My eyes, my eyes! I had to work quite hard to make this as a) I don’t own any of those filter programs and b) I don’t do this kind of hyper toned, overlapping HDR. The actual, final version of this image is at the end of the article.
HDR/ High Dynamic Range photography is perhaps one of the greatest blessings and curses of the digital age of imaging. On one hand, we have retina-searing rubbish that’s put out by people who for some odd reason celebrate the unnaturalness of the images, encouraged by the companies who make the filters that make doing this kind of thing too easy – and on the other hand, there are a lot of HDR images out there that you probably wouldn’t have pegged as being anything other than natural. There is, of course, a way to do it right, and a way to do it wrong. I use HDR techniques in almost all of my images – I live in the tropics, remember, and noon contrast can exceed 16 stops from deep shadows to extreme highlights – we simply have no choice if you want to produce a natural-looking scene.
I originally wanted to explain my recent digital B&W conversion epiphany, but I realized that I couldn’t do it without explaining the whole concept of dynamic range and briefly touching on the zone system first. All photography centres around light, and how that light is represented in a way that captures the scene to represent and convey the artistic intentions of the photographer. Taking the quality and quantity of light in the scene as a given, it always then becomes a question of how you map input luminance to output luminance, especially when one exceeds the other.
We also need to understand something around how human vision works: our eyes are nonlinear, and this is partially because our brains are extremely sophisticated processing devices, and partially because of the way we see: the eyeball scans rapidly many hundreds of times a second, building up a detailed picture that’s then composited together by the brain. This is known as persistence of vision, and is the reason why cinema at 25fps still appears to be mostly smooth motion even though we are seeing discrete frames – our brains fill in the blanks. While it’s doing that, it’s also dynamically compensating the iris and the signal processing to maximise dynamic range: the upshot is that whilst we almost never have to deal with blown highlights – no matter how bright a scene is, we can almost always make out some luminance gradient – we don’t see so well into the shadows; seeing pure black with almost no detail is normal. This is a consideration of perception.
A photograph is a static scene: we view it and the brain doesn’t get any additional information from scanning it again with a larger iris or while collecting more light. We therefore need to ensure that the limited tonal range contained within a static image – be it backlit and transmissive as on a screen, or reflective in a print – represents the actual scene in such a way that the observer’s brain can reconstruct the relative tonal relationships. I put heavy emphasis on ‘relative’ here; again, because our eyes scan the image and the brain uses persistence of vision to reconstruct the whole (see these two articles – part one, part two – on psychology and how we view images for more information) – the absolute difference doesn’t matter; only the relative difference. So long as the image maintains an overall semblance of separation, and the right relative separation to adjacent areas, then all is well – and the image appears natural.
This is a good thing, because even if our cameras can capture 16 stops of dynamic range – none of our output media can display it; digital or print. We therefore need to find a way of allocating input to capture, and capture to output. The final stage isn’t so much of an issue as the nature of the technology tends to take care of this for us – the extremes of the range will become compressed, but they will never overlap. It’s the input to capture portion that one must be extremely careful of. Of course, all that follows applies only to scenes where you are not in control of the light; if you’re using a controlled lighting setup in studio and have to use HDR to control your dynamic range, you are an embarrassment as a photographer.
Here’s what normally happens: input (top) goes to output (bottom). The gray wedge represents the tonal/ luminance scale; it’s grossly oversimplified as there’s one for each colour channel, but this is purely for purpose of explanation. In the process, there’s some tonal clipping – the areas represented by the red triangles is usually lost and compressed to either extreme of the tonal scale; i.e. everything below a certain luminance level goes to pure black only, and everything above goes to pure white only. The same process is generally true of the digital file (or film negative) to final output process, except it’s the digital file on top, and your output medium of choice at the bottom.
Assuming we extend the recorded dynamic range of the scene by bracketing and compositing or using grad ND filters or some other process – we are still going to be left with more input dynamic range than output dynamic range. We need some way of allocating the extra information, preferably in such a way that it a) doesn’t look unnatural and b) is useful – i.e. opens up the shadows slightly, or tames the highlights for a smoother rolloff. What is typically referred to as ‘HDR’ is this allocation process.
Your typical HDR image has tone mapping that has undergone a process that looks like this. The Roman numerals are zones; a zone is basically a luminance band/ range. The problem here is that the allocation results in overlaps: input zones 0-IV are output as zones 0-VII; but zones VII-X output as IV-X. Thus zones IV-VII become this ambiguous soup: we have highlights that are darker than shadows/ midtones, and shadows that are brighter than midtones/ highlights. And this simply looks unnatural – it’s also what I’ve done with the first image in this post. In case your retinas are still intact, here it is again:
You’ll notice that there’s something not quite right with the naming convention of the zones: I suspect this is the root cause of all of the bad-looking HDR is partially because whoever is using or writing the software makes the mistake of thinking that there are as many output zones are there are input zones: there simply aren’t. (This is why we have to do HDR in the first place: we cannot accurately capture or represent the full input tonal range; maybe our monitors don’t go perfectly black, or bright enough, or it’s because our sensors are limited, or because paper can’t get any brighter than zero ink density.) On top of that, most of the tone mapping is performed in the full RGB channels, instead of luminance only: hues doesn’t change when it gets darker. This results in hue/ colour shifts and the rather strange palette you see above. To understand why this is the case, we need to consider how the software works: you put in a number of images taken at different exposures; for each given pixel address, the program calculates an average luminance value for each RGB channel based on the input files. It may then run those through a curve to bias input/output. This of course is is a purely mathematical approach – it has to be, as every situation is different, and there is no such thing as an ‘ideal exposure – it varies based on artistic intent – it simply cannot beat personal, perceptual adjustment on a case by case basis.
Now, here’s how to do it right:
First, remember that everything is relative: there are fewer output zones than there are input zones. Golden rule: there should be no tonal overlaps. The input tonal band is always wider than the output band; even within the bands we must make sure there are no tonal overlaps. This way, the relative brightness transitions across a scene and between the subjects in the scene are preserved.
Second, and perhaps more importantly, HDR gives us the ability to choose the bias of the tonal allocation: whether we have priority given to the shadow areas, or the highlights, is up to us; this can still be done without overlaps:
This method allows us to maintain the smoothness of the transitions, especially at the shadow and highlight borders – areas where our eyes are particularly sensitive, and digital becomes binary – an area is either white and detail-less (i.e. no contrast because it’s fully desaturated) or it isn’t. A good HDR starting point should appear very flat: i.e. little apparent output contrast between adjacent areas in the image – because this gives you the flexibility to put the contrast where you want it later on, i.e. the ability to do the above allocation by means of a curve or local adjustments like dodging and burning.
This is the same image, also run through a HDR process, but I think you’ll agree that it looks far more natural: firstly, there are no tonal overlaps. Secondly, colour remains natural and accurate to the scene (assuming a calibrated monitor). Thirdly, if you look closely, you’ll see that there are no large clipped areas in the image – there are small areas to allows the viewer’s eyes to automatically calibrate and get a feel for overall scene brightness only. Finally, pay close attention to the deep shadow and extreme highlight transitions: they’re smooth, and natural. But here’s the thing: if I didn’t tell you it wasn’t a single exposure, would you have known? In my mind, the benchmark of good processing – HDR or not – is that the first thing you should notice is the subject: not the post-capture adjustments. MT
There will be updated Photoshop Workflow videos available in the near future, including one specifically covering black and white conversions and preparation for print – I have made a lot of improvements and refinements to the process in the two years since the first video…
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