ESFotoClix Blog

I finished reading an article regarding countershading from Outdoor Photographer a couple of nights ago. Here’s a key quote:

“Countershading is the technique of introducing a gradual change in the background illumination, let’s say from light to dark, so that light foreground elements placed against the dark part of the background gradient look brighter than they actually are. Countershading relies on two principles. The first is that our visual system is much more sensitive to abrupt changes in luminosity than gradual ones. The second is that surrounding a tone with a darker tone makes the original tone seem lighter; surrounding the original tone with a lighter one makes the original tone seem darker.” [Reference Link]

This is challenging some of my conceptions about dynamic range (DR). For instance, since the display (monitor) or print medium has a limited DR, should I bother with various techniques (VGND, HDR) that give me greater dynamic range than the sensor provides? For example, if the best print paper only gives me 6-7 stops, does it matter that I generated a 14-stop HDR image, then converted to JPG, etc? Or, alternatively, do I need to play closer attention to how I process my HDR (or whatever) image to maximize the countershading effect and thus give the impression of greater dynamic range in the final display or print medium?

I have a feeling that the answer will fall somewhere in the middle of these two alternative views, though perhaps more heavily weighted toward the second. It won’t be enough just to crank up the DR with whatever method one chooses, but rather, in post-processing one will have to take care to achieve the effects of countershading to ensure the viewer believes she is indeed experiencing greater dynamic range than the camera’s sensor originally captured. I see some experimentation and learning in my future.

Yell, “HDR!” in a crowded theater, and someone will turn you in to Homeland Security. Such are the negative connotations and powerful emotions High Dynamic Range (HDR) elicits. Why such opposition to what should be another useful technique to expand the quality of your photographs? “Over-cooked, impressionistic cartoonish clap-trap images,” will be the raw answer you might get from certain folks. The feeling even among those who would hold a softer stance is that HDR abuses everything we know and understand about a photograph. In some ways, this may relate to some of the discussions we have had regarding realism and its relationship to photography.

Before we address whether HDR is a valid photographic technique, let us establish what it is and what it does. Simply put HDR seeks to increase the dynamic range (DR) of today’s digital cameras through the combination of under-exposed, over-exposed and just-right-exposed captures. To throw some numbers around, today’s DSLRs feature a DR somewhere between 7-9 stops, depending on who you ask and how the camera is being tested. As an example, a 3 frame HDR capture that deviates by 1 stop from the center exposure increases the dynamic range of your image by 2 full stops, so your 7-9 just became 9-11 stops. And if you took 5 frames, also separated by 1 stop each, now you have an increase of 4 stops, and you have achieved a 11-13 stop DR.

Now contrast this with film, and let’s go all out with Tri-X B&W film, which is reported to provide 18(!) stops of dynamic range, while regular B&W film features around 14 stops. You can see that with the 5 frame HDR example you’re in the ball-park for regular B&W film. Some observations come to mind.

First, if you hate HDR, realize that Ansel Adams with his black negatives was doing HDR before many of us were born. Second, if you realize that a 16-bit image doubles the number 2 16 times, 16 stops is a good approximation(1) of the range it can contain. If you want to do a 9 frame HDR capture, either reduce the frame-to-frame stop separation (say, to 1/2 stop) or you will need to generate a 32-bit image. The third observation is that the best print paper provides 6 stops in image reproduction capability, and if you’re displaying it as a JPEG or PNG file for Web consumption, you’re back down to an 8-bit image. Take that into account before you spend a lot of time producing images with extreme dynamic range. Yes, having more range gives you more latitude to process the image … until you have to make your final image.

All that to say, “easy does it,” and “everything in moderation.” If you want to get artistic and do some impressionism or reality-bending stuff, go for it. But if you want to capture a photograph, take care of your image by processing it in such a way as to produce results one would expect from a photograph, as opposed to what a button-press-and-go software package would give you.

Pop quiz: which of these is the HDR image?

Answer: who cares? All three aim at being photographs, and shouldn’t be judged on whether they were processed with HDR or some other method. If you must know, all 3 are HDR images.

Footnotes:
(1) Theoretically, a 16-bit per channel image can contain a much larger number of stops (up to 2 raised to the 16th power). The question hinges more on what is practical in terms of what can be perceived by the human eye and what can be represented without loss of fidelity in the image’s display medium.

References:
http://www.luminous-landscape.com/tutorials/hdr.shtml
http://www.northlight-images.co.uk/article_pages/16_bit_black_and_white.html

If you have ever dealt with hot sky that led to over-exposed highlights and/or under-exposed shadows and have wondered whether you should run out and buy an Graduated Neutral Density (GND) filter, you might want to see whether an alternative solution is available to you. We have all seen terrific exposures made possible through the judicious and skilled use of GND filters, but GND filters can’t handle every situation or require compromises to be used for some scenes. For instance, if the dark-to-bright line occurs along a jagged mountain top, how would you align the filter’s straight dark-to-bright line? Complicated geometries will require compromise or abandoning the use of the filter altogether. What if we used a post-processing technique that allows us to achieve a Virtual Gradual Neutral Density (VGND) filter?

Let’s look at this next sample image as test case. The first image is the middle exposure for 3-shot HDR bracket, and a compromise between sky and shadow areas has led to some problems. The second image is the HDR composite image: better in the shadows, but still not quite there, and pushing out the shadows (not shown here) any further leads to image degradation. The final image is a straight horizon virtual GND image, where below the buildings we use the bright frame from the 3-shot bracket, and for the sky we use the middle image in the bracket (i.e., the 0EV image), using two layers and a masking technique to blend the two exposures. The dark exposure is used in the background layer and the bright exposure is used for the foreground layer. By selecting and removing the sky from the bright exposure in the foreground, we are left with a composite that includes a brighter foreground (water, sand, etc.), and a well-exposed sky, for a more balanced composite exposure.

Original	HDR	VGND

As you will notice, the VGND version shows more detail in the foreground while preserving good balance in the sky’s exposure. When compared to the HDR version, it also shows less blurring in the surf. And for what it’s worth, this particular image used the equivalent of a 1.3 stop GND. Have you seen any of those on ebay?

Now let’s look at a far more complicated example, a very challenging exposure of a bright early morning scene. Again, the first two images show the original, as-shot middle exposure and the HDR version blending a 3-frame bracket. The third image uses the VGND technique, but with as complicated a mask as you can concoct, isolating the shadow areas in the foreground from the bright areas in the middle and background. Once more, the shadow areas use the +1.3EV frame in the 3-frame bracket, while the midtones and highlights use the 0EV (centered) frame. Combining the two yields nice detail in the shadows in the foreground and calm highlights in the rest of the scene. The only trick here is that your masking technique has to be spot-on, and unfortunately, the author is still fine-tuning his skills in that department.

Original	HDR	VGND

If you go pixel peeping, I’m sure you can find some imperfections in these VGND images, but don’t attribute those to the method or concept behind it. With better masking technique, even better results are possible. Bottom line: when you encounter a difficult exposure, namely one with a lot of dynamic range, take a 3-5 frame bracket, and when you get home, try both the HDR and VGND approach and see which you like best.

Updated Aug 5, 11:20 PST with italic and underlined text.