Saturday, July 9, 2011

The Velvet HDR Underground. HDR, DRI, LDR, Black Card, Layer Masking compared.


Figure A: Kitschy yet iconic at the same time!
Although one of the catch words these days in photography is HDR (High Dynamic Range), the problem with it is it starts to look like cheap velvet paintings of the 70's (actually all velvet paintings look cheap). In case there are those out there that have no idea what a velvet painting looks like, here's figure A.

Regardless, HDR was invented in the 50's for a very important and specific reason which was to image difficult to visualize subjects like what it looked like in the heart of an atomic explosion with the entire image of the explosion intact. It has other applications, specifically one of those reasons is to simulate what the human eye sees and not what a camera sensor or even film sees.

The human eye is actually quite a sophisticated imaging device. It has the ability to see well in the dark while at the same time it also sees what's in the light. This is all simultaneously and mostly instantly. The problem with most photographs is that the medium isn't able to render the same range as what the human eye can see. The big misconception by many is that the camera sensor or film cannot see more than what it represents. The recording medium can see quite a bit more than what one might think it can see (at least with modern sensors and to dispel the notion that film has a way higher dynamic range than modern sensors; most modern sensors have a dynamic range of 10-12 stops, while film has a range of 8-12 stops). The real problem is the display medium that we view them on. I won't go into details about the differences between additive and reflective light/colour because there's plenty of articles online about colour theory.

In a nutshell, reflective colour or photographic display medium can only display a certain range of light and colour to maintain a certain level of smooth photo-realism (when the medium is incapable of displaying the range you'll see banding in things like blue skies or worse, blown out highlights). One of the tricks to 'increase' this range is to employ a technique called tone compression. This is what this article is going to mainly focus on.

Figure B: Typical exposure - balanced for the entire scene but doesn't represent what my eyes really saw.
Tone compression merely takes the darkest areas and 'compresses' the range so that subtle details are more visible. The same is also applied to highlighted areas. In reality, regardless if you like HDR images or not, what most people see on the internet or their computer screens is merely some sort of tone compression not true HDR (in reality, there is no known medium that can display HDR properly. It is merely an intangible range that an image can have, but requires optimization to be displayed on a medium that is incapable of displaying the full range).

Figure C: Single RAW file compensated by ±2 stops in exposure. 
In figure C tonal recovery is done by adjusting the exposure in Lightroom from the single RAW file. The thing to note is that even though the sensor is able to see 10-12 stops of dynamic range, very subtle details and colours are still lost. So this is where multi-bracket exposures comes into play (figure D).

Figure D: Three bracketed exposures.
Note the differences in the rocks on the left to figure C and the sunset colours and cloud details on the right.
In figure D, the amount of subtle detail differences become apparent in both examples, compared to pushing the file as in above. HDR software attempts to combine all images to give an image that comes closer to what I saw.

Figure E: HDR Efex Pro from Nik Software combines all exposures together
 to create an image that comes close to what the human eye sees.
HDR software like HDR Efex Pro from Nik Software does a pretty good job of combining the images using the tone-mapping functions, however there can be a few issues with colour, saturation, and if you're not careful sometimes it creates these awful halos around areas of strong contrast. Understanding how to use HDR software takes practice. Generally speaking, I found that with all HDR software, I still have to make adjustments after processing to tone down the effect. Often, I found that most HDR software is too contrasty, and sometimes too much saturation. Figure E has been adjusted slightly with contrast and saturation as an example.

Another option to traditional tone-mapping is using something called exposure fusion found in programs like Photomatix. Although Photomatix traditionally is an HDR program, exposure fusion is actually not HDR but rather it is LDR (low dynamic range sometimes referred to as DRI or Dynamic Range Increase). Here's a great link on the subject but basically like HDR in that it combines images, it uses a much better algorithm that compares the multiple images and takes the best balance of saturation, contrast, exposure and recombines the images. Unlike HDR images which tend to have a certain fuzziness or glow about it (hence the velvet painting feel), exposure fusion keeps things sharp looking and much more balanced. Figure F demonstrates how fusion works.

Figure F: Exposure Fusion via Photomatix Pro.
Note the details in the sunset and the rock details.
Exposure Fusion is an excellent option to maintain a level of realism that HDR software has always seemed to struggle with. Note also how the sky has been balanced from the darker areas to the lighter areas. Your eye would see a far better consistency across the scene, personally I think this is a touch too much and with some more tweaking and adjusting I probably could have balanced it better across the scene.


