Actually, my title is probably misleading. To determine when you need flash is subjective, until you post your image and then all the gurus will yell at you saying, “you should've used flash.”

In this short blog, I am going to just talk about one very important thing, and that is the perspective by which we approach the need to use flash.   We all know that photography is mostly about “light” and we know that there are basically two categories of light (Ambient and Flash (albeit from a strobe, a speed light or the addition of a constant light)).  So now, all we really have to concern ourselves with is which do we need to light the scene.

In normal photography, whether your light is sunlight, room light, or some other kind of light, already available, we can approach the scene from a standpoint of “managing EXISTING light.”  In Flash photography, your approach has to be different, because you are introducing another source of light into the scene.  With that, your new responsibility is the management of this new, additional, or foreign source of light and understanding its impact on your particular scene. 

Therefore, the real question is:  "how do I introduce this new light?"

This is the preception that determines how you're going to make your foreign light source either the dominant source, a blended source, or a Fill source.  With this paradigm of thought lingering in your professional mind, you have to decide how to apply your new light to the scene.  It will require an understanding of the complexity of the categories of light present in your scene; because you will be managing the two lights independently to produce the desired outcome.  Let that sink in for a second.  Your new responsibility is to manage both sources of light independently. 

So, if you remember that when you introduced Flash to the scene, your approach (using the same Exposure Triangle that you are now familiar with) has to be modified to account for the new equipment and/or the additional functions you must use for the light you are adding.  In normal photography, while managing existing light, your understanding of the Exposure Triangle allows you to use any combination of the values to control the primary source of light.  But, in Flash Photography the roles of the Exposure Triangle factors, take on a different meaning. (If you are not yet familiar with the Exposure Triangle, then stop and go back to other Tutorials to learn that before going forward.)

The ISO will control the overall brightness/darkness of the image; like a master exposure slider in your favorite Post Processing software.  This function is often the least manipulated setting.  The basic role follows the premise that if it is bright outside then you should use more of a native ISO (somewhere around 100); but if it is darker out, then you may want to increase the ISO.  So, this is simple:  Brighter lesser, darker more.

However, in contrast to what you know about Speed, it now has a slightly different role.  The new role of using Speed in Flash Photography has now shifted to controlling Only the existing light in the scene.  It will not impact the Flash output or power, but it is the primary tool for controlling the existing light.  (More on this with when we talk about High Speed Sync (HSS))   So, in order to be successful using flash, you must understand that distinct difference; you must understand the different roles Speed has in normal photography versus Flash Photography.  Most of what you know from the Exposure Triangle theories, changes here.  

Understanding the role of Speed versus Aperture is the key factor that empowers you to separate the control of Ambient Light from the Flash output.  This is highly important and requires your devoted attention.  As we've introduced two sets of lights into our scene, you must understand how to control those lights separately.  From hence forward, you must know that Speed will control the Ambient light and Aperture will control the Flash light. 

Now if that doesn't confuse you, let's add some additional things Aperture can control also.  As we remember from the Exposure Triangle, Aperture also controls Depth of Field (that area of Focus surrounding your subject).   In Flash Photography, Aperture maintains this role.  Aperture still controls Depth of Field (DOF) in the Image as it has always done, but its new role now is to primarily control the output of your new light source—the Flash...and, this is especially true in TTL (an automatic setting that meters the light passing through the lens to the sensor)  The Aperture still maintains its dominant role by controlling depth, but its other primary function it to control the output of flash power in your scene.   

So, when you approach your scene, you should first determine what DOF you want to use.  Once you establish your desired DOF,  then if using TTL, you do not have to worry about the Power output.  The communication between the camera and the flash based on Aperture, will determine the amount of light that will strike the subject from the flash and return to the camera.  Stop here and let that sink in.  If you are using TTL, then the Flash is practically in auto mode and its strength or output power is based on what you set for Aperture.  It will add just the amount of light to complement the existing lighting in the scene.  

Well, Perfect, right?  TTL handles adding light to your scene and usually manages this perfectly.  Now, if you are paying close attention, really close attention, then you have figured out that when using Flash in TTL it is really easy to set the camera mode to Aperture Priority.  In Aperture Priority, you set the Aperture and ISO, and the Camera sets the speed.  Well, if you also set your camera to Auto ISO, (A/ISO), then the camera will use your Aperture setting to expose the image, and it will have the power/authority to adjust ISO and Speed to get an accurate exposure.  

