Friday, February 15, 2013

View of the Milky Way with Your Naked Eye

Milky Way over The Watchman - Zion National Park (light on mountain from Springdale city) ~ © Royce Bair
Simulated naked eye view
Histogram of a "naked eye" exposure
What the eye sees vs. what the camera sees. When people see my NightScapes for the first time the most common question is, "Is that what the Milky Way really looks like with your naked eye? Can you really see that many stars in the night sky?" The answer is, "Yes and no." You really can see it quite clearly with your naked eye (if you are in an area with very little light pollution), it's just not as bright as I can see with my camera.

The reason is because my camera is manually controlled to take the images at a longer exposure (typically 8 to 30 seconds, depending on the lens I'm using), so it gathers more light for its sensor than my eyes can. Also, the human eye has an iris aperture that is rated at about f/4.0, whereas my typical night photography lens is rated at f/2.8 or wider (letting in twice as much light to the sensor).

Histogram of raw camera exposure
Raw camera exposure
Above, is a simulated view of what my eye saw of the Milky Way over The Watchman in Zion National Park — this is about two stops darker than what my camera recorded (to the right), using an f/2.8 aperture setting, an exposure time of 15 seconds, and an ISO of 6400 or 8000. (Notice that the "toe" of the exposure histogram is just passing the midway point, whereas the toe of the simulated "naked eye" exposure histogram doesn't even come close to the midway point. This exposure is two stops less than what it should be —post production corrections can correct a 1-stop underexposure, but not a 2-stop underexposure. This means that if you only have an f/3.5 or f/4.0 lens; or your camera only goes to ISO 3200, you're still going to get an acceptable exposure, that's within the range of post production correction —provided you shoot in the camera "raw" mode, which typically has 16-bits of information, rather than the camera JPEG mode which carries only 8-bits of information per color channel. Here's a tutorial I created on the benefits of 16-bits raw images vs. 8-bits images.)

Post Production: The raw camera image has all the brightness and detail needed for a great photo, but it is flat or lacking in contrast. My final NightScape images are created in post production with very simple steps in Photoshop. All the stars are there in the camera's raw image, but a contrast gain in the sky is necessary to make them more apparent. My goal is give the same clarity you expect to see in an astronomical observatory (taken with a huge telescope) image, but with a wide-field view, coupled with an interesting landscape feature in the foreground — something the big telescopes cannot do!

After adding curve adj.
"S" shaped adjustment curve
The powerful "S" Curve. Here's the main Photoshop post production step I use to increase the contrast in the sky: I first select the sky, using the Magic Wand tool (I usually have to do a little Laso tool work to get all the bright stars into the selection). I then create a channel of the selection that I can use later. The second and main step is to turn that saved selection into a "Curves" Adjustment Layer that I can add a contrast-increasing "S" shaped adjustment curve. (The shape of this curve is controlled by the two anchor points so that the curve pinches or brightens the large highlight area to the right of the histogram "mountain", and darkens the shadow area to its left.) The more vertical the line becomes between the two anchor points, the more your contrast gain.

After color changes
Slight color changes via Curves
Color Balance. Every evening sky has a difference color to it, depending on the atmospheric conditions for that night. I shoot in the "AVG" (average) color balance mode, which produces somewhat of a neutral coloration to the sky. Some photographers will use a daylight WB (white balance). Others will use the "Tungsten" setting for a deep blue look. Some will play with the color balance in Photoshop's RAW Converter (changing the Kelvin color temperature). I prefer leave the AVG WB close to neutral (usually around 4500ยบ Kelvin) and do minor color changes in Photoshops Curves.

In the adjustment curve, you'll notice that I've also change the color slightly in the red, green, and blue channels (represented by the three diagonal colored lines). I've added a bit more blue than the other colors (mainly in the shadow areas). Be careful to not over do this — as a little color goes a long way. The final coloration depends on what you remember seeing in the sky that night.

Adjusting the landscape. The final step is to use the same saved sky selection (channel) and inverse it so that it is now selecting the mountains, instead of the sky. Once this is done, another Adjustment Layer (you can use "Levels", or "Curves", but Curves is more powerful) is created from that selection in order to change the color of the sodium vapor lights (coming about a mile away from Springdale city) —so that The Watchman mountain is a more pleasing "red rock" sandstone color.

Histogram after all adjustments
Extended tonal range. The purpose of all these adjustments is to not only increase the contrast of the sky, but to extend the tonal range of the final image. Compare the histogram on the right to the histogram of the raw camera  exposure, above. Note: In the final, top image I added just a little "vibrance" to the image. Color vibrance (Image > Adjustments > Vibrance...) is often a better way to add a little pizazz to your images than using the Hue / Saturation adjustment).

Compare: You can use this slideshow viewer to enlarge and quickly compare the differences between the "naked eye" simulation, the "raw camera exposure", the "curve adjustment", and the final image with all the post adjustments.

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12 comments:

  1. Very interesting, thanks for sharing.

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  2. Are you able to do a similar adjustment using the tone curve in Lightroom 4 or is there a way to do this adjustment in Photoshop Elements? Thanks.

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    1. You should be able to do something similar in Lightroom's tone curve (although I'm not as proficient in LR as I am in PS. You can also do curve adjustments in Elements via the Enhance menu, although you cannot set custom points on the curve as you can in the full PS --you can only move their preset highlight and shadow points up and down. Although Elements has adjustment layers (allowing for editing of an adjustment or enhancement), I don't believe "Curves" is one of their options, only levels, Hue/Sat, and a few others. This is why the full PS cost about $700, and elements under $100 :(

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  3. Thanks Royce. I will play with the Lightroom Tone Curve settings. I think the main difference will be that the adjustments will be made to the entire photograph whereas in PS you are adjusting just the relevant part of the photograph.

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  4. Great post! I was out this last weekend shooting some night sky. I typically use Picasa 3 to post process my photos, therefore shooting in RAW is not very beneficial to me. I do have Photoshop, but I don't know it very well. This is a great explanation on how to process these kind of photos. I've got a couple photos that I really didn't like because they were RAW that I will process using this method. Thanks Royce!

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  5. I love how your post title is "View of the Milky Way with Your Naked Eye" but the biggest, most noticeable picture right under the title is a camera/software-enhanced one. You have to scroll farther down and find the tiny one that actually says "Simulated naked eye view." The reason I clicked this post is because I expected the main picture to be what the title says.

    Great blogging. I give you 0 points.

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    1. Wow. This guy must have it rough in life. Must have been confused by all the words.

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  6. I just discovered this blog and I love it!

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  7. Hi Royce,

    The best site on starscapes I have ever come across. is there anyway you can go into more details on some of your other post processing steps? I have found after taking nightscapes for the last two years that the easy part is the picture taking- the settings are actually relatively straightforward. However if you look at work of yourself, Lincoln Harrison's, David Kinghams, Ted Gore etc, it would be very interesting and very educational for them to write an article on step by step post production as that is ultimately the determining factor. Please do this! thanks. T, London, UK

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  8. Thanks for an interesting & informative article.

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  9. Just found this site and I am very impressed! It truly is cool that you are willing to share your talents, I really appreciate it!

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  10. Does the portion of the Milky Way that's visible differ much as you move farther south? All I seem able to see here is about the top half of what you captured over the Watchman... am I just stuck at the wrong latitude? Time of year?

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