Sunday, July 29, 2012

Notes on Dynamic Range, Gamma Correction and the Importance of Shooting RAW

Some months ago I wrote a quick blog post titled "Tones and Dynamic Range. Why You Should Shoot RAW". In that post I quickly analysed the shortcomings of files with low bit depths (such as 8 bits) and tried to give a valid reason why a photographer should always shoot RAW. Unfortunately, the limitation of the blogging platform and the time restrictions I've got make writing this kind of content awkward, at a minimum.

Here's a document featuring the same content, although better organized.

Notes on Dynamic Range, Gamma Correction and the Importance of Shooting RAW v. 1.2 (PDF)

Monday, July 23, 2012

Adobe Photoshop Lightroom Tutorial - Part XXIII - Understanding Channel Mixing to Achieve Effective Black and White Photos

Part I - Index and Introduction

Converting a photo to black and white may seem one of the easiest thing you can do with your photo editing software of choice. Unfortunately, nothing could be further than the truth, and if you don't do it correctly you may end up with dull images, very different from what you thought you'd get.

The Basic Fallacy: Zeroing the Color Saturation (Depending on the Tool You Use)

An approach I see very often to convert an image to black and white is zeroing the color saturation. What's worse, I sometimes hear theories about not-well-defined advantages of this technique. You get a black and white image, of course. But that image doesn't represent the luminance your eyes are seeing, let alone the intuitive result you think you'd get. Depending on the colors that are present in the shot and their saturation, differences can either be subtle or very deep.

What's interesting to note, as we'll see in a minute, is that most differences will be noticeable on deep blues and saturated reds. Think about: skies and some skin tones. Indeed, this doesn't look like a great deal.

Without going into the technical detailes about concepts like relative luminance or luma in colorimetric spaces, a photographer should understand that the luminance values of pure RGB colors aren't all equal to the human eye. In fact, many standards try to model the behaviour of the human eye and most photo editing programs provide us the tools we require to achieve predictable results, according to how the human eye works. Just for the sake of example, here's the transform matrix from RBG to CIE 1931, where Y is the luminance:

As you can see, red contribution to luminance is approximately 4 times greater than green's (0.17697 vs. 0.81240) and more than 16 times greater than blue's (0.17697 vs. 0.01063).

Another example can be seen in the transform matrix from RGB to the Y'UV color space:

We can see the contribution of each RGB channel to the value of Y' (luma, a gamma compressed luminance). Once again, the contribution of red is smaller than green (0.299 vs. 0.587) and bigger than blue's (0.299 vs. 0.114).

What can we infer from this? That green brings the greatest contribution to the luminance value, followed at a big distance by red, and finally by blue. In other words: given three pure RGB colors with  the same component:
  • Green will be much brighter than the others,
  • Red will be much darker than green but slightly brighter than blue,
  • Blue will be the darkest of all.

What happens, then, zeroing the RGB color saturation?

What happens is simple: the smaller the saturation, the more colors will tend to equal a specific grey value. We're not interested in knowing which grey tone, but the important thing is that each channel will then have the same luminance.

Here's a visual example, better than a thousand words: in the following images you can see the same color chart with different values of color saturation: 100%, 50%, 25% and 0%.

Color Chart - Saturation: 100%

Color Chart - Saturation: 50%

Color Chart - Saturation: 25%

Color Chart - Saturation: 0%

What's clear from this example is that zeroing the color saturation is not what you want during a conversion to black and white. In fact, to achieve balanced and realistic results, or results that at least would faithfully represent what you're eyes are seeing, you'd expect relative luminance between different RGB colors to be respected. That is, red should be a darker shade of grey than green and blue an even darker shade of green.

What can you do, then? Simple: use the tools provided by your photo editing program and forget about the saturation adjustment.

To say the truth, this also depends on the tool you use. The saturation control of some photo editors, such as Lightroom, behave differently than this and, in fact, apply a default mix when desaturating colors. Even so, even basic photo editing programs have got a "Convert to black and white" feature which will hopefully do a better job than you can do with the saturation slider. More sophisticated program may offer better tool, the "channel mixer" being the most interesting and flexible from a photographer's point of view.

Channel Mixing

As we've seen, it's necessary to "mix" RGB values with certain coefficients to simulate the behaviour of the human eye, as far as relative luminance across RGB color is concerned. That's exactly what the channel mixer can do for you, and that's why it's an omnipresent feature of good photo editing programs, although little known to many amateurs.

