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Author Topic: Finally, a raw histogram?  (Read 24149 times)

madmanchan

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Re: Finally, a raw histogram?
« Reply #40 on: December 30, 2010, 01:08:28 pm »

tho_mas, I believe even the Phase backs' LCD histograms would display the red channel as clipping in the example I described, even if the WB is dead-on. (I am reasonably certain of this, at least with a P65+, having tried this specific test myself.) It is true that the raw data itself for the red channel may not be clipped, but the point I am making is that the LCD feedback shows that it is ... thereby making it impossible for the photographer to know in the field whether or not it actually is.

The usual advice is to reduce the exposure until the LCD histograms do not indicate clipping. Unfortunately, this usually then results in an underexposure by a stop or more. In this context, by "underexposure" I mean that the photographer could have increasing exposure (e.g., by increasing the exposure time) by 1 or more stops, without clipping any raw data.
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Eric Chan

tho_mas

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Re: Finally, a raw histogram?
« Reply #41 on: December 30, 2010, 01:14:46 pm »

tho_mas, I believe even the Phase backs' LCD histograms would display the red channel as clipping in the example I described, even if the WB is dead-on. (I am reasonably certain of this, at least with a P65+, having tried this specific test myself.) It is true that the raw data itself for the red channel may not be clipped, but the point I am making is that the LCD feedback shows that it is ... thereby making it impossible for the photographer to know in the field whether or not it actually is.

The usual advice is to reduce the exposure until the LCD histograms do not indicate clipping. Unfortunately, this usually then results in an underexposure by a stop or more. In this context, by "underexposure" I mean that the photographer could have increasing exposure (e.g., by increasing the exposure time) by 1 or more stops, without clipping any raw data.
Eric, I understand all that.
My question is: what do you do afterwards (in post) with your "accurately" exposed capture (i.e. the one that shows clipping only the LCD but not in the raw chanels)? How do you white balance it without introducing clipping?
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fdisilvestro

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Re: Finally, a raw histogram?
« Reply #42 on: December 30, 2010, 02:00:01 pm »

Eric, I understand all that.
My question is: what do you do afterwards (in post) with your "accurately" exposed capture (i.e. the one that shows clipping only the LCD but not in the raw chanels)? How do you white balance it without introducing clipping?

You will have to adjust exposure and/or brigthness. If it is a color outside of output color space then maybe vibrance or saturation
If the raw converter used white balance coefficients less than 1, then there would be no clipping, but blown out areas could get a color cast.

hjulenissen

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Re: Finally, a raw histogram?
« Reply #43 on: December 30, 2010, 02:12:48 pm »

Eric, I understand all that.
My question is: what do you do afterwards (in post) with your "accurately" exposed capture (i.e. the one that shows clipping only the LCD but not in the raw chanels)? How do you white balance it without introducing clipping?
The sensor/camera is the limitation in many aspects. It seems to make sense (from a pedantic, numbers-oriented perpective) to make sure that the sensor works at its optimal, then worry about processing later on.

No matter how good exposure/WB is out in the field, you might (I sure do) want to adjust them later on. Doing curves/levels or pretty much anything else could also alter (reduce) the output channel peaks (even image scaling could in principle have some effect). In that case, you could get something (slightly better SNR) "for free". If not, you could always reduce exposure and have similar results to what you have today.

-h
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bjanes

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Re: Finally, a raw histogram?
« Reply #44 on: December 30, 2010, 02:50:07 pm »

no, sorry... I never look at those DxO charts :-)

Tho_mas, you seem remarkably resistant to reasoning and scientific data, and perhaps you should pay attention to such authoritative sources as DXO and Eric Chan.

The DXO charts show how ISO is handled by some Phase One backs. The P40+ and the P60+ rate the sensor 1 stop below nominal, and apparently make up for this with the tone curve applied by the camera or the raw converter. As Eric stated, if you expose so that the histogram is at clipping, you have effectively underexposed by one f/stop. That gives you highlight headroom and protection from blown highlights in the raw file, but shadow performance will suffer.

The P45+ does not alter the amplifier gain, and handles ISO totally in the raw converter or camera. If you expose at a given f/stop and shutter speed at ISO 100 or ISO 800, the results in the raw file will be the same with this camera. Hasselblad also handles ISO in this fashion. ISO is merely appended as metadata with the Hasselblad backs. With some current dSLRs such as the Nikon D7000, one can either set ISO on the camera or use exposure in the raw converter and expose everything at base ISO. This is possible, since read noise does not vary with ISO on this camera.

I can't replicate those values with my P45 and P21+.
I've just shot a BasICColor Grey card (see attachments).
My meter showed 30''/f4 for ISO50 (60''/f4 for ISO100 on the P21+).
Unlike other grey cards the BasICColor card reflects 25% grey (see attachment).
The respective Lab value is L*60.

