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09-18-2015, 02:54 AM   #16
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QuoteOriginally posted by stevebrot Quote
I like your Crocosmia example. In many ways they are my nemesis flower, particularly the lucifer variety. The red is intense and very true and is painfully easy to block up. The below photo is a good example. I shot perhaps 20 exposures of this bunch of blooms, but only got four that were usable and that was shooting in open shade
Yeah it's tricky - but my 'inferior' point-and-shoot seems to produce better results.

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09-18-2015, 05:38 AM   #17
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Yesterday I did a test using the Pentax K-01 to see the actual levels in the wells.
These levels are stored the camera raw file, and they can be read using utilities.

In bright sunlight I set 100 iso, and used the -M 100mm macro lens to photograph first a gray card ( LAB(50,0,0) )
and then a bright red t shirt fabric. I used the macros lens to completely fill the sensor frame with similar values.
Both photos were near sunny 16, 125th and f/11 and the camerea was set to raw+jpg. The camera jpgs were well exposed and the histograms were between 2/4 and 3/4

I then pointed the camera towrd the sun and took a way overexposed photo. That was to saturate the wells for a check of the 100% values.
The K-01 is a 12 bit adc so its 100% well values are Hex16 #FFF0.

Using the bright red photo, it was possible to deduce which wells in the row, column array were red, and following that, which wells were blue and green.

The snapshot of the resulting spreadsheet is below. Note that on the gray card, the red wells are only about half the levels of the green.
Note that the sensor levels for all RGB channnels in both of the well exposed photos are around 8% to 15% of full well capacity.

This is necessary in design so that the camera has 2 or 3 stops (~20dB) of overhead to handle bright spots in the typical images.
(Both of the images I took were about 3.5 stops below saturation, perhaps I could have made slightly higher exposures)

When the camera makes the raster jpg image, it applies the Bayer demosaicing coefficients to the values of the RGB raw well values.
It also applies a gamma correction which boosts the low well values and compresses the high values.
This correction might be 0.45 to comply with ITU standard 709 so the camera jpg image is compatible with various monitors etc.

When the camera displays the histogram, ( I believe) the 3 histograms for the RGB cvhannels are not based on actual sensor well values,
they are based on the resultant pixel levels of the jpg raster image after the coefficients and gamma are applied.

That means, the 3 camera histograms give us an indication of how close to saturation the raster image will be when eventually displayed on the monitor.
The camera histograms do not give a direct indication of how full the sensor wells are.
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