The channel mixer simply lets you change the coefficient with which each color contributes to the luminance. Raising a color's coefficient means that its contribution will be bigger, and thus will appear brighter. On the contrary, lowering a color's coefficient means that its contribution will be smaller, and thus will appear darker.

Adobe Lightroom 4 applies a default "mix" when converting to black and white which I found pretty acceptable. However, since the tool is there to use it, I almost always tweak the mix a little bit to achieve the results I desire. For example, I often raise the value of the red and orange channels in portraits in order to reduce speckles and imperfections and to slightly brighten the skin.

Here's the result you achieve in Lightroom 4 when desaturating the reference color chart:

Lightroom 4 - Saturation: -100

As you can see, it has done a much better job than Photoshop in this case (beware: I say "much better" from a photographer's point of view, not from a theoretical one). Now, reds and blues are darker than greens, as expected.

On the other hand, if you convert it to black and white, here's the result you get:

Lightroom 4 - Black & White - Default Mix

and here's the default mix applied by Lightroom:

Lightroom 4 - Default Black & White Mix

The result is somewhat less contrasted, and you can see how blues are slightly brighter (+9) and greens are slightly darker (-27).

The point is there's no right or wrong here: the Black & White mix is a tool you can use to fine tune your shot and is much more flexible than just zeroing the saturation. For example: if you want a darker sky, for example, just decrease the Blue contribution. if you want some brighter reds, just increase their contribution.


The bottom line is: the channel mixer is pretty flexible and you can use it at your advantage, for example to simulate some B&W filters in post production. For example, I often increase the Red and Orange channel in many portraits, especially those taken during the summer, to get less contrast in the subject's skin thus getting it brighter and removing many imperfections. In the following shot, for example, you can see how simulating a red filter has given the photo more contrast in the red highlights and resulting in a smoother and brighter skin:

Red and Oraange channels were increased to simulate a red filter

The same effect was used in this shot to achieve a similar result:

Red and Oraange channels were increased to simulate a red filter

In the following shot, the subject was very tanned and the shot, when converted to black and white, had a look I really didn't like. Once again, using an appropriate mix, I could deliver a more natural skin tone and get rid of all the subject's speckles too.

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.

Saturday, July 14, 2012

Nikon Creative Lighting System Tutorial: The Basics

Nikon Creative Lighting System Tutorial: The Basics v. 1.1 (PDF)

Nikon's Creative Lighting System, in Nikon's word
offers photographers new and unprecedented levels of accuracy, automation and control.
Looking past the marketing jargon, Nikon CLS is a set of technologies (and automations) that enable photographers taking the most out of their flash systems with the minimum effort. The technologies making up CLS include:
  • i-TTL balanced fill flash.
  • Auto FP High Speed Sync.
  • Flash Value Lock (FV Lock).
  • Wide-Area AF-Assist illuminator.
  • Flash Color Communication.
  • Distance-Priority Manual Flash.
  • Modeling Flash.
  • Advanced Wireless Lighting.

As it happens with most automatic mechanisms, an incomplete understanding of their behaviour might negatively (or at least counterintuitively) affect the obtained result. In my opinion, the behaviour of several Nikon CLS technologies isn't properly documented in the camera and flash manuals and, "unfortunately", you will be using some of them each time you use a flash, including the pop-up flash of your Nikon camera.

The purpose of this document is to describe the fundamental behaviour of the basic technologies that make up the CLS technology, such as i-TTL balanced fill flash (TTL-BL), regular iTTL flash (TTL in this guide) and flash value lock (FV lock).

Flash photography implies multiple exposures

As we've seen in Flash exposure tutorial: the basics, any time you're making a photograph, you have to deal with multiple light sources which, in the context of this article, we will divide into two categories: ambient and flashIn this section we'll quickly recap the concepts exposed in that tutorial.

Even if extremely faint, you will always be dealing with ambient light, which you may consider as the amount of light coming from continuous light sources outside of your control (the Sun, environmental lighting, etc.). That's the light you're used to meter when taking a shot without controlling any flash.

As soon as you turn on a flash, you're introducing new variables into the equations. The first thing you've got to realize, apart from the distinctive traits of flash light we've already covered in the previous tutorial, is that you've got control over that light source and this is where the CLS technology comes into play.

Every time you shoot a picture using flashes the resulting image will be the combination of two exposures: an exposure coming from the ambient light and an exposure coming from the flashes. Depending on the results you want to achieve, you will have to meter both light sources and configure both your camera and your flashes to achieve the desired ratio a/f between ambient light a and flash light f that's going to be received by your sensor.

