High Speed Sync for Flash at Any Shutter Speed

Use Your Flash in Daylight Without Overexposing

Ralph Paonessa

January 1, 2015

1. Introduction

High Speed Sync lets you flash at high shutter speeds. Find out if you've got it and how it can improve your shots.

Some participants on my recent Costa Rica and Falkland Islands workshops noticed a problem when they turned on their flashes to add fill light. We were photographing birds with long lenses and Better Beamer Flash Extenders, and were shooting wide open (e.g. f/4 or f/5.6) to maximize shutter speed, minimize motion blur, and soften the background. Typical settings at ISO 200 in full sun were around 1/2,000 sec at f/5.6.

Gentoo Penguin and chick, Falkland Islands
Gentoo Penguin and chick, Falkland Islands

EOS 1D Mark II, ISO 100, 500 mm lens, 1/700 sec @ f/8, fill flash @ -1.7 with Better Beamer Flash Extender and Speedlite 580EX in High Speed Sync mode. Fill flash softens the shadows the parent casts on this tiny chick.

When participants turned on their flashes for fill light, the result for some was very overexposed shots. (Thanks to digital, the blown out shots were immediately obvious.) But what was causing this problem, and how could we fix it?

Maximum sync speed

What we experienced is an object lesson in the significance of a camera's maximum flash sync speed (sometimes referred to as simply "sync speed"). Most photographers know that better cameras have higher maximum sync (synchronization) speeds, but the value of this feature isn't always obvious.

A camera's maximum sync speed is the fastest shutter speed at which you can use a "standard" flash. (I say "standard" because some systems now offer a "high speed sync" mode that allows you to use flash at any shutter speed. More about that key innovation below.)

A standard hotshoe flash gives out a very short pulse of light. The flash duration can vary from around 1/1,000 second to 1/50,000 sec or shorter. The less light that's needed, the shorter the duration; that's how TTL flashes reduce their light output below full power.

Maximum sync speeds range from as low as 1/60 sec on entry-level SLRs to 1/250 or even 1/500 sec on top-of-the-line models. Why is there a limit? It comes from the way SLR shutters work. The focal plane shutter on your SLR consists of two curtains which work in tandem and move up and down across the frame to expose it to light for the required amount of time (shutter speed).

For a long exposure, say 1 second, the first curtain quickly opens to expose the film or sensor. After 1 second has elapsed, the other curtain quickly closes to end the exposure. And that's the way it works, until you exceed the maximum sync speed.