Here is a great page on digital cameras from Jerry Lodriguss. He points out that our senses actually work more logarithmic than linear. Our eyes are very good at exposing very bright areas against very dark areas, digital cameras are not. This is the reason a lot of digital photographers have gotten into High Dynamic Range photography. If you manipulate the data right, you can end up with a result that is more closely related to what the eye sees, and it can be easily overdone to make a very manipulated looking result. http://astropix.com/HTML/I_ASTROP/HOW.HTM That is after all the idea behind a camera, to pick up light and present it, as we see it. (Obviously CCD's can do this differently by picking up light from specific gasses and then assigning color to those, or wavelengths we are not sensitive to). I'm talking mainly about normal RGB photography meant to represent the world as we see it. Another issue is when you start stacking photos, you end up with way way more data than you actually need. Also, increasing the sensitivity of the chip (gain/ISO) is doing a sort of stretching by mapping less of the signal over the same dynamic range, enhancing the little bit of signal that was captured. Some cameras are made to represent the world as most humans see it. I think they do a good job, but have limits. The camera can over time pick up a much larger dynamic range of data than our eyes can. When you take long exposures of deep space, your collecting photons over time. This data to start with is nothing like we see it as our eyes don't pick up light over 30hz. When I take a landscape photo with my DSLR in raw format, the data is nothing like what I actually see. The image comes out very unsaturated, and very gray looking. This is because I have not let the camera do any auto stretching or color adjusting to the image. The idea here is to gather as much data as possible for manipulation later, so I can do the manipulation to get a result more to my liking. I do agree that there is an artistic license to the processing, but the idea for me is to get as close to what we actually see as possible. This wouldn't be possible without processing, as the camera has picked up a lot of data that needs to be stretched and chopped down a bit to better represent how we would see it. A lot of cameras do this automatically and give you no control at all. I think it is impossible to separate digital photography from artistic license completely. It takes a bit of manipulation to represent data more like we would see it. I don't know for sure if our eyes, were they able to expose like a camera, would blow out the core of M82 and show the faint details, or would not. But, I think that masking and stretching provide a more aesthetically pleasing result. One more point as well. Our eyes have evolved the way they have, and how they work we don't have much control of. They are just one calibration to an infinite combination possible of the electromagnetic spectrum. So, who's to say that calibrations and manipulations we create are not valid? They represent valid data, maybe not exactly how we would see it, but the digital image isn't "creating" anything out of nothing, it is capturing and presenting objects that actually exist, just in a different way. So I guess a definition of what is natural or not comes into play. I think that our ability to create devices that capture this data so we can manipulate it and present it, is a natural thing. Cheers, David Rankin On 3/30/2010 11:16 AM, Tyler Allred wrote:
<Joe Bauman said...
<Magnificent, Patrick! In response to your question, you need to mask and burn etc., to show the core details as well as the faint arms. But<in a way that's cheating. It's like a photo at night with a car's bright headlights shining at the photographer: you can print the negative<to show the general scene, people on the sidewalk, with washed-out blazing headlights or you can darken the whole scene and show the<headlights as sharp round orbs and the people hardly visible. But to show both you have to manipulate the image, "burning in" the headlights<with the enlarger or PhotoShop. That results in an unnatural view of the scene. You shouldn't have it both ways. I feel somewhat the same way<about manipulating astrophotos. The center is magnitudes brighter than the arms and a photo that in effect dims the center isn't a true<report. -- Joe
Hello all. Joe brings up an interesting question, and I want to respond to this issue of whether or not it offers a "true report" if an image is stretched to show the full range of detail. The issue of stretching astronomical images often comes up and I hear the so-called purists argument that you shouldn't manipulate the data because it is somehow "cheating". I will try to debunk that argument now.
Let me start by asking a question...
Question: Why do scientists present data on non-linear plots (such as log-scale plots, probability plots, pie charts, etc.)? Answer... Because the real data often covers a range that is too wide to see and evaluate without manipulating the presentation of the data.
So, is it "cheating" to show data on a log-scale or probability plot? Or for that matter, to plot log-transformed data? I think the answer you would get from scientists is universally a resounding "no". Is the presentation of data in astrophotos really any different? I think not. If you present the data without stretching, then real data is not even visible. That data represents real structures within an object. Is it a true report to allow real structures to remain unseen, simply because the range of data is too large to represent on a linear scale? Again, I think the answer is a resounding "no".
I normally process my images by applying a log-log stretch to the linear data. That means that any value in the mostly linear representation of the data (directly off the chip) has a mathematical transformation applied that allows the full range of values to be better represented in the image. I rarely use the burn tool to alter my images. In fact, I do very little individual pixel manipulation, and most of what I do is removal of defects from hot pixels and dead pixels on the CCD, which I would not characterize as cheating. In my image processing, I try to show the variability in the actual data, and to accentuate the subtle details that allow the true structure of the object to be seen clearly by the viewer. I don't consider that to be cheating. In fact, I see it as quite the opposite... as revealing the true character and structure of the object.
I thought I would offer these thoughts for anyone who is interested. Cheers, Tyler
PS - Don't worry Joe... I am not upset, but rather I appreciated the chance to discuss this issue. :)
________________________________ From: Patrick Wiggins<paw@wirelessbeehive.com> To: utah astronomy utah astronomy listserve <utah-astronomy@mailman.xmission.com> Sent: Wed, March 24, 2010 4:02:38 AM Subject: [Utah-astronomy] M-82
I was between data taking projects tonight so I refocused the scope for the warmer temps that may finally be arriving.
Once finished I shot five 30" test images of M-82 and stacked them.
http://users.wirelessbeehive.com/~paw/temp/M82.JPG
I'm satisfied with the focus and I like the spiral arms and the detail near the core but wish I knew how to keep the core itself from burning out.
patrick _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php Visit the Wiki: http://www.utahastronomy.com
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