This posting is going to seem a little dry, and overly techy … but it pertains to questions that I am often asked.  So hang in there and let’s go over some of the basics and see how it all fits together.

There are two distinct ways in which flash is handled.  Manual flash or Auto / TTL flash.
(For the purposes of the explanation here, Auto and TTL flash can be grouped together wrt D-SLRs.)

.

.

Manual flash

Firstly, flash could simply be a constant amount of light that is emitted from the flashgun.  In the case of manual flash, there is NO control by the flashgun or camera, over the intensity or duration of the pulse of light from the flash unit.

This is manual flash.  Photographers can control the output of their flashguns by adjusting the settings in fractions of the maximum possible output, eg, ¼ power, 1/16th power.   It should be obvious that the absolute value of ¼ power will vary from flashgun to flash gun, as each model and make of flashgun has a different maximum power.  Manual flash exposure is most easily measured by a handheld flashmeter.

With manual flash,  four things control the exposure metering of our subject:
 - the actual output level from the flashgun, (ie the ratio of the full power),
 - distance from our light source to the subject,
 - aperture,
 - ISO.

Any of these four things can be used to control the amount of light falling on your subject.

.

.

TTL flash

The second way to control flash exposure, is as an automatically controlled flash burst. This flash output can either be controlled by the flashgun itself (usually called Auto mode), or by the camera in conjunction with the camera’s metering system (usually called TTL flash).  When the flash is controlled by the camera, as then measured as the amount of light coming through the camera’s lens, it is called Through-The-Lens flash metering, (hence, TTL flash.)

For most purposes in understanding the basics of flash exposure with D-SLRs, there is little difference between Auto flash and TTL flash.  So, as mentioned earlier, for simplicity’s sake here, we could group Auto flash and TTL flash together.  (There are differences with pre-digital cameras and older flashguns, in how Auto flashguns interface with the cameras, and then the explanation is slightly more complex.)

Anyway, with  TTL flash, the flash output is varied and controlled by the camera’s metering system. This means that for a certain range, our chosen aperture or ISO, or distance to our subject, does not influence our TTL flash exposure.

This is such a crucial point to understand about TTL flash, that I want to mention it again for emphasis. 

With TTL flash, our chosen aperture or ISO (within a certain usable range), doesn’t affect our exposure - and in a sense becomes transparent to our exposure metering. What does affect our exposure, is the reflectivity of our subject, and how large our subject appears within our frame.

This is something that is difficult to comprehend at first, but is easily verifiable with your D-SLR.  There, at home, you can photograph any subject in your immediate surroundings using a TTL capable flashgun.  You can change your aperture from f4 to f5.6 to f8 … and your exposures should look the same.  Similarly if you changed your ISO, your TTL flash exposures should look the same.  The reason for this is that your camera and flashgun’s TTL flash metering system takes care of the basic flash exposure … and can do this within a certain range of chosen apertures and ISO settings.  Your flashgun will emit more or less light as required for correct exposure, dependent on your settings … but your exposures should appear similar.

So, looking back at the four things which affect manual flash, you’ll notice that none of these, not distance, nor aperure, nor ISO setting, seem to have an effect on our flash exposure. (Within reasonable range of course.) 

And it is in this, that the huge difference arises beween approaching metering for manual flash, and TTL flash metering.

The only way to control TTL flash metering, is with flash exposure compensation.  You could control manual flash exposure with any of the four variables mentioned earlier on, but with TTL flash, you would have to change your flash exposure compensation.

.

.

In these differences, the individual strengths of using manual flash, or using TTL flash, can be seen.  And this will most likely affect which of the two kinds of flash lighting we end up using. 

With manual flash, since the flash is a specific level, our subject’s reflectivity or our choice of composition (ie, how we frame our subject) has absolutely no impact on our metering.

This is a hugely important aspect of manual flash.  Once we have our lights set up at a specific distance, and determined our aperture and ISO … then the subject’s reflectivity (ie, how much lighter tones or darker tones there are), have NO effect on our flash exposure.

However, with TTL flash, the subject’s reflectivity / tonality, and our choice of composition, WILL affect our flash exposure.   And hence, we often need to control our flash exposure with the flash exposure compensation. 

More on Flash exposure compensation

.

.

The concepts explained in this section are so essential to our further understanding in flash photography, that I would strongly suggest re-reading this section until it makes sense.  Also check other webpages or books on this topic.  It is also important to your own understanding of flash photography, to try and figure this out with your own camera and flashgun. 

In  thoroughly grasping of the differences between manual flash and TTL flash, our approach to balancing flash with available light will become so much easier.  But that is subject matter for a future series of topics.

There is a fundamental principle in lighting :
the larger your light source, the softer your light.

Using any of the myriad of flash modifiers that are on offer, helps in achieving that - spreading the light from the on-camera Speedlight much wider, thereby creating softer light that direct flash would’ve given.  However, (and this is a big however), these flash modifiers also throw light forward.  Ultimately all flash modifiers do the same thing - they disperse a lot of light around the room, while throwing some measure of light directly forward to lift shadows under the eyes and bring a sparkle to the eyes.

That is a huge step up from using direct flash - (or poorly bounced flash.. ie, flash at 45′ or 60′ forward) - but won’t be as good as directional light.  Directional light falls onto your subject from a specific angle.  This direction can very often be carefully chosen even when you use an on-camera flash indoors.