In the previous tutorial we've seen how common settings (such as ISO sensitivity, shutter speed and aperture) differently affect a and f and we've also quickly described how TTL metering automatically changes the flash power output and, in turn, how it affects a/fThe physics behind it is easy, but the resulting mechanisms are not intuitive, and that's why it's important for a photographer to know them, at least their broad outline.

Nikon CLS system is a set of technologies that further assist the photographer in getting the results he wants more quickly and more easily. Once again, though, it's important for you to know how that technology works and the assumptions it makes: that way, you will be able to get the most out of it and you will avoid being "trapped" into situations in which you're getting results you can't explain.

Which is your main light source?
Failing to correctly recognize which your main light source is often is the beginning of a novice photographer's problems. No matter the lighting condition, if the main subject is poorly lit, you turn on the flash and hope the camera metering system will solve the problem for you. Whilst this is not so bad an assumption (after all, that's why the TTL and CLS technology are there), you must realize that your gear is going to make an educated guess based on its assumptions and the lighting conditions it determines. That's a starting point, but it seldom is the correct guess.

In fact, look at the name of one of the most misunderstood CLS technologies: i-TTL balanced fill flashFill flash implies that the flash is not the main light source or, said in other words, that ambient light is stronger that flash light. Unfortunately, although understandably, i-TTL balanced fill flash is the flash mode Nikon cameras and flashes use by default.

One of the important things we've learnt is that flash is a nearly instantaneous light source whilst ambient light is continuousAs a consequence, you can adjust the ratio a/f of their contribution, at least to a certain degree. This fact allows you, for example, to get properly exposed subjects and properly exposed backgrounds, where properly means "according to your will". TTL metering makes this so easy because it automatically changes the flash power output to compensate a change in other parameters (such as aperture or ISO sensitivity) and get a properly exposed subject.
To summarize:
Ambient light exposure is controlled by the camera metering system, while flash exposure is controlled by the flash metering system.
Said in other words, they're decoupledThe only "problem" with TTL metering is that novice photographers are often unaware of it, and wonder what's going on when results aren't as expected or when they're ready to make a step forward and get more creative.

To understand the different nature of the two situations and the kind of issues you may run into, let's make a quick summary of what we've seen in the previous article, trying to distinguish between them.

Flash is the primary light source
When flash is the primary light source, things are pretty easy. If you want your subject to standout over an underexposed background, just reduce the contribution of ambient light (for example, reducing the shutter speed) and the ratio a/f will decrease as well. On the contrary, you can increase the contribution of ambient light and the ratio a/f (for example, lowering the shutter speed or raising the ISO sensitivity) if you want to get a more exposed background.

Ambient light is the primary light source
When ambient light is the primary light source, flash can be used to balance shadows of a poorly lit subject and this technique is usually called fill flashThe camera will be set to correctly expose the (lighter) foreground and the flash metering system will fire the flash at the correct power to correctly expose the subject. You could argue that the sum of the two light source could eventually overexpose the subject: it's true, and that's one of the aspects that the Nikon CLS system takes care of.

In Nikon's jargon, TTL (regular TTL) and TTL-BL indicate how the metering systems of your camera and your flash will "react" to the lighting conditions you're shooting in. Unfortunately, the difference between the two systems is not well understood by many users and I recognize that Nikon is not making its best to clarify the differences between the two modes in its manuals.

As we've seen, common exposure parameters may have different effects on different kind of light sources and on the lighting conditions you're shooting in. If you shoot in regular TTL, you can separately manage the two exposures (ambient and flash) and get the results you want.

In TTL-BL mode, the metering systems will assume you want to balance the two exposures. Basically, you're telling your camera to assume that the subject is darker than the background. Nikon CLS has been improving over the years and I do recognize that you can get great results even when blindly shooting in TTL-BL all the time. However, you may sometimes get weird results when shooting TTL-BL and not meeting its assumptions, and it's important to understand why.

In the next sections, we will quickly recap how TTL and TTL-BL modes work, the assumptions the metering systems make and the decisions they take. Since I haven't found yet conclusive official documentation about the Nikon CLS internals, please take all of this with a grain of salt.

TTL flash

When using the flash in TTL mode, you're basically telling your camera metering systems to independently manage the two exposures: ambient light and flash light will be metered separately and no (or little) compensation logic will be applied.

The behaviour of the TTL metering system isn't always intuitive and, once again, it is not properly documented. When using this mode, the two metering systems will meter ambient light and flash light independently.