Now, these are the shots from the P45 and P21+ without any adjusments made in Capture One (only WB); processed to TIF and opened in Photoshop.
Well, if you look at the Info palette in Photoshop I would say my meter works as supposed to... and so do the digibacks.
I don't use the meter anyway (at least rarely)... I mostly expose short of clipping highlights.

Don't know if this clarifies something with regard to DxO charts... it's simply just the outcome based on light metering and C1 processing...

Your system does appear to give an appropriate pixel value for gray cards. However, you don't know what saturation is obtained by the sensor and how much exposure compensation has been applied by the camera or raw converter. The actual reflectivity of the card does not matter if you meter from the card: you will get the same result for a white card, a gray card, or a nearly black card. The exposure meter merely reads luminance. It can't differentiate a dark card illuminated in strong light from a gray card illuminated by weaker light.

You can either keep your head in the sand or learn :D

Regards,

Bill
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tho_mas

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Re: Finally, a raw histogram?
« Reply #45 on: December 30, 2010, 02:52:38 pm »

I think I should start to shoot flowers or so... I think I have never seen clipping in colors, only in highlights. And when there is clipping in highlights I sure stop down.
If I would discover clipping in colors on the LCD-histogram I would most likely ignore it as I can deal with such a clipping in the Color Editor later on...
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tho_mas

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Re: Finally, a raw histogram?
« Reply #46 on: December 30, 2010, 02:57:45 pm »

The actual reflectivity of the card does not matter if you meter from the card:
I didn't measure the card, I did measure the light (so with the white calotte).
When you use the BC greycard for metering you have to compensate 1/2 stop.

Quote
You can either keep your head in the sand or learn :D
 ;)
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eronald

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Re: Finally, a raw histogram?
« Reply #47 on: December 30, 2010, 03:11:27 pm »

Bill, I think Phase rate their sensors *above* its measured rating ... otherwise clipping would be a cert.

As for ISO, they may not change anything but metadata, but they may include some additional calibration data for use at higher ISO.

Edmund
« Last Edit: December 30, 2010, 03:13:26 pm by eronald »
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joofa

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Re: Finally, a raw histogram?
« Reply #48 on: December 30, 2010, 04:43:20 pm »

So that would mean that for some colors Adobe RGB blue primary will be less than (normalized) 1, but the ProPhoto RGB blue primary is a little more than 1. If a color processing is such that it clips at 1, then it will clip it here.

Joofa

Let me elaborate a little here. The above mentioned applies to standard white points of D50 for ProPhoto RGB and D65 for Adobe RGB, and I think this corresponds to the first image in tho_mas' message above where profiles are shown. However, if the direction of primaries are kept the same, but the white points are changed, which means that the magnitude of unit stimulus changes, then the following situation emerges:

Fraction of unit stimulus blue ProPhoto RGB primary needed to match unit stimulus blue Adobe RGB primary:

(1) Adobe RGB white point=D65, PropPhoto RGB white point=D50, Fraction needed=1.2
(2) Adobe RGB white point=D65, PropPhoto RGB white point=D65, Fraction needed=0.91
(3) Adobe RGB white point=D50, PropPhoto RGB white point=D50, Fraction needed=0.88
(4) Adobe RGB white point=D50, PropPhoto RGB white point=D65, Fraction needed=0.67

So, except (1) other modes offer a scenario where no clipping needs to happen. However, (1) is the standard mode of specification of white points for both Adobe and ProPhoto RGB, and this does seem to clip. I think either (2) or (3) correspond to the second image shown by tho_mas in the above message.

Sincerely,

Joofa
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madmanchan

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Re: Finally, a raw histogram?
« Reply #49 on: January 11, 2011, 10:03:47 am »

Eric, I understand all that.
My question is: what do you do afterwards (in post) with your "accurately" exposed capture (i.e. the one that shows clipping only the LCD but not in the raw chanels)? How do you white balance it without introducing clipping?

Reviving an older thread (was traveling last week and offline ...)

Generally, the image looks too bright on input, so I just reduce the exposure. Since the raw channels were not clipped, no data is lost during this process.

The resulting image will have less noise than had I captured an image with a shorter exposure.
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Eric Chan

Bart_van_der_Wolf

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Re: Finally, a raw histogram?
« Reply #50 on: January 11, 2011, 12:21:39 pm »

Reviving an older thread (was traveling last week and offline ...)

Generally, the image looks too bright on input, so I just reduce the exposure. Since the raw channels were not clipped, no data is lost during this process.

The resulting image will have less noise than had I captured an image with a shorter exposure.

Hi Eric,

So I assume Exposure correction is effectively applied before Whitebalance in ACR, correct?

Cheers,
Bart
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