Rethinking Contrast

The trouble with Adobe Lightroom and other slider-driven applications is that their meticulously ordered collections of sliders are perfectly primed for the kind of unhinged fiddling that can sometimes get results but doesn’t necessarily impart understanding. Even today, I struggle against the temptation to start adjusting without first thinking through what it is that I am trying to achieve, and for years I simply swiped sliders back and forth between their arbitrary limits, stopping only when the thought, “Oh, that looks cool!” casually popped into my head. To be fair, there are many photographers who create exceptional works using the oh-that-looks-cool approach, but for those of us without the natural talent necessary to fashion masterpieces by the seat of our pants, each photo we wish to finesse into an artwork requires forethought, and that in itself necessitates an exact knowledge of what each slider does.

During my dark days of anti-intellectual photo editing, there was one adjustment slider that came to epitomise my ignorance, the Contrast slider. Conceptually, the innocuous Contrast slider seems like a straightforward tool. Nudge it to the right and the dark tones in an image get darker and the light tones lighter. Move it to the left and the opposite occurs, the light tones get darker and the dark tones lighter – or do they? The thing that perplexed me was why the colours in an image were also influenced when adjusting a slider that is a senior statesman in the Tone group. Increasing contrast by pushing the slider to the right made the colours in the photos more vivid and punchy, and why should that be if tone, by definition, only affects how light or dark a pixel is and not how colourful it appears?


The answer to my question, as well as the flush of satisfaction that accompanies true understanding, came when I recognised that colour can be expressed in many ways. Today, all electronic devices that are in the business of either capturing or rendering images, do so by exploiting the RGB colour model. The RGB model is a ubiquitous representation of colour founded on the idea that every colour in the universe can be produced by simply adding together different combinations of red, green and blue. This theory is enormously useful to the electronic wizards who bolt your camera together because all these geniuses need at pixel level is a red, green and blue light source and they can, by fiddling with the respective intensities, reproduce any colour that ever existed.

The trouble is that RGB is an excellent tool for engineers but far less useful if you are grappling with the intricacies of contrast. Instead, a decidedly more intuitive way to think about colour is as a combination of hue, saturation and luminance (HSL). This may sound unfamiliar and possibly even prohibitively complicated but, before you throw in the towel, Lightroom itself hints at the power of this model by employing it in its own, more advanced, HSL adjustment panel. What Lightroom fails to do is highlight the parallels that exist between contrast and HSL, something that is rather inconvenient for those of us lacking a PhD in colour theory.


A lot can be said about the HSL colour model and its cognitive benefits but there are two specific points that are important to anybody on a quest to gain contrast enlightenment. The first of these matters is that just like RGB, every single colour that has ever flashed into existence can be represented as a combination of hue, saturation and luminance. No colour ever escapes RGB and, likewise, no colour escapes HSL either. The second thing to recognise is that tone, the very same tone that the Contrast slider manipulates, is defined by a pixel’s luminance value. Altering a luminance value is precisely the same as altering its tone and, given that luminance is one of the three parameters that constitutes a colour, it should now also be easy to understand why fiddling with tone impacts upon colour too.

To understand the real-world implications of all this luminance malarkey, it might be a good idea to quickly contemplate contrast’s cohort – exposure. When you move the Exposure slider about, you are effectively either adding to or removing from the luminance value of every pixel within an image. The true mathematical function is, in fact, multiplication, meaning that when you increase exposure you are, in reality, multiplying the luminance value of each pixel by some value greater than one. Naturally, when you decrease exposure you are multiplying each and every pixel by some fraction of one. The key thing to take away from this interlude is that the multiplication is applied in the same way to every last pixel on the screen.  


Contrast, on the other hand, uses a differential approach. When you increase the contrast, all the darker pixels in an image have their luminance values decreased while the lighter ones have their luminance values increased. It is this action that results in the darkening of dark tones and lightening of light tones which characterises added contrast. It is also this action that results in deeper, punchier colours and I hope, by this point, it is obvious that the opposite is also true – decreasing contrast lightens dark tones and darkens light tones, resulting in washed-out colours.

If you are at all like me, then this bombardment of colour models and mathematics is bound to leave you asking why one can’t just get the contrast right in the camera. The reason is that contrast, unfortunately, is an optical phenomenon and, unlike exposure, which a photographer can toy with to their heart’s desire, it is largely fixed, a product of the lens and sensor’s manufacture. Broadly speaking, the amount of native contrast that one is likely to receive from their camera is determined first by the quality of their lens and second by the properties of the camera sensor. It is incredibly unlikely that you are going to achieve adequate contrast directly from your camera even if you are privileged enough to be armed with the finest honed glass and state-of-the-art sensor.

A High-Contrast Photo of Big Mamma Dune – Sossusvlei, Namibia

A High-Contrast Photo of Big Mamma Dune – Sossusvlei, Namibia

It is for this reason that all cameras include picture styles as part of their operating systems. Picture styles are, in effect, the camera’s very own version of Lightroom within which it attempts to make automatic adjustments to, among other things, the contrast. The objective of the camera is to better satisfy the photographer’s immediate expectations through its own retroactive adjustments to the captured image data. In some sense, the camera itself is trying to compensate for its own intrinsic limitations.

However, if you have suffered this far through commentary tediously articulating the complexities of contrast, then I feel confident suggesting that you, like me, believe that manual adjustments are always superior to those of the programmed variety. Despite the rise of artificial intelligence, I still cannot foresee a day when a machine will create art, and until that day dawns, a detailed understanding of contrast and how its humble little slider works will be required by everyone pursuing that elusive photographic masterpiece.


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