Before we begin to specifically talk about flash, we should have a short discussion about light in general.  There are three basic principles of light you should consider in any situation.

First, let’s talk about the color of light.   You probably know about white balance but we should include it in the discussion.  It can be particularly important with flash because of the possibility of mixed colors of light.  Your flash is probably daylight balanced.  Most are.  This means the color of light coming from the flash is probably around 5,500 Kelvin.  However, what color is the ambient light where you are shooting.  Chances are that it’s not the same as the flash.  However, this is probably OK as long as it’s not too much different.  A big color range in light could make your image look a little strange.  If this happens you may want to put a gel on the light to match the ambient and then adjust your camera to the correct white balance.

The second principle we should talk about is the direction of the light.  Where the light is coming from can make a huge impact on the mood of the image.  This is true whether it is coming from a natural source or from your flash.  The difference is that you can control the flash a little easier.  For example, backlight will put a glow on the subject and cause it to be somewhat of a silhouette.  This look has a totally different feel to it than if it were lit any other way.  Side lit images will give you more contrast and tend to be more dramatic.  For what I call traditional shots where you want to see all of the front of the subject you typically want some sort of front lighting.  Most of the time you want to avoid having the light in the same direction of the camera.  When you place the light in the same direction of the camera you basically remove the possibility of shadows.  Shadows show depth and without depth the image is flat.

Source:  Pixelogist

Finally, we should talk about the quality of light.  Quality of light is measured by how quickly the transition from highlight to shadow is.  When the transition is very short the light is very harsh, but when the transition is very gradual the light is considered very soft.  So if you’re looking to soften the light how can you do it?  There are basically two things that affect the intensity of the light.  First, is the size of the light source.  The bigger the light source in relation to the subject the softer the effect.  So if you move the light closer to the subject, it becomes much bigger and, therefore, softer.  Second, is diffusing the light.  This can be done by placing diffusers in front of the light, but can also be done by bouncing the light.  Bouncing the light is the primary method used by natural light photographers.  They often use buildings, or walls, or concrete sidewalks to bounce light from the sun.

Now that we have talked about light in general, let’s talk about some topics that are particularly important when shooting with flash.  First, I think it’s important to understand how camera settings affect the light for determining exposure.

One thing you should always keep in mind is that Shutter Speed controls the ambient light while Aperture controls the light of the flash.  It can be a difficult concept to get at first, but it really makes sense when you think about it.  Consider that the light from the flash only travels so far and when you have it correctly measured on the subject the background tends to be unaffected.  So changing the power of the flash to better light the background really doesn’t make sense.  The flash also only last for a specific duration, so changing the speed of the shutter doesn’t really play a part in how much it exposes the image.  Conversely, changing the shutter speed to change the light on the subject doesn’t make sense because while it will change the exposure on the subject it will also completely change the exposure of the background where the light from the flash can’t travel.  This is why you first expose for the background and then set the light to match that exposure on your subject.

Although shutter speed doesn’t really affect the light on the subject very much you need to pay attention to it when shooting with flash.  This is because your camera has a specific shutter speed that you should use when using a flash.  You should use this speed because the camera and flash must be in sync so that the flash fires when the shutter is open and exposing the sensor.  If the flash fires at a different time the sensor cannot see the light and will not be able to expose the image properly.  The term used to describe this is called “Sync Speed”.  In short, it is the fastest shutter speed you can set your camera and have the flash expose the sensor properly.

How does sync speed work?  To understand this you must understand how shutters work.  Shutters work on a system of curtains.  Think of a show on stage.  The curtain opens, the show occurs, and then the curtain closes.  Unique to the camera, however, is that we have two curtains and they both move in the same direction.  Using the show analogy again, assume the curtain opens from top to bottom.  When the front curtain opens completely, the show begins.  After the show, a second curtain closes from top to bottom and the exposure is complete.   When your camera shutter speed is very fast, these curtains are very close together.  The front curtain is not opened completely before the rear curtain begins to close.  This is the only way the camera can expose the sensor properly.  Imagine a shutter speed of 1/1000.  When the first part of the sensor has been exposed for 1/1000 the rear curtain must begin to close.  This is the only way the sensor can be exposed properly (see image below).

Source:  Illustration used with permission from B&H Photo

There are also two different ways that the camera can be set for syncing with the flash.  What we have talked about is what is considered “front curtain” sync.  This is where the flash fires when the front curtain is completely open.  Most cameras default with this setting, but have the ability for you to change it to “rear curtain sync”.  With this setting the flash fires just before the rear curtain begins to close.  So what’s the difference?  If you are shooting at the camera’s sync speed there is really no difference because the rear curtain will begin to close as soon as the front curtain has completed the opening process.  However, while you can’t shoot faster than the camera’s sync speed you can shoot slower (called “dragging the shutter”).  This is because the light from the flash will be exposed on the sensor as long as your shutter speed is at or slower than the sync speed.  If you drag the shutter with flash and use front curtain sync any motion in the image will have motion blur in front rather than behind the action.  Rear curtain sync will give you a more natural trail of motion when you drag the shutter.  This may be the only time you should consider changing the setting to rear curtain sync.

