Joe McNally is not your average Joe. With a career spanning over 30 years and including assignments in more than 50 countries, Joe McNally has shot everything everywhere in every thinkable sort of way. In addition to being an exceptionally experienced photographer, Joe is also known for his remarkable ability to share that knowledge. In this video he explains what TTL is and how to use it to improve your photography. Here is how it works, in Joe’s own words.
So we’re here in a photo studio today discussing the principles of TTL flash using the Profoto Off-Camera Flash System.
If you watch the video, you will see that there is a Profoto Softbox RFi 5′ Octa behind me. It’s fitted with a Profoto B1 Off-Camera Flash, which is a battery-operated system, right? And I’ve got TTL control over it.
If you see that size of light, especially in a studio, historically you would say: “Oh, that’s a manual light! I have to go over and adjust that light.” But the beauty of the Profoto system is that not only has it got this tremendous array of Light Shaping Tools, it also has TTL technology.
WHAT IS TTL?
TTL stands for Through-the-Lens metering. It means, basically, that the camera is judging flash exposure relative to what the camera actually sees.
So the only information that is being translated actively into judging the exposure of the flash is, in fact, the frame of the camera. In other words, the camera discerns the information that it views through the lens and then uses that to factor the flash exposure.
WHAT IS A PRE-FLASH?
There are two flashes that are involved in every exposure. One is called a pre-flash, which emanates from the flash prior to the actual exposure.
What that does is that it illuminates the field of view of the camera and gives the camera’s brain information about color and reflectivity and all that kind of stuff that the camera needs to ascertain to be able to in turn tell the flash how to behave.
So what the camera and the flash are doing basically is having a conversation. The pre-flash pops out and the camera judges that. And then it sends a single back to the flash. This is how much light to put out, or how little light to put out. Then, finally, the flash emits the actual flash needed to get just the right exposure.
So I’m not exaggerating when I say it’s like a conversation. That’s exactly what it is!