The image at the top is from a wedding I photographed a few weeks ago.
I touched up the WB in RAW, and that’s that. Simple, and it looks just great.
The light is soft, and the baby wasn’t bothered by any direct flash … since there was NO direct flash at all.

[edited to add on Dec 15 : I originally  used the term 'scrim'. 
I have since changed it for the more correct description of 'half-snoot'.]

The way I achieve directional light from my flash is by adding what is in effect, a half-snoot to my on-camera flash.  The half-snoot will partially block the light, and also direct it.  I also very often when shooting in tungsten light, use a filter on my flash to bring the cold light of the flash closer to the warmer tones of tungsten light. 

I simply stick a piece of gel over the head of my Speedlight with some gaffer’s tape. It is low-tech, but it works.

By gelling my flash for tungsten I change the grungy orange backgrounds to a more pleasing warm tone.

The gel used in the image of the baby above, was full CTO.  I keep my camera’s WB to Tungsten.  And in post-production I fine-tune the WB, since by bouncing my flash, it picks up an additional colour from the walls and ceiling.  The gel shown in the illustration here is 1/2 CTS.  I then keep my WB of my camera to 3800 K, which is still much closer to Tungsten, than the 5400K of flash.

The black half-snoot that I add to my Speedlight is just as simple.  It’s a piece of thin black foam bought from an arts store, and then cut smaller.

I keep the piece of black foam tied to my Speedlight with a hair band that I stole from my daughter.  Yup, low-tech and simple … and it works!

This piece of black foam around my Speedlight has two advantages:
1. Directional light.  I can now much more precisely direct where I want my light to come from.
2. Less annoying to others.  In turning my flash to the side or to point behind me, I would risk blasting other people directly in the face with flash. This piece of black foam keeps that from happening.  I now direct my flash over people’s heads, and no flash hits people directly in the eyes.

And thatis the flash modifier I most often use. Total cost is less than $2.oo However, if you feel the need to splash some money on hardware that is custom-made for you, send $40 to my Paypal account.  I will cut a piece of foam for you, add the hair band, and even sign it with silver pen. Oh, go on, you know you want one. It will surely become a collectible.

The additional advantage with this light modifier is that you retain the cool all-black stealthy ninja-photographer look. This is Elisabeth, who often works with me as a 2nd photographer. She’s cool.

.

.

The only other light modifier that I use, is a Stofen cup.  This I use when I am in a situation where I do need light thrown directly forward. I also use it in rooms where the ceiling is too high to successfully shoot with the black half-snoot.

However, I still want a measure of control over where I direct my light.  Therefore I cut a hole in the top of the Stofen so that the majority of light can still be thrown in a direction of my choice, instead of being scattered all around. (I do keep a spare Stofen on hand that is unblemished, as well as an Amber coloured Stofen.)

To be able to flip the Stofen in and out of position, I keep it down with a piece of gaffer’s tape.  So if I want to take a shot without it, I simply flip it over.

.

.

And there they are - the only light modifiers that I use when I use on-camera Speedlights.
In my weddings and portrait work,  I often do use off-camera lighting, whether Speedlights or Q-flashes.  But with on-camera strobes, these are the only light modifiers I use.  And they are all I need.

Several photographers have asked me how in much of my photography has flash been used.  And I would say that about 80% of my work when I photograph weddings and environmental portraits, has flash of some kind.

Whether nearly all flash :

(a single on-camera speedlight, gelled to tungsten, and bounced over my left shoulder,
1/25th @ f4 @ 1600 iso)

.

(Once again, a single on-camera speedlight, gelled to tungsten, and bounced over my left shoulder,
1/100th @ f2.8 @ 2000 iso)

.
… or a healthy mixture of flash and available light …
(both these next images also had the flash gelled for tungsten, but I  
dialled down appropriately to match ambient light the way I wanted to.
I also took care in controlling the direction of my flash.)

.
…. or just a touch of fill-flash.
(juuuuust enough to lift the shadows from her eyes, or lift the contrast levels … so that I have to spend very little time in post-production, editing these images.)

(flash directly forward, but dialled down to -3EV to enable just a touch of fill-flash.
1/250th @ f4.5 @ 160 ISO)

.

(flash once again directly forward, but dialled down to -2EV to enable just a touch of fill-flash.
1/200th @ f5.0 @ 200 ISO)

.
Or sometimes only the available light when the existing light is just perfect.

(1/250th @ f4 @ 640 ISO)

.

(1/320th @ f2 @ 640 ISO)

.
Whatever needs to be done to create a flattering image.

It isn’t just a about sense of aesthetics:
1. Reducing my post-production time is a strong consideration whether I should use flash or not. At best, I only want to touch up WB and exposure and still have images that look great.
2. I want to give the bride the best photos on the day that I can get, without clinging to any particular rigid ideology.

.
Oh, and can we just dispell one silly notion here … that being able to shoot at a high ISO enables you not to use flash. The decision to use alternate lighting (whether flash or video lights or reflectors) has more to do with the quality of light, than the light levels. Those are two distinctly different aspects .. quality and quantity of light.

.
Back to the topic … In portaiture and wedding photography, my intention though is always to create flattering images, combined with a story-telling approach, and one that is not intrusive.  And within certain contraints, I am always considering the direction and intensity of the flash that I use.  And I am always aware of the ambient light levels, and the quality of the ambient light … and this shapes my decision as to how I use flash, and how much.  And thereby using juuuust enough flash.