I briefly mentioned about my frustration of realizing the moonrise feel in the post processed image. Here is characterisation of the problem further and a challenge to you to explore and learn/share more. Let me first share the original unprocessed, resized color image and its histogram/curves.
- Moonrise Unprocessed
- moonrise_boulders_1_challenge.jpg (530.11 KiB) Viewed 2058 times
The main problem is human eye has far more dynamic range (beyond 20 stops) while camera systems of today it is about 14-16 stops at best. Increase of every stop of dynamic range is like doubling the range. So difference between 14 stops vs 20 stops is not 6 times but it is 2 to the power 6 times! That is 64 times more. When I saw the moon at night my visual system easily separates the tones of sky from tones of boulders and tones of moon. In this particular scene boulders and sky intensity levels seem to be too close for comfort (that is how the night scene in general will be). Further these tones fall closer to darker side in the tonal range from black to white.
Let us look at the curves of the above image.
- Moonrise Curve
- moon_challenge_curves.jpg (122.97 KiB) Viewed 2058 times
Note where boulders' pixels end and where sky's pixels starts. This is the root cause of the issue I have (even if I decide not to convert the image to B&W). The two intensity ranges seem to overlap a bit too. This is what camera saw, not my eyes and there is a conflict!! The conflict is difference in intensity levels of boulders and sky look a bit unnatural. How do I separate it to match my mind's tonal curve? Since our vision has far bigger dynamic range we now have the task of mapping larger set to a much smaller set. What this means is multiple values will map on to the same numbers in the smaller set. The essence of post processing in this case is how to cheat the human mind such that feel of the night is still maintained by playing around with where to keep boulders and sky pixels in the histogram. Also, please note intensity levels of moon spreads across a longer range after the tonal range of the sky. They need to be attended to, though it is less of a challenge than handling tones of boulders and sky.
If you want to attempt post processing this please feel free to download the color image, process it and upload your version of the B&W image and its curves for us to enjoy and learn from. First of all you need to ensure your display system is calibrated. Make sure you see well separated distinct 16 different patches of colors/grey in this below image, especially initial darker patches need to be well separated on your screen.
- Tonal Range
- color_band.jpg (24.25 KiB) Viewed 2058 times
Here below is the curve of the above bands.
- Curve of the Tonal Ranges
- band_curve.jpg (117.58 KiB) Viewed 2058 times
Compare this curve to the curve of the moonrise image above. Carefully watch where ranges of boulders/sky start and end. You may note that boulders and sky tones end well before half of the full range, that causes additional challenges. It needs very well calibrated display to manage tones at the darker side of the spectrum.
I hope this gives a glimpse of the challenge related to post processing subtle images made when ambient light is very less and when we have the ambition of matching the earth with sky!!
