Wednesday, May 14, 2014

What is TTL Flash and do I need it?

In my last blog post I explained there are three things that affect the exposure when using flash.
  • Flash Power—How bright the flash is will influence if the picture is over, under or properly exposed. You can control the Neewer flash from 1/128 to full power in 1/3 stop increments.
  • ƒ-stop/Aperture—You control how much of the light is coming into your camera by the camera iris called the aperture. These are fractions. The focal length of the lens over how wide the opening of the lens is. 
  • Flash Distance to Subject—The closer you put the flash to the subject the brighter the subject and the further away you put is the darker it gets. This is assuming your Flash Power and ƒ-stop are constant. 
TTL—Through The Lens

TTL metering has been around for a while with cameras. Your camera helps make the proper exposure settings based on what light hits the sensor of the camera. The TTL flash is a very sophisticated flash system, which sends out a flash that tells the camera what to set the camera and then the flash fires again taking the photo at those settings. This happens so fast that to the human eye it looks like one flash went off.

The most accurate way to take a reading for any photo is using an incident meter. This is where you put the meter where the subject is located and take a reading of the light falling onto the subject. The white dome needs to be where the subject is to get an accurate reading. 

Most of the incident meters have a available light setting and flash setting where you can measure the light. 

TTL metering is a reflective reading. You are reading the light that is reflected off the subject. If you take a reflective reading of a White Wall, a 18% Gray Wall and a Black Wall all three photos will look like an 18% Gray Wall.

If you use the settings you get from the incident reading meter for all three walls then they will look like they should.

TTL Metering has a variable

As you can see the TTL metering system has one major disadvantage of using a reflective reading to set the aperture, shutter and ISO on your camera—the color and tone of the subject will influence the exposure and white balance whereas the incident reading is consistent.

Incident vs Reflective

As long as the situation allows you to use incident metering you will get the most consistent results. However, there are situations where this is impractical.

At parties where you are moving around the room getting a incident reading and then taking a photo may be impractical to get the "moment." This is where a flash with TTL can get you really close on exposure and white balance. You may have to check the LCD for histogram and make an adjustment to the EV of the camera and/or EV of the flash. This often is much more practical than incident reading.

When the subject is moving toward you quickly like a person on a red carpet walking towards you the TTL will adjust pretty quickly. If you are shooting RAW you most likely be close enough to fix in post if necessary.

Most seasoned pros have both TTL flashes, which are hot shoe flashes and some studio strobes that are not TTL. Most pros will have a incident flash meter to make the adjustments to the lights and camera.

If you just do portraits in a studio setting TTL flash isn't necessary, but if you shoot situations that are pretty fluid then the TTL system can help you get photos that may be impossible without them.