While shooting with flash may limit your shutter speed there are ways to over come this.  When shooting with flash the conditions may be that the background light is too bright, or you may want to record something that is in motion.  Your sync speed may be too slow to allow you to get the desired exposure.  One way to over come this is with the use of Neutral Density (ND) filters.  These filters are placed over the end of the lens to darken the scene.  These are kind of like sunglasses for your camera.  They come in different stop increments and are designed specifically to slow your shutter speed.   So if your sync speed is 1/250 and you want to shoot at 1/1000 your can effectively do this by putting a 2-stop ND filter on the end of the lens.  It darkens the scene by two stops allowing you to shoot at 1/250 and get the exposure you want. 

Source:  Wikipedia

This method is good for darkening the scene, but not for shooting action with a flash.  For that you need to use “High Speed Sync” (HSS).  This is a function of your camera and your flash.  Both must be capable.  Most high-end cameras these days allow you to shoot with HSS, but not all flashes do.  It’s usually an extra feature you have to pay for.  However, having this capability can be very useful.

When you set this on the camera, you don’t ever have to change it again.  Even if you are not shooting with HSS.  However, on the flash it’s usually one of the modes you will select each time you shoot.  This is good because HSS has a downside that you must consider.  It takes a lot of power.  This means two things.  First, you will drain your battery faster than usual.  Second, the output will not be as strong.  So if you are shooting through a modifier, you may not have enough power for a proper exposure.  Especially if you are using speed lights.

How does HSS work?  It’s pretty simple from a conceptual perspective, but very complicated from an engineering perspective.  The camera and flash must be synchronized so that the flash can fire multiple times as the small slit between the front and rear curtains move across the sensor assuring a complete exposure to the flash.  The faster the shutter speed, the smaller the slit, and the more times the flash must fire.  These multiple flashes are imperceptible to the naked eye, but are seen as a slightly longer flash than normal.   These many flashes are the reason HSS takes so much power.       

So now that we know these techniques what’s better, speed lights or studio strobes?  Well it depends.  Each has its own advantages and disadvantages.  The biggest advantage for speed lights is their mobility.  They are obviously smaller and lighter.  Not only are they more mobile, but they are getting so smart and powerful.  Even with more power their biggest downside is the lack of power when compared to studio strobes.  Battery life for speed lights is not near as great and the output is much less.  If you are in a situation where you need extra light you may have to use multiple speed lights to get the same exposure of a studio strobe.  There are brackets you can purchase for holding several speed lights on a single stand designed to give you a more powerful light source.  This may be especially necessary if you are using a modifier that reduces the light by 1, 2, or maybe 3 stops.  Using several speed lights to accomplish the task may actually be more expensive than using the studio strobes.

Studio strobes on the other hand are much more powerful and usually do not use as much battery as speed lights.  Many studio strobes also have HSS capability allowing you to shoot action images.  They also may or may not have built in battery packs and radio triggers that you would have to purchase separately and setup separately on location.  The major downside to studio strobes is the size and weight.  Carrying these around on location can be a huge burden.  Especially if you do not have an assistant.

Source:  Photography-on-the.net

Finally, let’s talk about how to set the power of your lights.  It doesn’t matter what kind of lights you have the amount of output is determined by the power setting on the flash.  This setting is incremented by stops.  As you probably know each stop of exposure either doubles or halves the amount of light for your exposure.  With that in mind the power setting on your flash goes from full power, to half power, to quarter power, to eighth power, etc..  Each increment is half or double the amount of light as the previous setting, depending on which direction you are changing the setting.

So if I take a shot and the flash is too much I can change the setting to cut the light in half.  Conversely, if there is not enough light I can double the light for a brighter exposure.  You can also move your light closer or farther from the subject, but that’s a discussion for another time.  Here we are assuming the light says stationary.

Is this the answer?  Just move the power up and down until I get something I like.  Well, you can do that, but there are a couple of ways to get it right the first time (or at least close to right the first time).  First, you can use a light meter.  Many photographers swear by them, while others never use them.  I always use them in a studio setting, but rarely use them on location.  Light meters are very accurate, but many photographers have used their lights so much they tend to know where to set the lights and the setting for the look they want.  Then they just tweak it a little or even bracket the shots.