This fact, as detailed in the previous tutorial, may lead to overexposure: if both metering systems are calculating a "correct exposure", if the lighting conditions and the scene characteristics are such that the two sources of light are not negligible, at least in a certain area of the scene (such as the very subject), then the two "correct exposures" will sum up and this may lead to a 1 stop overexposure in that area.

But besides these generic problems, Nikon CLS' behaviour may introduce new issues. Recent cameras, for example, may try to avoid overexposure risks by automatically dialing a negative exposure compensation in automatic and semi-automatic modes. This reduction seems to be somehow proportional to the intensity of the ambient light. That's why you may sometimes get a background darker than expected, especially when ambient light is very bright. This issue clarifies why TTL flash may not be the best mode to use for flash fill, especially when the scene is bright.

Another very important aspect of how the flash metering system works is: which part of the scene does it meter? It comes out that it meters the center of the frame. If your subject is not in the center when shooting, the flash metering system will be deceived and you may end up with an incorrectly exposed subject. This is the reason why Nikon CLS has got a flash value lock feature (FV lock) that we will see in the following sections.

Ultimately, in all the cases when you don't need TTL-BL (see below), you should switch to TTL. The quickest way to do that with Nikon cameras is selecting the spot metering system. You will then have total control over your photo and, following the advices of the previous tutorial, you will be able to get very good results, especially being able to tune the relatively intensity of ambient and flash lights in your shots: using the camera common exposure settings (ISO sensitivity, aperture and shutter speed) you will tune how much ambient light is detected by the sensor and using flash exposure compensation you will tune how much flash light will light your subject.

TTL-BL flash

Fill flash is a technique in which you use a flash to "fill" the shadows in the subject when the ambient light is brighter than the subject itself. For example, if you shoot a backlit subject, such a person in front of a bright sky or a window, you may need to fill the shadows in the subject face using a flash. This is the use case Nikon invented TTL-BL for: TTL-BL is meant to balance ambient light with flash and get a properly exposed and balanced background and foreground.

TTL-BL is the mode used by default (unless spot metering is used) with both the pop-up flash and hot-shoe mounted flash units compatible with Nikon CLS. As stated in the introduction, modern TTL-BL flash implementations work really good even when using them when flash is the primary light source. However, you may get unexpected results at times; that's why you should learn about both flash modes and learn to choose and use the more suitable depending on the shoot you're taking.

When using TTL-BL, the two metering systems will coordinate together in order to achieve the desired balancing of ambient and flash light. Roughly speaking, when using TTL-BL, the two systems meter the light and exchange the information required in order for the flash to be fired at the power that will achieve the balance. Once again, though, the camera will set its parameters as if the flash wasn't usedinstead, the flash metering system will lower the flash output at the desired level, taking into account the intensity of the ambient light. If the subject is not darker than the background, then, you will get an overexposed subject.

Flash exposure compensation can be used to fine tune the flash power output even when you shoot TTL-BL. Very often, in fact, you'd rather reduce the flash power output in order for your subject not to stand out too much in the shot or to achieve more creative moods.

Hopefully, it's now clear that the rationale behind TTL-BL flash is balancing a darker subject against a brighter background. No matter how smart your metering system might be, if you're not shooting under this assumption, you should switch to TTL instead.

Aperture priority mode in a bright ambient
As already seen in the previous tutorial, extra care must be taken when using aperture priority mode with a flash, especially in bright ambient light. In fact, aperture has a direct effect on both ambient light and flash light reaching the sensor and, above all, on the flash power output required to correctly light the subject. If you recall the definition of guide number, your flash will be able to properly light a subject at a certain distance (see PDF version for more details).

Why does this fact matters so much? Because if the light gets brighter, or if the ISO sensitivity being used is increased, then the shutter speed selected by the camera will be increased to compensate for it. But there's a maximum shutter speed that can be used when using a flash (wnless your camera can use high speed flash sync) which can be as slow as 1/200 s. If the correct exposure is given by a shutter speed faster than this value, the camera won't be able to select it and you'll get an overexposed shot.

In bright light, such as when shooting in sunlight, that's a boundary that you'll hit very soon: the sunny 16 rules gives:
i = 100
s = 1/125
a = f/16

An aperture = f/16 is right at the limit. If you open it one stop, you'll get
i = 100
s = 1/250
a = f/11
and you've just hit the maximum shutter speed for flash sync.

I bet many people won't be usually using aperture priority mode at a = f/11 (or smaller) at they'll surely get an overexposed shot.

