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Suk Lee
Post Laureate
Reged: 10/07/03
Posts: 4314
Loc: Pleasanton, CA
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Hi all. I've noticed some photos posted here have their black points set too aggressively, meaning the sky background is too black and fine greys are lost, meaning transitions from light to dark are too abrupt, leading to cartoonish looking pictures.
It's not just a matter of taste - if you're clipping the black point on your pictures, you're actually throwing away some of the photons you've worked so hard to capture.
The reason for setting the black point incorrectly is usually doing it visually on a monitor where the black level hasn't been set correctly. There are two steps to fixing this.
Step 1) set the monitor black point correctly using either Adobe Gamma (a utility that comes with Photoshop) or using a hardware color calibrator. The latter is the best method - they used to be quite expensive but you can get them for around $150 these days. In addition to setting the black point correctly, it'll calibrate your colors correctly so that there's no guesswork - what you see is accurate.
Step 2) use the histogram to take the visual guesswork part out.
On the following picture you can see the histrogram in the upper right. It rises up at the left from "zero", showing that no light greys have been lost. The result is a natural looking picture, with lots of detail in the greys, and natural looking edges to the stars (if you can' see the detail where the left arrow is pointing, your monitor black point is set too low.)
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http://www.siliconvalleyskies.com
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Suk Lee
Post Laureate
Reged: 10/07/03
Posts: 4314
Loc: Pleasanton, CA
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Here's an example where the black has been set too aggressivly. You can see on the histrogram that it doesn't rise from "zero" - at black there's already a bunch of levels.
In the picture you can see that the stars have taken on an unnaturally hard edge, that subtle details (some of the comparison is masked by the 60k JPEG limit we use in the forum) in the horsehead have been lost, and the overall picture has taken on a cartoonish look.
Pay attention to that all important histogram -- it's on of your most important astrophotography tools! -- and you'll be getting all the detail out of your picture that you can.
Cheers,
Suk
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http://www.siliconvalleyskies.com
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ClownFish
Post Laureate
Reged: 04/26/05
Posts: 5562
Loc: Islamabad, Pakistan
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Nice job Suk Lee. Thanks for posting!
CF
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Keep it simple, keep it cheap! Meade LXD75 SN8, Orion Guidescope, Olympus OM-1 and STI Stiletto. Celestron 15x70 Binos and SkyWindow. Learn all about POLAR ALIGNMENT with my Drift Method Tutorial and simulator!!
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Ossi
member
Reged: 11/30/05
Posts: 55
Loc: Groningen, The Netherlands
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I am sorry for saying this, but i think it IS a matter of taste!
It is true what you say about loosing detail, but in IMO that's worth it. 
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Gregk
scholastic sledgehammer
Reged: 11/19/06
Posts: 930
Loc: Gilbert Arizona
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Good Point and Presentation ... But I might add that in very light polluted areas it is quite difficult not to clip the black point to remove unwanted LP...Greg
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www.azspaceblog.com/wordpress
SkyWatccher EQ-6 Goto mount
Celestron ED80 Piggy back with william Optics 66mm APO
Meade LX-50 Piggy Back with william Optics 66mm APO
Celestron ED80 Piggy back with Canon Xt
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