Beyond HDR or Fusion

One thing I hope to clear up is that you don't need specialized HDR software to get your images to get this appearance.

One of the more typical examples of compressing or balancing your scene is to employ the use of gradient filters while shooting your images. Grad filters do this by slowing the exposure time down in a specific area of your scene. Think of this as the tinted top portion of your vehicle windshield (or even sunglasses). By darkening the scene in the bright areas, you can compensate your exposure settings so your shadow areas get more time to expose. This can also be done easily through Photoshop or Lightroom through the gradient filter option (figure G).

Figure G: A soft gradient effect
For most applications, a gradient effect or filter covers most things, however many gradient filters or even just the basic filter is either flimsy, requires additional hardware, too linear, has colour casts and may have light leaks. They can be costly, or the effect is too strong, or not strong enough. Software is a great solution, but if your scene has a difference of 6 stops from the sky to the foreground, then you need to consider other options. One of my more favourite methods in the field is to use a black card (here's a good link describing how it's done). Like a gradient filter, it slows down the exposure in the areas that are too bright, however there's the added benefit of having a stronger effect when you need it. You can also feather the effect across the scene, by painting the effect you desire (figure H).

Figure H: Black Card Exposure
Black Card exposure does take lots of practice and experience to get right so it's not for everyone. It does require a good understanding of exposure and good memory (to know where to dodge your exposures blindly in front of the lens). The tradeoff to black card is that it can create a surreal feel to your images, Especially in scenes where long exposure shows water milky, but your skies which typically move as being crisp and sharp. I like this effect personally but for some it might be too extreme.

Sometimes in the end, nothing is better than just doing a layer blend of multiple images within Photoshop. Figure I demonstrates the combination of using bracketed exposures, layer blending, and some other minor adjustments done in Lightroom. Of course it does mean you need to set up for three exposures to do so and some knowledge of Photoshop.
Figure I: Exposure blending, layer masking of 3 bracketed images.


I personally have enjoyed using black card to do most of my work. I certainly am comfortable with using all sorts of software, but in the end to me the most important aspect is achieving the closest possible image to what my eyes adjusted and saw. Your RAW images from your camera is certainly more than capable of capturing that kind of vision without resorting to HDR or multiple exposures (Figure J), but it of course means you expose your image correctly so you can do the necessary recovery in the shadow and highlight details at a later date.

Figure J: Sometimes you can just use your original image and just do all the corrections in Lightroom.
Whatever method you choose to do your tone recovery, what should be apparent in this exercise is that all methods employed here can be used to recover an image to hopefully your liking. Do you need to have HDR software or Photoshop to do things? Not for the scenes that are less extreme. This example used in this article certainly pushed the range at 4 stops of difference from the highlight to the shadows. Beyond that, even HDR software has it's limitations and black card would be about the only option here. But with 4 stops, you have a lot of flexibility and every method demonstrated here today is more than capable of doing some sort of tone recovery to an image.


Mouse over the links below to compare images.
(Just discovered Blogger supports inline javascript - more posts with this feature in the future)
Original | HDR |Fusion/LDR/DRI |GRAD Black Card Layer Mask Lightroom

5 comments:

  1. Great blog. I just can't understand how you are not followed by lots of photographers! Outstanding! Congratulations.

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  2. Thanks Archerphoto. I appreciate the compliment. I'm not sure who reads my blog, but I do get pretty good traffic for it. Since I started this in January, I've had over 13,000 visitors, with 70% of them being new, and the other 30% repeat.

    I try to keep it informative and interesting when I can.

    Cheers
    Terry

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  3. I for one follow your posts and find them extremely informative. I also follow you in flickr. Thanks a million for sharing.

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  4. Very informative Terry. Thank you for an excellent read.

    I haven't tried HDR Efex Pro, but used Photomatix. Its really hard to get the tone mapping (the HDR part of Photomatix) right, and get a natural looking HDR. I also have used layer blending (Auto Align, Auto Blend) option in Photoshop to work better in certain shots. Some times, I use Photomatix to get a HDR, use it as top layer add original bracketed images below it, set opacity differently to get more natural look. After reading this post now I need to experiment with LDR side.

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  5. i often use HDR in my photos, but still there are many different moments i dont understand, and your tutorial explained it to me, so thanks, also you can read my tips and guide http://softwarehdr.com/raw/ here!enjoy

    ReplyDelete