Well, Voila.  You can now begin to add Flash to your images by just controlling the DOF, while letting the camera make the remaining decisions on how much Flash to use.   Then you can focus on the Quality of Light, Direction of the Light, and the Temperature of the light...while the camera decides the power based on your Aperture setting.

After you have mastered this technique, to change the amount of light striking the subject from the Flash, you can make adjustments to Exposure Compensation (EC) or the Flash Exposure Compensation (FEC).

Note: It is important to note that this function is “coupled” together in TTL mode; because TTL is an automatic function that attempts to use your camera’s sensor as a meter-receptor.  I will not discuss those details, but there is a complete mathematical algorithm that is used to determine the exposure values.  In manual, Aperture still controls the light, but controlling the flash output is decoupled from an automatic setting and therefore a hand-held meter may be needed to measure the specific exposure value, which must then be set in the camera.

When you have a full grasp of this concept, then you will be better equipped to use the functions of TTL.   Shooting in TTL  is the equivalent of shooting in Automatic in normal photography; the communication between the camera and the Flash works to produce a correct flash exposure.   And, it works most of the time, but it relies heavily on your camera’s metering algorithm. (By now, you should be familiar with your camera's metering system, so I will not explain that.)

So now, let's do a brief recap of the camera's metering system.

Whether you have your camera set to Matrix, Center-weighted, or Spot metering—the mixture of Flash and Ambient light will heavily depend on that setting.  This can be confusing, since I already said that it is your responsibility to manage both categories of light, independently.  So, you must remember that the tools you have available to you are TTL or Manual; where TTL will use Matrix, Center-weighted or Spot metering to determine the flash out.  

Therefore, understanding the potential outcome and the variances that each can create in the scene is extremely important. 

Matrix and Center-weighted are more Flash controlled settings where the Ambient is averaged at various locations on the sensor where light strikes;  whereas Spot tends to allow the Photographer to make her own assessment of where the camera will measure the light, based on a single Spot in the scene—then it will automatically measure that spot.  So, for Matrix and Center-weight, it will attempt to measure the existing light and balance the potential Flash with the current ambient light.  For Spot, it will only measure at that Spot.  

The most important thing to note here is based on manufacturer and camera type, the Spot metering may be decoupled from the focus dot.  Whereas, if you place the focus dot in the corner 3rd of your composition, Spot metering can still measure the scene at the Center Spot in the viewfinder--causing a potentially undesired reading.  Therefore, if you are unaware that your camera decouples the Spot Metering from the Focus Point; TTL is most likely to give you a meter reading that is not what you intended. This is where TTL overall gets it wrong sometimes. 

Full manual flash, decouples this functions, allowing the Photographer to meter anywhere in the scene she wants.   She has full control over the output of flash.  This is where understanding the Inverse Square Law and the Power output settings of your Flash are paramount.

Sidebar:

In Flash photography, using Speed to control the ambient light has some limitations inherent in every style of camera.  To fully illuminate the sensor, when the flash fires the shutter blades must be fully open.  This is called, "sync speed." (Sync Speed is the maximum speed by which both shutters blades are fully open). 

In many cameras, that sync speed is 1/250 of a second; any amount of time faster than that, then the shutter blades begin to close, causing a gradient bar on your image.  This is undesireable.  So, as solution, some cameras and flash can be synced using a technique that allows the flash to pulsate its beam as the closing shutter blades move across the sensor.  This illuminates the sensor similar to how a copier or fax machine operates. This is called High Speed Sync (HSS).

HSS allows the photographer to achieve Speeds as high as 1/8000th of a second.  By pulsating the flash rapidly and consistently, such that as the shutters move across the sensors, it becomes evenly exposed by the pulsating light.  Now, since Speed controls the Ambient light, HSS allows photographers to shoot in bright daylight while still using shallow DOF or very wide open Apertures. 

Many seasoned photographers have also developed other tricks to overcome those limitations too (I will not discuss those now), but HSS is usually the easiest way to do it.

Note: When using HSS it is important to remember that the camera, the flash, and the triggering system must all be compatible for HSS use.

So the most important thing to remember when you use Flash photography is the Perspective of the Approach. You must remember that because you are introducing new light into the scene that the Exposure Triangle takes on a different role.  In addition, you must know and understand both the functions and limitations of your equipment.  "To Flash or Not to Flash," is really based on what you know about light and how you want to use it.

Next Time: Dramatic Lighting, Controlling Falloff, and Understanding Shadows.