What is TTL and i-TTL?

One of the fundamental differences that separate a DSLR from a typical point-and-shoot is what we call TTL. TTL means "through the lens", what you see in the viewfinder is the actual composition through the lens, its the picture you are going to make. On the other hand, when you use a point-and-shoot, you're often looking though a viewfinder that is placed adjacent to the lens. 

While the distance it it placed away from the lens is small, it will introduce errors in composition and distortion from what you are photographing. This is called a parallax error, as you can imagine, the closer the object, the greater the impact of a parallax error. Basically, you'll get a different picture than what you see. 

When the light enters your lens, it hits a mirror, a portion is sent to the focus and exposure sensor and a portion is bounce up into a prism that flips the image and is viewable through the viewfinder. This is TTL, its exactly what your lens is pointed at.

Now what is i-TTL? This is Nikon's smart flash metering (Canon has a similar system called e-TTL, though not as advanced). It's a system that uses exposure information that comes from the lens to set the proper flash power. It serves to balance ambient light with light from the flash. It will analyze the composition of the scene and in a fraction of a second, send out pulses of light to evaluate the entire scene. While this system is only compatible with flashes made from Nikon, its a powerful tool that all photographers should learn how to use. 

There may be some of us who love to fuss with the settings of our lights, but the majority of us would rather opt to spend this time shooting and spending time with our subjects. Within a few seconds you are able to dial-in flash settings for your shoot and not have to worry about them for the rest of the day. Its a simple, yet extremely powerful system! 

You can read more about Nikon i-TTL here and the Canon e-TTL here.

Happy shooting.