
separate histograms for each channel, while you are thinking about a
camera with a single-channel histogram. It's a case of serial
misunderstanding!

I agree with you, I also had the same experience with single-channel
histograms.

 
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picture taker
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      07-31-2006
thank all of you for your replys , i haven't been able to get back to
the group until now.

acl wrote:
> (E-Mail Removed) wrote:
> > I get lots of single channel clipping, although it may be specific to
> > my camera model (Canon 300d), even though the histogram on the camera
> > indicates that I should not. It could even by that my specific camera
> > is the culprit. But I can correct a lot of the single channel clipping
> > in the camera just by doing a custom wb. This is because I use the
> > histogram as a way to gauge where to set the exposure. Probably a good
> > photographer does not do that. I am not saying that I am modifying the
> > raw data only how the histogram is displayed and that the resulting
> > display i.e. after custom wb, is less likely to cause me to expose in a
> > way that results in single channel clipping.
> > I wish you were here and i could show you what I am talking about. It
> > would be so easy to show you.

>
> I think I know what the problem is: I am thinking of a camera with
> separate histograms for each channel, while you are thinking about a
> camera with a single-channel histogram. It's a case of serial
> misunderstanding!
>
> I agree with you, I also had the same experience with single-channel
> histograms.


 
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