On the other hand, if you're aware of what's going on (your camera meter will indicate the overexposure) and close down your aperture, you may soon get your flash out of range. Professional speedlights (such as Nikon SB-910) have guide numbers around 34 meters which, at = f/16, give a maximum distance r from the subject of approximately 2 meters:
r = g/a = 34/16

Clearly, a very short flash range.

Why manual mode is a good choice when using TTL flash

There are several reasons why manual mode is a good choice when using a flash, especially in TTL mode.

Manual is not that challenging
The first one is that manual mode is not that challenging when shooting with a flash. The reasons are manifold. First of all, as we've stressed several times, the camera metering system pretty much ignores the flash, even more when used in TTL mode. As a consequence, manual mode lets you freely use the camera metering system to quickly evaluate ambient light conditions and achieve the effect you want. On the other hand, the flash metering system will do its job and will properly expose your subject.

The flash freezes the movement
The nearly instantaneous burst of light emitted by the flash will freeze the movement of your subject and, in a reasonable settings range, you won't have to worry too much about shutter speeds and about getting a blurred subject. If you remember what we've seen in the other tutorial, the shutter speed usually has no effect on the amount of flash light reaching the sensor. This is a "degree of freedom" when using manual mode with flash: you can set ISO and aperture according to the flash power output needs you have and then using slow shutter speeds, even when no tripod is used. In fact, you may get interesting and creative results: when using wide apertures a slightly blur in the background will be nearly indistinguishable with shallow depths of fields.

No need to use "slow sync"
When using automatic or semi automatic modes with a flash, cameras usually limit the shutter speed to a minimum which is usually around 1/60 s. Such a speed is often insufficient for the sensor to gather sufficient ambient light and you get the typical "white ghost" (your subject) over a dark background. To override this behaviour, you need to choose the slow sync mode: your camera will then choose slower shutter speeds.

But why? I think the reasoning behind that behaviour is that the camera prevents you from getting some ghosting in your shot. 1/60 s is sufficiently fast a speed for freezing a standing still subject when shooting without a tripod. It's sort of an "error prevention" mechanism you can override if want to.

However, in the previous section we've seen you can use the instantaneous flash light to freeze your subject movement when using slow shutter speeds. Should you get unacceptable ghosting, just increase your shutter speed in manual mode and you're done.

Use flash exposure compensation and exposure compensation interchangeably
This is an advantage in terms of ergonomics I like when using manual mode with TTL flash. As we've seen, photographers can use two mechanisms to compensate exposure: exposure compensation and flash exposure compensationThe former acts on both light sources and effectively changes camera settings to accordingly reduce ambient light and flash power output. The latter acts only on flash power output, and is commonly used to fine tune the ratio between flash light on the subject and ambient exposure.

But what happens when you're using manual mode? When using manual mode, exposure compensation just changes the value shown by your camera meter, but effectively has no effect on camera settings (beware that you must also disable auto ISO). As a consequence, exposure compensation will only affect flash power output and will give a result similar to what you'd obtain using flash exposure compensation instead.

Flash value lock (FV lock)

No matter which flash mode you're using, the flash metering system always meters the centre of the frame. This is a very important thing to know if you want to get predictable results when shooting with a flash.

The problem is somewhat analogous to what happens when you set a parameter and then recomposeMost photographers are aware of the risks of recomposing when using some automatisms such as exposure meters and autofocus systems. Unless you tell the camera somehow what your subject is, you won't get predictable results.

The same thing happens with the flash exposure metering system. As it measures reflected flash light at the centre of the frame, when your subject is not in the centre, its exposure will likely be incorrect. This problem can be amplified by the fact that very often, after recomposing, the centre of the frame contains a farther background or a nearer foreground object. In either case, the flash metering system will be deceived and, for the effect of the inverse-square law (see Flash exposure tutorial: the basics), its reading can greatly differ from the correct one. As a consequence, you may get strongly overexposed or underexposed subjects.

Similarly to what happens with exposure lock and focus lock, a new kind of lock is provided: flash value lockUsing flash value lock when pointing at your subject lets your camera meter a flash burst and lock the flash power output level. You can, then, recompose your shot and get the proper flash output. Furthermore, while the flash value is locked, you will be able to take a burst of photographs using exactly the same flash power output obtaining consistent results.

Depending on your camera settings, the locked flash value will be retained until
  • you unlock it, pushing FV lock again,
  • the metering system timeout elapses (by default, it's a few seconds on most cameras),
  • the camera is turned off.