The second way to get the light right the first time is to use the Guide Number for the flash (GN).  Using the GN makes it easy to get you very close to the proper exposure immediately.  Every flash has a guide number.  This number is an indication of the flash output under certain conditions.  These conditions include an ISO of 100, the flash set at full power, and if you are using a speed light (or a light that zooms) a specific zoom setting (35mm for my Nikons).  So if you are shooting under these conditions you can easily determine the distance the light needs to be from the subject and at what aperture to set your camera.  Here’s how it works.  Divide your GN by the distance from your subject and it gives you the proper aperture to set your camera with your flash set at full power.

Source:  Shortcourses.com

Now it’s pretty typical that you wouldn’t shoot at the exact conditions, but if you can keep the ISO on the camera and the zoom on the flash constant the rest is pretty easy.  For example, the GN on my Nikon SB-910 is 111.5.  So if I divide my GN by 10 feet I get 11.15 for an aperture of F-11.  So if I put my flash at 10 feet from the subject and at full power, zoomed at 35mm with my aperture at F-11 and ISO at 100, I should get a good exposure.  So what if I want to shoot at F-2.8.  I can do that a couple of ways, but one way is to leave everything the same except the flash power.  Changing my aperture from F-11 to F-2.8 is adding 4 stops of light.  If I change my flash power from full power to 1/16 power I am reducing the light from the flash by 4 stops.  Therefore, I should have exactly the same exposure with a much narrower depth of field.

Instead of changing the power on the flash to adjust the flash you could also move the light.  The Inverse Square Law (also a topic for another time) says that if you halve the distance of the light to the subject you pick up two stops of light.  Conversely, if you double the distance you lose two stops of light.  So in the example above I could have moved the light from 10 feet of the subject to 20 feet from the subject and lost two stops and then lowered the power by two stops to ¼ power and maintained the same exposure.

One thing to note about this method is that when you use modifiers (like soft boxes or umbrellas) you will lose 1 – 3 stops of light depending on the number of diffusers (soft boxes sometimes use both an inner baffle and an outer diffuser).   Also if your are bouncing light off of a wall or a reflector you should estimate the distance to the bounce object and then to the subject and then subtract about a stop of light for diffusion.

All these numbers!!!  I hate Math!!!  As you may have expected there is an alternative.  One of the most popular ways of shooting with flash, especially with speed lights is with using Through the Lens (TTL).  This is an automatic mode where the flash determines the amount of flash to expose on the subject.

To use TTL both your camera and flash must support and be set to this method.  Most flashes now support TTL, but the ones that do are usually a little more expensive.  You probably won’t find it on low dollar budget flashes.  You must also have a triggering system (ex. Pocketwizard or Radio Poppers) that can send and receive the proper information to and from the flash.  Once you set your camera to TTL you don’t have to change it regardless of whether you are using it or not.  Then when you are ready, set your flash to the TTL mode.

Source:  Cloud9Design

So, how does it work?  When all of the settings for TTL are in place you only need to worry about your exposure settings on the camera.  You still must set your shutter speed to the sync speed or slower, but your ISO, flash zoom, and aperture can be whatever you want.  When you press the shutter your flash transmitter sends a signal to the flash for a preflash on the subject.  The flash fires a preflash that basically bounces a signal on to the subject and back to the flash indicating the amount of light on the subject.  The flash then fires the proper amount of light to light the subject depending on the aperture your camera is set on. 

The major problem with TTL is that the light falling on the subject is reflective, just like the meter in the camera.  Therefore, it doesn’t always get it right.  However, there is the capability to use a flash compensation.  This setting is much like the exposure compensation you have available when shooting in one of the Auto exposure modes (ex. Shutter Priority).  With this you can approach it much like shooting with manual flash.  Just let TTL get you close and then tweak the setting with the flash compensation.   

Source:  SLR Lounge

To sum up, how should you approach shooting with flash?  First, determine the look you are going for.  Lighting direction, lighting quality, and DOF are the things to think about first.  You really can’t think about camera or flash settings until you have determined these things.  Once you decide these things you can then worry about the mechanical side.

After determining look you want the first thing you should do is expose for the background.  Take a couple of shots and make sure the background is exposed the way you want.  This exposure should include the aperture setting you want and need for the flash.  The idea is to get the ambient light exposed the way you want and then use the flash to expose the subject to match.

If you’re using TTL there are no worries for setting the power on your flash, otherwise, you can use the GN number mentioned above for that.  Take some shots with the flash.  You should be very close to where you want to be.  Make some tweaks if necessary either by changing the flash power, or by using flash compensation if you are using TTL.  You should not have to change your shutter speed or ISO.  If you do the background exposure will change and you have already set it the way you want.