The Sleeping Giant's Sea Lion

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Let me explain the question. I've been making a lot of HDR photos, most of them in places where the difference between darker and lighter zones is no to big, and the transition from one zone to other is slight. And I get quite fine HDR photos.

But when I take photos in night, where you have a very big range between lighter zones (lamps) and darker zones; and the transition is strong, I can't get good HDR photos.

I mean, the dark zones close to the light ones, get an ugly glow that can't be removed. Even in the source images that will later compose the HDR image.

Is that just "light nature" or something else?

What do you do in this cases?

EDIT

In these cases, I tried 7 stops with +/- 1 EV., but I still can't have good HDR photos.

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This question I asked may be helpful, but I'm not sure if it's exactly what you want: photo.stackexchange.com/questions/1805/… –  mmr Oct 18 '10 at 2:04
    
Mmmm... In that case you had the luck of not having the most lightened object (moon) close to a very dark zone with details. So, simply editing 2 frames would solve the problem, IMHO. –  tomm89 Oct 18 '10 at 2:15
    
Well, what do your current results look like? Is the glow you're talking about from the high brightness images having spillover (ie, pixels that would be black or dim in faster exposures are accumulating more light from nearby light sources in the slower ones, throwing off the HDR processing?) –  mmr Oct 18 '10 at 2:19
    
...are accumulating more light from nearby light sources in the slower ones... That's what I mean. Look at the example below. goo.gl/dSmO –  tomm89 Oct 18 '10 at 3:11

2 Answers 2

up vote 7 down vote accepted

When it comes to HDR, it is important to understand the true nature of what your are doing. High Dynamic Range images contain a floating-point dynamic range that is nearly infinite. No device that exists is actually truly able to render the full dynamic range offered by a 32bit HDR image. To be able to view your HDR images, you ultimately have to downconvert them to a smaller integer bit depth, such as 16bit or 8bit. When you do so, you are mapping tones from the full range provided by the 32bit floating point image to the limited range of 16bit or 8bit integer images.

Even though a 32bit HDR image can technically contain and represent an immensely wide dynamic range, its beyond visibility on any modern computer screen, and well beyond what any printer is capable of. In cases of extreme contrast, such as photographs with the sun in them that also include deep shadows, or in your case, night photographs that include artificial lighting via lamps and the like, there is simply too much contrast to compress into a 16bit or 8bit integer image or print. If you try to tone map such an image, you'll undoubtedly get things like posterization, harsh edges around highlights, undesirable color shifts, excessive noise in shadows, etc.

There may be some things you can do to improve your images when taking the shot. Providing extra lighting for the shadows is the first thing to try. Flash can be useful for filling in some light into deep shadows. You may need to bring along some extra lighting. Try following the ETTR technique, or Expose to the Right, where you overexpose your shots as much as possible without clipping highlights (or in your case, you may want to clip them a little bit because of the extreme contrast difference). This essentially dedicates more of the sensors dynamic range to capturing shadow detail. You can recover highlights in post-processing, and effectively achieve a more balanced shot. Try combining ETTR with some fill-flash as well to get the best shots possible.

You might also try Exposure Fusion, which is an alternative process to HDR. Expose a few shots, one to capture the bright highlights of street lamps and the like, and a few overexposed shots to capture as much shadow detail as you can. Using a tool like Photomatix, you can "fuse" the exposures together to merge shadow detail into shots that contain highlight detail. The result is similar to HDR, but simpler and often produces more natural results.

Finally, some of the exposure stacking tools for night sky photography might be helpful with images that have had their shadows lightened but display too much noise. Using burst mode of your camera, snap a rapid sequence of shots of the same scene (best done with a tripod/cable release to avoid changes between frames) that covers as much contrast as you possibly can in a single shot (ETTR might help here as well). Recover shadows and fill light with a tool like Lightroom, and merge a sequence of a single scene together in a photo stacking tool designed for stacking astrophotography shots. These tools excel at merging images and eliminating noise in the darker parts.

