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Capture NX Dynamic Range |
Question: When you start using NX Adjustments, like Contrast, Brightness,
etc, do you have full access to the entire dynamic range (12 or so EV)
present in a NEF?
Answer: No! The answer is surprising, and maybe one day
Capture NX will allow it, but as of now (NX 1.3.2), the
answer is a surprising no, you do not, and a simple test (below)
will prove this to you.
The details: Instead, Adjustments only have access to the
dynamic range of the RGB color space of the photo you are editing
-- a photo which was derived from the NEF. The upper end
of the dynamic range is truncated (lost).
Capture NX by default truncates the upper EV range
from a NEF.
Why is this? The NEF has overexposure headroom built into it.
This means that the dynamic range of a well exposed photo inside a
NEF is less than the NEF dynamic range. The only Adjustment that can
access this 'overexposure' dynamic range information is 'Exposure
Compensation' under 'RAW Adjustments' (details below).
NEF to RGB process: The process whereby Capture NX creates a RGB photo
for you to edit and apply Adjustments to is illustrated to the upper right.
The steps are:
- First, White Balance corrections
can end up changing reds and blues (and sometimes green) in the NEF linear color space.
- Next, Exposure Compensation
selects which part of the NEF dynamic range will be used to create the RGB photo
to edit.
- Optionally, there is Active D-Lighting and other RAW Adjustments, which for
purposes of illustrating this point, don't matter.
- Finally, a RGB photo is generated from the values in the NEF as follows:
But the problem is that RGB pixel values can only range from 0 to 255, which means
some of the upper EV in the NEF is lost. It is truncated, and gone.
Testing upper end EV truncation:
A very simple test will clearly illustrate the truncation
of the upper EV end of the dynamic range. Take a picture inside a house towards a
window, with proper expsoure for inside the house. The result is a 'blown out'
window. Then edit in Capture NX and 'View' / 'Show Lost Hightlights' (everything
non-black is 'blown out' or has a maximum/255 pixel value). No amount of non-RAW
Adjustments can recover the detail in the blown hilight.
Now change the Exposure Compensation under 'RAW Adjustments' to -2EV and you
can clearly see that some of the lost highlights have been "recovered". Clearly
the information is present in the NEF, since it was 'recovered', but was not
available to you for editing via Adjustments prior to the exposure adjustment.
Only 'RAW Adjustments' access the full NEF dynamic range:
Even within 'Base Adjustments', the only Adjustments which have access to the
full NEF dynamic range are located under 'RAW Adjustments'. This means that all
of the other non-RAW Adjustments under 'Base Adjustments' are working on the derived RGB
photo (and its smaller dynamic range), and not the NEF (and its larger dynamic range).
The Solution:
Adjust exposure down a little (as needed) in RAW Adjustments, using 'View /
Show Lost Highlights' to
assist you in seeing how much is lost.
The lesson learned: Before making any edits to your NEF in Capture NX,
first go to 'RAW Adjustments' under the 'Base Adjustments' and
adjust/correct (if needed):
This will maximize the quality of the dynamic range in
the resulting photo that Capture NX allows you to edit.
If you ignore this advice and adjust exposure or white balance via
non-RAW Adjustments, you will impact the quality of photo. Just
how much depends on the type of adjustments you make.
Even more information ... about the internal workings of Capture NX 1.3.2:
- Internal 0-255 RGB value clipping: When performing adjustments on RGB pixel
values, any result lower than zero is set to zero, and any result greater than
255 is set to 255. This is clearly seen if you take a +4 EV test photo's NEF,
apply a Contrast Adjustment (of +98), and then try to recover with a
negative Contrast Adjustment. All you end up with is gray. Proving the
first contrast adjustment clipped pixel information (it would have been restored
if there was no clipping).
- Fast vs Slow Algorithms: I was playing around with
Capture NX making adjustments, resulting in nearly a black image, walked
away, and came back later to find the black had turned to white. Very
unexpected.
After much investigation, I discovered that NX apparently attempts to provide
instant image feedback (apparently using low quality algorithms or bit
depth for speed), but then behind the scenes, uses higher quality algorithms
(or bit depth) to calculate the 'final' image, which is then displayed,
possibly much later, possibly even after the exclamation mark
in the title bar has gone away. If the RGB histogram contains 'bars' and is
not a smooth curve, NX has not finished updating the photo behind the scenes.
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