Sunday, March 4, 2012

Adobe Lightroom Tutorial - Part XX - Exposure vs. Brightness

Part I - Index and Introduction
Part XXI - Making Photo Books

One of the questions I'm most frequently asked for by both blog readers and friends is:

"What's the real difference between Exposure and Brightness."

The answer to this will soon be irrelevant: the Brightness slider has been removed from the new development process (2012) shipped with the upcoming release of Adobe Lightroom. I quickly described some of the changes that we'll see in Lightroom 4 on a previous blog post and I described the major differences between the current development process and the new one. Also, I stated I'm really glad that both the Exposure and the Brightness tools have been merged into a new tool, still called Exposure, whose behaviour I find more intuitive and predictable.

However, here's a quick summary of the major differences between Exposure and Brightness in Lightroom and how you could take advantage of both to improve your results.

First of all, let's have a look at the authoritative "answer", although not satisfactory, that comes directly from Adobe. The Lightroom 3 official documentation, that you can find here, states the following (this is an excerpt):
  • Exposure: Sets the overall image brightness, with a greater effect in the high values.
  • Brightness: Adjust image brightness, mainly affecting midtones.

Lightroom documentation also suggests you adjust the overall tonal scale before adjusting the brightness.

Information provided by the documentation is crystal clear, but clearly insufficient. Although hinting at a major difference in how they alter the tonal scale of a picture, it fails to explains the real difference between them, leaving Lightroom users experimenting and trying to learn by trial and error. Furthermore, but this is just a personal opinion, the documentation only tells an half-truth. Based on my experience, in fact, the description is both incomplete and deceiving. In the following sections we'll see why.

In this blog post I'll use the following photo (unedited) as a test bench:

Grey Gradient - From Zone IX to Zone 0

Exposure and Brightness Behaviour

For small adjustments, both exposure and brightness will produce very similar results: you will begin seeing some differences when you push these sliders farther, on either end.

Let's begin increasing either adjustment:

Exposure: +2

Brightness: +100

The effect of the exposure increase on the image is greater than the effect of the corresponding brightness increase throughout the range.

The first thing we notice is something we already knew: as I've often stated in this blog, exposure is much less forgiving than brightness and burns the highlights very quickly. In fact, both zone VIII and zone IX are burned when increasing exposure by 2. On the other hand, increasing brightness has a much gentler effect on the highlight: in fact, no zone is yet burned out.

If you push these adjustments further, this behaviour is even more evident:

Exposure: +3

Brightness: +150

After increasing exposure by 3 steps, Zone from VI to IX are completely blown out. On the other hand, when increasing brightness to 150, zone IX is burnt (and zone VIII is very close to being so).

A similar behaviour can be observed when reducing a negative adjustment with these tools:

Exposure: -3

Brightness: -150

When decreasing their value, the effect of a brightness reduction is now greater than the effect of the corresponding exposure reduction. Once more, the overall effect of a brightness adjustment throughout the range is gentler and more uniform than the effect of an exposure adjustment. Also, a brightness reduction has a stronger effect on the shadows than an exposure one although, in this case, no zone has gone into zone 0 (pure black).

We can draw the following conclusions:
  • The overall effect of a brightness adjustment is gentler and more uniform throughout the dynamic range in both directions.
  • Increasing the value of either tool has a stronger effect on the shadows.
  • Decreasing the value of either tool has a stronger effect on the highlights.
  • Increasing exposure quickly burns the highlights and has a stronger effect throughout the range.
  • Decreasing brightness has a stronger effect throughout the range.


This basically mean, also, that the effect of either tool is not linear throughout the range, peaking at a specific value: the shadows, when increasing their value, or the highlights, when decreasing it. However, the overall effect of a brightness increase is more gentle and uniform throughout the dynamic range.

This asymmetric behaviour has interesting consequences that, once understood, can be incorporated into your workflow to improve your results:
  • If you need to reduce the highlights of a picture (relatively to other ranges), you should consider reducing the brightness. 
  • If you need to increase the shadows of a picture (relatively to other ranges), you should consider increasing the exposure.

