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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Exposure Compensation in Post Production Using Layer Blending Modes

Very often I hear people complain about what should be, apparently, a very easy thing to do: modifying the exposure of an image in post production. Truth is that if the adjustment you need is relatively small results are going to be satisfactory. Unfortunately, as long as you need to push the exposure adjustment a bit farther, results are often going to disappoint you.

Why?

Usually because of two factors:

• Your image has not sufficient dynamic range to achieve the result you want (that's why you should shoot RAW if you can).
• Because people don't clearly understand the physics of the human eye.

As far as it concerns the first factor, you cannot help but repeating it over and over again: you should shoot RAW if you can. Double check it. Even if you think you cannot. Some Canon point and shoots, for example, can be tweaked to shoot RAW.

As far as it concerns the second factor, here is yet another quick wrap up.

A Word of Warning: I'm a mathematician but if this is not a rigorous post. This is a quick tutorial for you to better understand some tools that are often misunderstood.

The Human Eye as a Sensor

As we stated many ways, one of the things that photographers often overlook is that the human eye is a logarithmic sensor. But what does it mean? It basically means that whenever the quantity of light doubles, whichever its frequency in the visible spectrum, you'll notice a comparable "brightness" improvement.

This also means that if you need to modify the exposure in post production, you cannot overlook this effect. You cannot just bump the channels up and down because luminance ratios must be preserved. If you don't, the dynamic range of your image will suffer and you'll end up with a flat, hazy image.

The Problem with the Brightness Tool

The problem with the Brightness tool is that, despite the name, it is not suitable to apply such a modification. It depends on the tool you're using but as far as it concerns the software most commonly used (Photoshop and The Gimp), a brightness adjustment does not what you think it does.

Roughly speaking, the brightness adjustment just adds a correction to the pixel values:

n = o + d

where o is the old value and d is the selected brightness adjustment. It might look right, doesn't it? It does shifts the histogram left or right (depending on the sign of d). But it does not preserve ratios and, as such, it is not the tool you need to tweak an image exposure preserving its dynamic range.

Here's what happens trying to fix an artificially underexposed TIFF image using just the Brightness tool (in this post I'll use The Gimp):

Original Underexposed Image

Brightness raised to 100

This test image is badly underexposed but the detrimental effect introduced by the Brightness adjustment on an image dynamic ratio won't ever be avoided.

If you like Mathematics, try to bring d to infinity and think at what happens to the (absolute value of the) ratio between two pixels value: it just goes down to 1. That's why your images will soon start to lose contrast.

Many people will then try to fix the contrast but that won't work, either.

What Can Be Done?

Preserving the ratios! To preserve the ratio, you need to correct a pixel values using a multiplier. That's what the Multiply and Screen blending modes are made for:

• If you need to lower the exposure of your image, you can blend a layer with a copy of itself using the Multiply blending mode.
• If you need to raise the exposure of your image, you can blend a layer with a copy of itself using the Screen blending mode.

What do these blending modes do?

Basically, they're effect is:

• Screen lightens a pixel in the lower layer proportionally to the "brightness" of the corresponding pixel in the upper layer.
• Multiply darkens a pixel in the lower layer proportionally to the "darkness" of the corresponding pixel in the upper layer.

What happens if you blend a layer with a copy of itself using these blending modes? It applies the very effect we were looking for!

Again, if you like Mathematics, just think about the definition of the exponential function. What happens when you're modeling a phenomenon whose variation (derivative) depends solely on its intensity? Refine this concept: is there any function whose derivative in a point x is only a function of the value of the original function in the point x? Yes: the exponential function.

We're applying an exponential transformation on a matrix of points that will be fed in a logarithmic sensor (the human eye). Just what we were looking for.

Here's what happens when we screen the test image we used before with itself (twice):

Screen Blending Mode

This is a much better result indeed.

If you're new to layers and blending modes, well, it's time you get serious about them. Here's a screenshot of The Gimp layer palette of the previous image:

The Gimp - Layer Palette

Conclusion

The Brightness and Contrast tools are often misused and it's important you understand what's going on under the hood. If you need to apply an exposure compensation, they're not the right tools. Instead, take some time to learn about layer blending modes. The Multiply and the Screen blending modes are just the beginning. Here are some additional tips:

• You can mix and match them to achieve more sophisticated results and you can fine tune the effect tweaking the layer transparency.
• If screening or multiplying two layers is too much compensation for your image, just make the layer a bit more transparent.
• If you're looking for a way to raise the overall contrast of an image, try to screen and multiply it.
• You may also want to take a look to the Overlay blending mode, which is a mix between the Screen and the Multiply (it screens dark colors and multiplies bright ones).

If you want to help me keep on writing this blog, buy your Adobe Photoshop licenses at the best price on Amazon using the links below.