Flash Duration Quiz: t.1, t.5, IGBT and Motion Freezing

t.5 is the time the flash output stays above half its peak brightness. It is the figure most often advertised because it is short, but light continues after it, so it understates how long the flash really lasts.

t.1 is the time the flash output stays above 10 percent of its peak brightness. It includes the long tail of the pulse that t.5 leaves out, so it is a more realistic measure of the full flash duration.

t.1 measures how long the flash stays above 10 percent of its peak, so it includes the dimmer tail of the pulse that can still record a faint blur on a moving subject. t.5 counts only the brightest part and makes the flash look faster than it really is, so t.1 better predicts how well motion is frozen.

In a dark studio, the shutter is open long enough to capture the entire flash pulse, but ambient light contributes little. The flash duration, not the mechanical shutter speed, determines how long the subject is actually illuminated and therefore how sharply motion is frozen.

In bright surroundings, ambient light contributes much of the exposure, and it records for the full time the shutter is open. Even if the flash is brief, any ambient-lit movement during that time blurs, so shutter speed sets the limit on freezing.

Many speedlights adjust power by cutting the flash discharge short. At lower power the pulse ends sooner, so flash duration gets shorter, which helps freeze fast motion.

High-speed sync pulses the flash on and off many times as the shutter curtains travel across the sensor, ensuring every strip of the sensor receives light during a fast exposure. The tradeoff is reduced effective power compared to a single full burst.

In high speed sync, the flash fires a rapid series of pulses for the whole time the shutter travels, so it behaves like a continuous light. It no longer freezes motion with a brief burst; only the shutter speed does.

IGBT flashes control power by cutting the discharge off early. Lower power settings end the pulse sooner, which also shortens the flash duration and helps freeze motion.

How flash duration changes with power depends on the design. In IGBT units, lower power shortens the burst, while in many traditional voltage-controlled packs, lower power lengthens it, so full power or special action heads give the shortest flash. Checking the specifications for each unit tells you which setting freezes motion best.

Ghosting appears when the shutter is open long enough for ambient light to also expose the sensor alongside the flash. The flash freezes a sharp version while the ambient component records a blurred secondary image over it.

Ambient exposure is controlled by ISO, aperture and shutter speed, so lowering ISO, closing the aperture or using a faster shutter within sync speed all cut ambient light. Since ISO and aperture also reduce the flash, raise the flash power to keep the subject bright.

In bright sun, a wide aperture at the camera's sync speed, often 1/200 to 1/250 second, lets in too much light. An ND filter cuts that light so you can keep the wide aperture and normal flash sync without needing high speed sync.

A flash's color can shift slightly with power level, since the tube's color depends on how the capacitor discharges. Many flashes warm or cool a little at low power, which can matter for consistent color.

When ambient light is dark, the sensor records almost nothing during the shutter except the flash burst. The short flash pulse then sets the effective exposure time, freezing motion more cleanly than any shutter speed can.