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Author Topic: Today's DSLR should have another exposure mode  (Read 26338 times)
PeterLange
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« Reply #80 on: June 11, 2006, 05:27:57 PM »
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I wouldn't say this is the final image. There's a slight blocking of shadows in the foreground which can be avoided. It's a good image to play around with though. I'm not keen on cropping it but I wish I could soften the abruptness of the blown part of the sky.
Ray,

In my eyes thatís an excellent rendition of this image.

And Iím glad that we could approach our views during discussion.

So long! Peter

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Ray
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« Reply #81 on: June 12, 2006, 07:50:39 AM »
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Thanks, Peter for sharing some Photoshop techniques. I actually like this photo despite the flaw of a large blown highlight. For me, of course, there's a background experience that I'm aware of, but the viewer is unlikely to be aware of.

The scene is in far North Queensland (Australia), in a World Heritage area, and the mist is rising from the Daintree river, barely visible on the extreme right of the shot. I was driving along an unidentified road going to a mystery destination in what I thought was a national park. I was therefore surprised to come across a couple of grazing cows.

I learned later that the road led to the farm house of a dairy farm that existed before the area had been declared World Heritage.
« Last Edit: June 12, 2006, 07:52:55 AM by Ray » Logged
Cyril
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« Reply #82 on: July 07, 2006, 07:28:33 AM »
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Interesting discussion all around.

For those using dcraw/ufraw, you might be interested by this webpage I put up a while ago:
Highlight recovery with ufraw

I find that I can recover quite a lot of information from the highlights this way, without getting those weird colour casts that tend to appear usually.

Regards,
Cyril
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Guillermo Luijk
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« Reply #83 on: May 16, 2007, 06:46:51 AM »
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But what about the centre of the sun? That white spot is bigger than a mere specral highlight. Whatever the setting in ACR, it's 255,255,255, even with -4EC, which is okay by me. I'd expect the centre of the sun to be a blown highlight, but I was curious as to what a linear conversion would reveal and was very surprised to find that even that centre white spot does not seem to be blown. It's just a neutral white. The image appears to be actually underexposed by about 1/4 of a stop. That's close enough for me†  .

[attachment=601:attachment]
[{POST_SNAPBACK}][/a]

Old but interesting post. I wonder if the sun in Ray's linear histogram is really not blown in any channel. The histograms look blown to me (see that little peak right in the end of the histogram line), but this does not happens in the end of the histogram as level values seem to have been corrected afterwards by some scaling (probably WB).
This happens sometimes if a RAW developer with less than 1.0 multipliers for the WB is used (like DCRAW for instance).
This is a sample of histogram with green channel clearly blown but using -H 0 (no-clip): WB multipliers in DCRAW {R,G,B}={0.586287, 0.421032, 1.000000}:




Another issue: did you know that scaling from 12-bit RAW to 16-bit in DCRAW uses a slightly greater than 16.0 multiplier?. Maybe this happens only on Canon cameras, but Dave Coffin confirmed this to me as I saw 16-bit histograms with peaks not equally spaced in 16 levels but a bit more (~17 let's say). That means (at least Canon cameras) don't make use of the full 12-bit: 0..4095 range.

If you want a tool to analyse detailed 15-bit histograms find it here:
[a href=\"http://perso.wanadoo.es/gluijk/soft/histo.htm]http://perso.wanadoo.es/gluijk/soft/histo.htm[/url]
« Last Edit: May 16, 2007, 08:14:56 AM by GLuijk » Logged

bjanes
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« Reply #84 on: May 16, 2007, 09:48:58 AM »
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Old but interesting post. I wonder if the sun in Ray's linear histogram is really not blown in any channel. The histograms look blown to me (see that little peak right in the end of the histogram line), but this does not happens in the end of the histogram as level values seem to have been corrected afterwards by some scaling (probably WB).
This happens sometimes if a RAW developer with less than 1.0 multipliers for the WB is used (like DCRAW for instance).


Another issue: did you know that scaling from 12-bit RAW to 16-bit in DCRAW uses a slightly greater than 16.0 multiplier?. Maybe this happens only on Canon cameras, but Dave Coffin confirmed this to me as I saw 16-bit histograms with peaks not equally spaced in 16 levels but a bit more (~17 let's say). That means (at least Canon cameras) don't make use of the full 12-bit: 0..4095 range.

[{POST_SNAPBACK}][/a]

For practical reasons, most digital cameras do not make use of the full 12 bit (0..4095) range. The pixel of a digital sensor can be likened to a bucket (or well) that collects electrons. When the pixel has collected all the electrons it can hold, this is called [a href=\"http://www.photomet.com/library/library_encyclopedia/library_enc_fwcapacity.php]full well[/url]. The output of a CCD sensor is voltage, which is amplified and presented to the analog to digital converter (ADC). A 12 bit ADC has possible outputs of 0..4095. This output is in units of analog to digital units (ADUs) and this represents the raw pixel value. The amplification is chosen so that at the full well of the sensor, the output of the ADC is at or near its maximum as explained in the reference. In practice, it may be difficult to obtain an exact match, so a little leeway may be allowed and the output of the ADC may not quite 4095. In addition, the sensor response may not be linear near full well, and only the linear portion of the sensor may be mapped to the ADC output.

To test the range of the ADC that is used, one can make a series of exposures until the sensor is saturated and the ADU value no longer increases. DCRaw is often used to examine the contents of the raw file, but I have found that Iris is more convenient, since it has a graphical interface and also has tools to analyze the resulting conversion. To use Iris, one loads the raw file and performs a demosaicing operation, which renders the raw file into an RGB 12 bit form. No white balance is applied, and one can examine the values in the raw channels by placing the cursor over the area of interest. If desired, a white balance can be applied with multipliers. The file can be converted to the Photoshop 15 bit format (0..32,768) by multiplying the channels by 8 or to a conventional 16 bit file by multiplying by 16.

With my Nikon D200 (which uses a CCD), the ADU output at saturation is about 4009, rather than 4095. In the case of Ray's sun disk, the sensor is most likely fully saturated and there is no "underexposure". Canon cameras use CMOS rather than CCD and the pixel outputs a pixel value directly than presenting a voltage to an ADC, but the principle is similar.

Bill
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