The Phoblographer Explains: How TTL Flash Metering Works

One of the biggest things that makes no sense to me as a strobist photographer is why we don’t have any sort of universal TTL flash metering system. Instead of that, every single camera manufacturer has their own for the sake of being able to compete with one another while delivering flashes that essentially all do the same thing. It’s a hassle for photographers moving from one system to another. To understand this and my reasoning, you need to understand how TTL Flash metering works.

And trust me; it’s a whole lot simpler than you think.

The Truth About Metering

Soft lighting Pro Tip: When shooting portraits, try spot metering for the face and overexposing by 1/3rd of a stop.

Soft lighting
Pro Tip: When shooting portraits, try spot metering for the face and overexposing by 1/3rd of a stop.

Here at the Phoblographer, we often test the way that camera’s meter according to the tried and true Sunny 16 standard which means that at f16 on a bright sunny day with no clouds, you should be able to photograph a scene at f16 and your shutter speed will be the reciprocal of your ISO. So to that end, you’re shooting at f16 ISO 100 and 1/100th. It’s a standard that has been around for ages.

More to the point, it hasn’t changed at all. Every camera system seems to honor this rule with 2/3rds of a stop for a margin of error. But that’s not enough to make us want to stop the presses though.

How a Flash Works with Your Camera’s Exposure Settings

Chris Gampat The Phoblographer Aperlite YH-700C flash review images (5 of 10)ISO 4001-50 sec at f - 3.2

When a flash is added into the scene or the creation of an image, things seem to change a bit.

  • Shutter speed controls the amount of ambient light in the scene. The reason for this has to do with flash duration–which essentially takes over for the motion stopping abilities of shutter speeds.
  • ISO sensitivity controls the scenes overall sensitivity to light.
  • Aperture controls and dictates how much of the flash output is affecting the scene. In manual flash output, if your flash is set to a consistent 1/4 power output, stopping down to f5.6 will lessen the effects of the flash compared to f2. At f2, you’ll have more light from the flash affecting the scene.

Now when TTL comes into play, the aperture settings kind of change.

TTL Flash Metering

Chris Gampat The Phoblographer Adorama Flashpoint Zoom Li-on Radio flash review (3 of 9)ISO 4001-60 sec at f - 4.0

When you connect a TTL capable flash to your camera’s hot shoe (or via radio or something else) the flash essentially looks at your camera’s aperture, ISO and the distance away from your camera the subject is (in some cases) and adjusts accordingly.

That’s it.

So if you have a manual lens with no exposure communication connected to your camera, your flash is bound to fire at full power instead.

Answer: TTL Metering is for the most part just looking at the ISO and the aperture and judging how much output it should deliver. If you use exposure compensation, it will act accordingly. This is all independent of whatever camera system you use.

To that end, I’m still not sure why we don’t have a universal TTL flash system except for the fact that there are many different communication pin ports on cameras preventing this. Still, it makes no sense.