Trying to understand Vuescan exposure settings. Can any expert confirm?

[Pedro Gomez]

Hi Everybody,

I'm trying to get a better understanding of how Vuescan works and I'd
like to ask a few questions to the experts around. My questions
concern the meaning of the exposure and WP/BP settings:

I understand that exposure clipping is used to determine the maximum
exposure time that will allow just % of the pixels to overexpose,
unless lock exposure is checked, in which case the exposure clipping
setting doesn't have any effect. This way, an exposure clipping of
0.1% would be used so that the exposure determined by Vuescan would
clip only 0.1% of the pixels in the crop. Is this correct?

I understand as well that Black Point % and White Poing % are used to
internally determine the WP & BP of the histogram unless lock image
color is checked, in which case Vuescan uses the (internal) WP & BP of
the last (locked) scan. Correct?

I assume that this internal WP/BP are then used by the color balance
algorithms. My next question is: Can BP/WP settings alter the color
balance when the Neutral CB algorithm is used? I presume that this
would be the case with White Balance algorithm, but I'm not sure about
Neutral. How does the combination of BP %, WP % and CB algorithm work?

All this would mean, if I understood correctly, that exposure clipping
is a pre-scan setting(i.e has no effect on a previewed/scanned image
unless we preview/scan again, along with RGB & IR exposure) while BP
%, WP % and Color Balance algorithm are post-scan settings (they do
have effects on a previewed/scanned image). Is this right? I assume as
well that the histogram and its BP & WP (internal, not %) are
determined only taking into account the pixels from the part of the
image which is cropped, correct?

Regards
Pedro.

 

[Ed Hamrick]

I understand that exposure clipping is used to determine the maximum
exposure time that will allow just % of the pixels to overexpose,
unless lock exposure is checked, in which case the exposure clipping
setting doesn't have any effect. This way, an exposure clipping of
0.1% would be used so that the exposure determined by Vuescan would
clip only 0.1% of the pixels in the crop. Is this correct?

That's correct.

I understand as well that Black Point % and White Poing % are used to
internally determine the WP & BP of the histogram unless lock image
color is checked, in which case Vuescan uses the (internal) WP & BP of
the last (locked) scan. Correct?

That's correct.

I assume that this internal WP/BP are then used by the color balance
algorithms. My next question is: Can BP/WP settings alter the color
balance when the Neutral CB algorithm is used? I presume that this
would be the case with White Balance algorithm, but I'm not sure about
Neutral. How does the combination of BP %, WP % and CB algorithm work?

BP/WP primarily affect the clipping of the darkest and brightest
pixels - they have nothing to do with color balance.

All this would mean, if I understood correctly, that exposure clipping
is a pre-scan setting(i.e has no effect on a previewed/scanned image
unless we preview/scan again, along with RGB & IR exposure) while BP
%, WP % and Color Balance algorithm are post-scan settings (they do
have effects on a previewed/scanned image). Is this right?

That's correct.

I assume as
well that the histogram and its BP & WP (internal, not %) are
determined only taking into account the pixels from the part of the
image which is cropped, correct?

That's correct.

Regards,
Ed Hamrick

 

[Bart van der Wolf]

That's correct.

As I understood from an earlier explanation, you then additionally reduce
the actual exposure by 5%. Is that still the case or has something changed?

Bart

 

[Ed Hamrick]

As I understood from an earlier explanation, you then additionally reduce
the actual exposure by 5%. Is that still the case or has something

changed?

Yes, that's correct. This provides a bit of safety margin.

Regards,
Ed Hamrick

 

[Bart van der Wolf]

changed?

Yes, that's correct. This provides a bit of safety margin.

Which makes a lot of sense, and I do like the ability to tweak the initial
exposure clipping %. This allows to finetune just about any type of image,
per image.

Bart

 

[Pedro Gomez]

Thanks Ed, I have a question, though

BP/WP primarily affect the clipping of the darkest and brightest
pixels - they have nothing to do with color balance.

I see, but I presume that it would change the way the color balance
algorithm behaves afterwards, wouldn't it? What I mean is that if I my
brightest pixel is, say (255, 250, 245) and this is clipped due to the
WP % to say (255, 255, 255) and I choose a White Balance algorithm
then this choice wouldn't have a very big effect on the overall color
balance of the image, since we already have a pure white. However, if
there is no clipping at all I assume that the WB algorithm would set
(255, 250, 245) as (255, 255, 255) and correct the rest of the colors
correspondingly, thus changing the overall color balance. Could you
please confirm me whether I'm right or wrong?, maybe I don't
understand the way clipping and color balancing occurs.

(Incidentally, If that is indeed the case, what would Neutral Balance
algorithm do?)

Best regards
Pedro.

 

[Ed Hamrick]

I see, but I presume that it would change the way the color balance
algorithm behaves afterwards, wouldn't it? What I mean is that if I my
brightest pixel is, say (255, 250, 245) and this is clipped due to the
WP % to say (255, 255, 255) and I choose a White Balance algorithm
then this choice wouldn't have a very big effect on the overall color
balance of the image, since we already have a pure white.

The color balance algorithm determines the ratio of the three colors
prior to the application of the black/white points.

You can easily confirm this by using white balance with varying
values for white point.

Regards,
Ed Hamrick

 

[Pedro Gomez]

Ok, now I understand.

Thanks for the response
Pedro.