Note that for almost all of these things, it is important to work in RAW through the whole process if you can. Don't convert to TIFF or DNG, use copies of the original RAW files from your camera. This is particularly important if you take the astrophotography stacking route, as those tools have a variety of pretty advanced algorithms that work directly with bayer array data to produce the most ideal, low-noise results when stacking.

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"High Dynamic Range images contain a floating-point dynamic range that is nearly infinite" To be more precise, 4,294,967,296 is the quantity of colors that an HDR image could have. Awesome! –  tomm89 Oct 18 '10 at 3:18
1  
Well, were not really talking colors, were talking tones. The number of "colors" that are representable by any color model, for 8bit, 16bit, or 32bit float, is about the same. There are two major modes of thought, but science shows that humans can only really see about 2-3 million (by one school of thought) to about 10 million (by another school of thought). On the other hand, the human eye is far more sensitive to different tones. It is not infinitely sensitive, but we are able to differentiate a pretty fine degree of tonal differences. HDR expands tonal range much more than color. –  jrista Oct 18 '10 at 3:26
2  
I think Exposure Fusion are simply trying to avoid any negative connotations of the word HDR. To say Exposure Fusion images aren't HDR, you have to also say 99.99% of the images people produce with Photomatix etc. aren't HDR either. –  Matt Grum Oct 18 '10 at 7:11
1  
@jrista I totally agree that the "enfused" images are LDR not HDR, but so are 99.99% of the images called HDR (given that jpegs can't store genuine HDR images, neither can monitors display them). So since niether are actually HDR it makes no sense to distinguish between them. Both methods take the same input, and produce similar looking output - the fact that no intermediate floating point representation exists is irrelevant. –  Matt Grum Oct 18 '10 at 11:06
1  
I guess another important point to make (Matt already made it, but to be clearer) is that the vast majority of "HDR" images are indeed NOT high dynamic range images...they are simply LDR images that have been tone-mapped from HDR images. That doesn't mitigate the benefits of using the alternative and simpler Exposure Fusion process to create improved LDR images, though. It just means that the final results of any process are all ultimately LDR images that can be viewed on the limited gamut's of computer screens and prints. –  jrista Oct 18 '10 at 17:20

Firstly, good HDR images are very difficult to produce, so don't be disheartened!

Whilst you can never gaurantee to capture the entire range of any scene, 7 stops will do the trick for most scenes you want to capture. However, your problem with multiple exposure night photos stems from the fact that you're often shooting directly into the lightsource, and when overexposing for the shadows you will get a combination of lens flare and sensor bloom (the latter occurs when a pixel goes so far beyond white that the charge 'overflows' and affects the pixels around it).

The only way round this is to either reduce the exposure of your darkest shot and accept more noise or less shadow detail, mask the areas yourself in Photoshop (or equivalant), or use an HDR compositing program that is aware of and can remove sensor bloom (sorry I don't have any recommendations here), or capture the scene in a single exposure.

When doing the latter you could use a flash to fill in the shadows as jrista says, but that will only work in confined spaces. If you want to take photos of larger scenes what you need to be doing is light painting with the flash (or a torch/flashlight). See the following blog post onthe topic:

http://strobist.blogspot.com/2008/01/using-speedlights-with-landscapes.html

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So, some things like lens flare or sensor bloom are just inevitable when you try to capture shadows without fill lights? –  tomm89 Oct 18 '10 at 21:56
    
It depends, bloom tends to be localised, as charge spilling over only affects pixels nearby, so as long as your shadows are not right next to the lightsource it can be managed. Flare is more problematic - it's always present in every image, but usually it is extremely dim compared to the light that makes up the image. However when you increase the incoming light far enough, in order to capture the shadows, it will inevitably become visible, and can affect the whole image, causing a lack of contrast. –  Matt Grum Oct 18 '10 at 22:15

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