Thoughts on achieving natural tonality

Riddle of the day: what do a good magic trick and a good photograph have in common – or by extension, a good prestidigitator and a good photographer?

The answer is of course both of them distort your perceived reality. The magician makes you believe you saw something that’s physically impossible, or at very least completely unexpected. The photographer presents you with either something you may not have expected, or could not previsualize. Both are technicians in a sense, but the best of both professions are more than that – they’re also psychologists. At this point, you are probably wondering what any of this has to do with tonality. Read on to find out.

Firstly, we need to define what exactly we mean by the term ‘tonality’: I suppose it is really the relationship between input and output luminance and color, in addition to the smoothness of luminance transition between spatially adjacent areas. Note that I’m very careful not to use the word zone because it has come to mean something very different from a photographic sense – the zone system is a way to describe the tonal thresholds in an image for proper exposure and printing. Tonality is therefore also related to contrast: high contrast/low contrast may describe the overall distribution of luminance within an image across all of its areas, but says nothing about whether the transition between these areas is smooth or not.

What we have above is a chart of all the possible options (from top to bottom, left to right):
– Moderate contrast and smooth tonality; high contrast and hard tonality
– low contrast and smooth tonality; no contrast and smooth tonality
– low key low contrast smooth tonality; low key high contrast
– high key low contrast smooth tonality; high key high contrast.

Note again that there is no ‘right’ or ‘wrong’ when it comes to these options; every photographic scenario is different, which means that some presentations may be suitable for some subjects/images, and completely unsuitable for others. It’s also a function of the quality of light: if you’ve got an undiffused pointlike and highly directional source, you will get hard shadows and high contrast; if you have diffuse light from multiple sources, then you cannot have high contrast. But both situations can be low key or high key. In summary: contrast is a property of the light; high key or low key is a property of the exposure choice. So where does tonality come into it?

Tonality is what we need to worry about at the transition points: the spatial zone where shadows turn into highlights and light bits into dark. The gradient of luminance change over spatial distance (or angle of view) is what we think of as contrast. And it is this very important portion of the image which I believe makes or breaks the difference between a very strong and coherent looking image and an amateurish one.

Remember that contrast is one of the things that cause us to sit up and take notice: and area with high contrast (luminance or color) is an area that’s immediately attention-grabbing to the audience because this is the way our brains are wired; it’s a holdover from when we needed to determine if an animal was brightly coloured and therefore poisonous, or where the land abruptly fell off into a possibly deadly fall. The psychology behind design, typography and advertising still relies on these principles to catch (or not) your attention. Think of camouflage: an object so covered does not contrast at all with its surroundings, and therefore does not stand out. Photographically, we want our subjects to stand out, and therefore some contrast is always desired – but not necessarily always maximum contrast. We just need the subjects to have more contrast than the surrounding area so they differentiate themselves.

Let’s take this a step further: if we have excessive contrast in an unintended area, then we are also going to be drawing attention to something that should probably not be singled out – this in turn removes the viewer’s focus from the intended subject and weakens the overall image. However, this is also something that has to be managed behind the scenes; if your eyes go precisely to the intended primary subject first and then only the secondary contextual items and not to some unintended elements in the rest of the frame, then chances are you won’t notice the well-managed contrast and tonal transitions unless you’re consciously looking for it. Remember the magician: the reason magic tricks ‘work’ is because they direct your attention elsewhere whilst the giveaway switch is being performed right in front of you. If a naked person ran through a restaurant, chances are you won’t notice the cockroach that just ran across your table.

This is relevant to photography because we often have to cheat a little when it comes to tonality; there is almost no situation in which the input dynamic range (i.e. real world) is less than the capture dynamic range (camera capabilities) and again less than the output dynamic range (i.e. screen or print). This means that we almost always have a situation in which there is more information than we know what to do with – i.e. tones to allocate. There are two obvious solutions to this problem: the first is to just discard any data that spills over the sides, which results invariably in clipping (abrupt transitions to black or white). The second is to use high dynamic range (HDR) techniques to map input to output dynamic range but allow for some overlap in the transition. Unfortunately, most of these tone-mapped HDR results look unnatural because it’s clear to see that you have areas or elements in the image which should be perceptually brighter than others, but aren’t. Again, in both situations, the unnatural contrast is the giveaway that something isn’t quite right – and in turn distracts from the subject.

Things get more complicated when you throw color into the mix, because we’ve now got to think about clipping and dynamic range across three channels; what the camera makers don’t tell you is that dynamic range isn’t the same across all three channels because of the nature of the filter packs, and because of the Bayer algorithm (or layering, in the case of Foveon) – there are always more of one color of photoreceptor than the others. On top of that, the native tonal response of the sensor may have been deliberately optimized for color or contrast or transitions in a nonlinear (and not always predictable) way. It’s also possible that for some colors, one or even two clipped channels do not matter so long as you don’t do too much tonal work there to emphasise the clipping – so long as there’s still some visible change in luminance, the eye does not perceive this as clipped. You also need to remember that our eyes read color differently at different luminance values: saturation and detail both decrease as things get darker, because our vision transitions from the high density and color-sensitive central region to the sparser luminance-sensitive outer region of the retina. It is therefore also important to adjust our output images accordingly – especially since unlike real life, they are usually presented with the same reflective or transmissive luminance across the entire tonal range.

But relax, because there is a solution for all of this. No matter how big the gulf between capture and output dynamic range, one very important thing has to be remembered: the eye notices contrast first, and everything else later. What this means in practical terms is that so long as your transition to complete black or white is smooth and gradual, chances are, it won’t be seen as clipping or a digital artefact, and neither will it draw unwanted attention to that area of the frame. A little judicious dodging and burning of the midtones is all that’s required to keep these transitions in check. By a similar token, even if you do have a smooth transition and the image has no overexposure, we must also remember that excessive luminance differences within areas that the eye can perceive at the same time might come across as clipping anyway due to the limitations of our eyes. In this case, a little bringing down (burn) of the highlights or bringing up (dodge) of the shadow areas can help immensely. HDR looks unnatural only because the adjacent zones we perceive were never mean to be adjacent or that close in luminance. Some differentiation is required – just enough to tell our brains that one area is meant to be brighter than the other. And if this differentiation matches our expectations (e.g. daytime sky lighter than area in the shade), it doesn’t even enter our consciously observing mind.

In conclusion: it’s all in the transition. The images in this post have all been chosen deliberately: they are images that have tricky tonality (very subtle) or high contrast and unavoidable over/underexposure. They of course look better at original sizes, but even at web size and with flickr’s horrible resizing algorithms, the transitions don’t stand out. Remember to take care of the areas of an image that aren’t your intended focus – in the opposite way to how the subject should stand out (light/contrast/color/texture/motion) – they should not stand out. MT

We go into significantly more detail on both capture and post processing for maximum tonal control (smoothness is up to you) in the Photoshop Workflow II and The Monochrome Masterclass workshop videos, as well as subject isolation in Making Outstanding Images Ep.1

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