Using this asymmetry at your advantage may signify improving the tone distribution of your image, especially when you want to push back the highlights towards the middle tones while retaining all of your shadow details and, on at the same time, preserving the natural look of your image.

Some Example Pictures

I took this picture on the Roman Bridge in Salamanca using matrix metering and no exposure compensation:

Original Image - Matrix Metering - Exposure Compensation: 0 EV

Clearly, the sky and the colour of the rock deceived the camera and the resulting image is pretty overexposed. If we try to fix this picture using only the exposure slider, using the church as a reference, an acceptable result could be the following:

Exposure: -1

Not bad, but I'm not satisfied with the tone of the shadows nor with the overall tone distribution. This is a consequence of how exposure works: decreasing exposure has a stronger effect on the highlights, but the effect throughout the range is less uniform than brightness' (as seen in the previous zones samples).

Let's try to reduce the highlights bringing brightness down:

Brightness: -10

Please beware that, at least when processing Nikon RAW files (NEF), the default Brightness value is 50, not 0. As a result, the brightness value of the previous picture has been decrease by -60. The result is much more satisfactory, as far as it concerns overall tone distribution.

Starting from this picture, we can further tweak it until we're happy with the result. In this case, I'd just apply a small exposure adjustment to bring some light back into the shadow range:

Exposure: +0.1 - Brightness: -10

The following picture suffers from the same problem:

Original Image - Overexposed

Trying to bring the rocky cross and the church into a satisfactory range using exposure gives this result:

Exposure: -1

Once again, the result is somewhat flat and the tone distribution doesn't feel natural. Using both brightness (to reduce the highlights) and exposure, we obtain:

Exposure: -0.2 - Brightness: 0


Look at the much more natural color of the rock: this is a much better starting point for further processing.

Clipping and Color Shift

One of the the issues a photographer is always struggling with is clipping and all its incarnations. This is true both on camera and in post production. The exposure sliders in the current Lightroom development process (2010, in Lightroom 3), is the risk of clipping some channel when increasing its value.

When working in the digital domain, tools may be less forgiving than film and paper are, since clipping is a phenomenon that occurs abruptly. Hopefully, this problem will be partially mitigated in Lightroom 4 but, in the meanwhile, we've got to live with it.

One of the problem that may arise when clipping a channel is color shift: when a channel fills up, the relative proportions among the channels cannot be respected and the hue of some colors may begin to drift.

That's the reason why I never push any tool any further when a channel start to clip. If I need further tonal adjustment, I'd rather use the curves instead, leaving the white point fixed.

Unexpectedly, color shift is another reason why you should consider brightness over exposure. This is a reference image I'll use to explain this problem:

Color Reference Chart

This color reference chart is made up of three rows, each of which contain:
  • A pure RGB color in the first square.
  • Two differently saturated versions of the same color on the remaining two squares.

This is what happens when increasing either exposure and brightness over a certain level:

Exposure: +1.5

Brightness: +150

It's evident that exposure and brightness treat distinct RGB channels in a different way. With a 1.5 exposure adjustment, you can clearly see a color shift. Indeed, if you take a measurement of the darkest squares, corresponding to pure RGB colors, you get:
  • Red: (220, 61, 68)
  • Green: (221, 255, 160)
  • Blue: (125, 76, 255)
On the other hand, measuring the color of the same squares when a brightness adjustment is applied, you get:
  • Red: (223, 91, 96)
  • Green: (223, 255, 211)
  • Blue: (122, 122, 255)
I'm not sure about the rationale behind this, but I suspect the exposure behavior is related to the characteristics of the human eye: since we're not equally sensible to light waves of different lengths, "lightening" a color is not a linear process amongst the RGB channels. That's pretty evident if you look at the relative luminance equations in colorimetric spaces.

If, on one hand, the exposure behavior may be justified, on the other hand you should be warned that strong exposure adjustment may lead to undesired color shifts.

Conclusion

Although similar, exposure and brightness have got completely different behaviour throughout the tonal range. Learning to use them is critical to master Lightroom and get the best out of your pictures. Also, every picture is unique and there's no rule but your personal taste when deciding which adjustments to apply in post production.

As we've seen, using one or the other may lead to very different results and even to unexpected (and undocumented) problems such as color shift.


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