Every external flash operates in one of two modes: TTL (Through The Lens) automatic metering, or manual power control. Understanding when each mode is appropriate, and how to meter effectively in both, eliminates the most common flash exposure problems.

How TTL Works

TTL metering fires a pre-flash, an extremely brief burst of light, milliseconds before the actual exposure. The camera’s meter reads the light reflected from this pre-flash through the lens, calculates the flash power needed for correct exposure, and sets the main flash output accordingly.

This happens automatically, every frame. If the subject moves closer or farther from the flash, or if the ambient light changes, the TTL system adjusts.

Advantages:

  • Fast. No manual calculation needed.
  • Adapts to changing conditions automatically.
  • Works well for dynamic situations where the subject-to-flash distance varies.

Limitations:

  • TTL can be fooled by the same things that fool camera metering: predominantly bright or dark scenes, reflective surfaces, and off-center subjects.
  • Flash output may vary from frame to frame as the meter reacts to subtle changes in the scene.
  • Flash exposure compensation is often needed to fine-tune the result.

How Manual Flash Works

In manual mode, you set the flash power yourself: full power, 1/2, 1/4, 1/8, and so on. The flash fires at exactly that power every time, regardless of the scene.

Advantages:

  • Consistent output, frame to frame. This is critical for studio work and any situation where consistency matters.
  • Complete creative control over the lighting ratio.
  • No pre-flash, which eliminates issues with subjects blinking at the pre-flash or radio triggers misfiring.

Limitations:

  • You must calculate or test the correct power setting.
  • If the subject-to-flash distance changes, you must adjust manually.

When to Use TTL

TTL excels in situations where speed and adaptability matter more than absolute consistency:

  • Event photography. Guests move around, rooms vary in size, and you are shooting rapidly. TTL handles these variables without requiring constant adjustment.
  • On-camera bounce flash. When bouncing flash off ceilings and walls at varying distances, TTL adjusts for the changing throw distance.
  • Run-and-gun portraits. Outdoor environmental portraits where you are moving between locations quickly.
  • Fill flash in daylight. TTL calculates the appropriate fill ratio against the ambient exposure.

Key technique: Learn to use flash exposure compensation (FEC). TTL tends to overexpose slightly in many situations. Dialing in -0.7 to -1.0 FEC is a common starting point for fill flash and bounce flash.

When to Use Manual

Manual mode is the better choice when consistency and precision are priorities:

  • Studio photography. Once you set your lights, the exposure is identical for every frame. This eliminates the frame-to-frame variation that TTL can introduce.
  • Off-camera flash with modifiers. When your flash is inside a softbox or bouncing into an umbrella at a fixed distance from the subject, the geometry does not change. Manual gives you repeatable results.
  • Multiple flash setups. When balancing two or more lights, manual control over each light’s output is essential for maintaining your intended ratio.
  • Creative lighting. Any situation where you want the flash to overpower ambient, create dramatic falloff, or produce a specific mood.

Metering Manual Flash

There are two approaches to setting manual flash power:

The Chimping Method

Fire a test shot, check the histogram and the image on your LCD, and adjust power up or down. This is simple and requires no additional equipment.

Start at 1/4 power and work from there. Each full stop change in flash power (1/4 to 1/2, or 1/4 to 1/8) doubles or halves the light output. Half-stop adjustments (available on most modern flashes) allow finer control.

Using a Flash Meter

A handheld flash meter (like the Sekonic L-308) measures the actual light falling on the subject. Hold the meter at the subject’s position with the dome pointed toward the camera, fire the flash, and the meter displays the corresponding aperture for correct exposure.

Flash meters eliminate guesswork and are particularly valuable when balancing multiple lights. You can meter each light independently, set precise ratios, and know the exposure is correct before taking a single photograph.

Balancing Flash and Ambient

Whether using TTL or manual, the critical skill is balancing flash output with ambient light.

Shutter speed controls ambient exposure. A faster shutter darkens the background; a slower shutter brightens it. Flash output is unaffected by shutter speed (up to sync speed).

Aperture and ISO control both flash and ambient. Opening the aperture brightens both. Raising ISO brightens both.

Flash power controls only flash exposure. This is the independent variable you use to set the ratio between your flash-lit subject and the ambient-lit background.

The standard approach: set your ambient exposure first using shutter speed, aperture, and ISO. Then add flash and adjust its power until the subject is properly illuminated against the ambient exposure you have already established.

This method works identically whether your flash is in TTL (using flash exposure compensation) or manual mode (using the power dial).