For this reason, check your camera viewfinder and be sure the lock hasn't be released before taking the picture.

Using the FV lock burst at your advantage
I find that the FV lock flash burst can also be useful to "prepare" your subjects' eyes to the main flash bursts. After the lock burst, you can have your subject blink their eyes and get used to it. Then, you can take the shot and reduce the chances someone has blinked just when the main flash burst is emitted.


Here are some examples to better understand how we can take advantage of both metering systems, of Nikon CLS technology and, thus, of our flash.

TTL-BL fill flash}

First of all, let's see an example of how we can use Nikon TTL-BL to get a shot with a well-balanced subject. The subject in the following figure was partially in the shadow of a palm tree and was strongly backlit. Since ambient light is the main source of light and our subject was darker (at least partially), we needed some fill-flash. Hence, I switched my camera in matrix metering mode, the flash in TTL-BL mode, I locked the flash value pointing at my subject and took the shot. Since ambient light was very strong and strong reflections were coming from the pool, I dialed an exposure compensation of -0.7 EV. The result is a well balanced image with a properly exposed background and the shadows in my subjects' faces partially lifted by the filling flash. In this case, I'd probably dial in another -0.7 EV to the flash compensation in order for my subjects not to ``pop out'', but that's just a matter of taste.

Matrix metering, TTL-BL, Exposure compensation: −0.7 EV 

TTL flash in manual mode
In the following figure we can see what happens turning on the flash and taking the photo using the settings suggested by the camera metering system in matrix metering mode and flash in TTL-BL.

Matrix metering, TTL-BL    

In this case, the ambient light was moderately strong, even if were in the shadow, and the subject was as lit as most of the background (excluding the sky). The poor child is much too lit: he's popping out the photo as if it were a ghost. Also, that background is ugly, so that we could take advantage of manual mode and TTL flash in order to darken it a bit and lower the ratio $a/f$ between ambient light and flash light.

In the following figure you can see the result of shooting in manual mode with flash in TTL mode locked onto our subject.

Manual mode, TTL, f/7.1, 1/125 s., ISO 320 

With the specified parameters (f/7.1, 1/125 s., ISO 320) the camera was metering an underexposure of almost -1 EV, so that the background would be 1 stop darker. On the other hand, the flash was locked onto the subject and flash compensation was set to -2/3 EV. The result is a dark background and a brighter subject, although not as bright as before because of the negative flash compensation I dialed in.

Why? Once again, just a matter of taste. Most of the times I prefer reducing the flash output to reduce the "ghost" effect you get when flash output is too strong. In case you need more output, just change the compensation, that's part of the beauty of Nikon CLS in manual mode.

In the previous example we wanted to achieve a darker background and we used the camera in manual mode with flash in TTL mode to modify the ratio between ambient and flash light a/f accordingly. In the following figure we use the same technique to \emph{increase} the a/f ratio. Since I wanted a bright background for this photo and had no other lighting equipment with me, I put the subjects in front of a white wall and lit by artificial light while leaving them on partial shade. On the one hand, I set the camera parameters to slightly overexpose the wall and get a bright white background and on the other hand I locked the flash value on the baby's face to properly expose it with a little fill.

Manual mode, TTL, f/7.1, 1/30 s., ISO 400 

Freezing action
In the following figure you can see how we can take advantage of the manual mode and the flash TTL mode to get:
  • a not-so-dark background,
  • a properly lit subject and
  • frozen action.

The child was moving in a low light situation and I couldn't have frozen the action without the flash without severe underexposure.On the other hand, manual mode lets us use shutter speeds as low as we want to gather the desired amount of ambient light: in this case, 1/10 s. was enough to get background light spots sufficiently visible. On the other hand, 1/10 s. is too slow a speed to properly freeze the movement of a human being at a focal length of approximately 60 mm, let alone a child who's playing. The flash burst duration, on the other hand, is much shorter (it depends on the flash power but it's approximately 1/1000 s.) and it can freeze action quite effectively. In fact, although you can still see a ghosting effect around the child's arms and shoulders, the flash has frozen the child's action pretty well.

Manual mode, TTL, f/8.0, 1/10 s., ISO 1600 

Since background lights were very dim, I had to push the sensitivity up to ISO 1600 to be able to properly expose them at a shutter speed of 1/10 s. Of course, I could have use lower shutter speeds, but the resulting ghosting effect would have been too strong to be acceptable.

Nikon Creative Lighting System Tutorial: The Basics v. 1.1 (PDF)