Camera Flash Tutorial (Part 2: Theory)

For part 1 go here: http://saritsblog.blogspot.com/2014/11/camera-flash-tutorial-part-1-terminology.html

Light Direction Matters
What's the difference between a mug shot and a flattering portrait?... Shadows!
How do you control shadows?... Light Direction!
Natural light doesn't stream directly from your eyes, bounce off the subject and get reflected back into your eyes. If it did, I'd get them checked, but also everything in the world would look flat and shadow-less to you. The fact that light usually illuminates a scene at a different angle from the direction you are looking at is what allows you to see shadows (which is what most people are accustom to seeing, aka "natural"). So what happens when you put the major light source (ie flash) right on top of your camera and use it to illuminate the scene?... You guessed it.. No shadows!

Making Shadows
To create shadows we have to make sure the direction of the light is different from the direction of the camera.
There are a 2 ways to do this:

1. Reflect the flash off something else and back onto the scene (aka "bounce flash")
2. Physically move the flash off and away from the camera (aka "off camera flash")

Bounce Flash
Bounce flash, although free (if you have a flash that can articulate) has a couple problems. First, there's not always a nice big wall or ceiling to bounce the flash off of. Second, whatever you use to bounce from better be neutral in color, otherwise your subject will look pretty funny. Third, since you can't really change the angle and distance of the wall/ceiling you have little control over exactly which part of the scene gets light. Its usually all or nothing. Finally, bouncing is a fairly inefficient use of your battery operated flash's power. Your flash is gonna have to use more power for each shot which translates into longer recycle times and fewer total number of pops.

Off Camera Flash
Off camera flash, however solves pretty much all the issues plagued by bounce flash. The only downside is you need more equipment. The simplest and cheapest approach, is to use a flash cord like this one:

Vello TTL-Off-Camera Flash Cord for Canon EOS - 3' (1 m)
I recommend that the cord you get support TTL which I will discuss later. This limits your flash to being within the distance of the flash cord. You can buy fairly long cords, but you will have to make sure the cord does not appear in the shot nor cause a tripping hazard. This makes long cords impractical for anything but studio type shots. A better approach is to use wireless flash.

Old Wireless Flash Technology
Until recently, the only type of wireless flash was one that utilized a coded sequence of flashes to communicate with and trigger remote flashes from a master flash unit. You will often hear terms like "optical master" or "optical slave" and that just means whether the flash can use these flashes to trigger other flashes (optical master) or if it can be triggered by an optical master (optical slave). However this optical technology has always been hit or miss. First-off, you need at least 2 flashes for this to work and the master flash has to be able to flash light on all the slave units optical sensors for this to work (i.e. direct line of sight). This severely constrains where and how you can place your flashes. Additionally, these optical sensors can be overwhelmed by direct sunlight preventing them from firing. All-in-all, the pain and price is probably more than many will bear.

RF Flash to the Rescue
Radio Frequency (RF) wireless flash is a whole new ballgame. Finally, you could put your flashes pretty much anywhere and the RF signal could penetrate or bounce around and reach the flash. You also didn't need a second flash, just to trigger the first one. A much cheaper RF flash controller mounted to your camera serves as the master trigger. RF triggers are much more reliable and completely immune to sunlight. RF systems are just so much better, there's really no reason to go back to optical. Even your old optical systems can be upgraded to RF which I'll describe later. Its also important to know that there are two types of RF systems: 1) RF Triggers and 2) RF Controllers w/ or w/o TTL. Triggers only do one thing, fire the flash. So you'll have to manually go over to the flash if you want to change something. Controllers allow you to change many if not all the flash settings remotely. The best controllers will allow TTL just like that flash cord I mentioned earlier.