Flash synchronisation

From Camera-wiki.org
Revision as of 05:07, 14 May 2007 by Fitzdocs (talk) (Links)
Jump to: navigation, search

Flash synchronisation, typically referred to as flash sync, is a means by which a flash head is fired at precisely the moment when the camera's shutter is at its peak opening.

Before flash sync, cameras had to be set to bulb (B) mode in complete darkness. A cable release would be used to hold the shutter open, and the flash would be manually fired. The photographer would then allow the shutter to close and the lights could be turned back on.


Types of Flash Sync

X Sync

X sync causes the flash to burst in synchronization with the peak opening of the shutter. For some manual cameras, the X synch speed refers to the maximum speed that the camera can synchronize with the flash. more info

M Sync

Some older cameras support M sync, which supports flash sync with (now obsolete) medium-speed electric flash bulbs. Because flash bulbs take some time to generate light, M-sync is timed to open the shutter at the time when light output from the bulb is greatest.

FP Sync

Because cameras with focal plane shutters use two curtains, there is a limit to the shutter speed you can use with traditional flash. This is because the rear curtain begins to close soon after the front curtain has finished opening. At higher shutter speeds, both curtains move simultaneously, forming a "slit" that moves across the film plane. In this case, standard X-sync flash will not work.

Focal plane high-speed sync, or FP-sync, fires the flash repeatedly as that "slit" moves across the film plane, which allows light from the flash head to expose the entire surface area of the negative evenly. more info


Types of Flash Sync Connections

PC Terminal

A Prontor-Compur, or PC, terminal is the standard method for connecting a flash head to a camera. When the flash sync is triggered, a signal is sent from the camera along a wire to the flash telling it to fire.

Hot Shoe

A hot shoe is an accessory holder (or accessory shoe) that has an electrical flash sync contacts. A small, portable flash that has a contact on its "foot" can be connected to a hot shoe, which will cause the flash to fire when you press the shutter release.


Shutter Curtain

Most focal plane shutters are composed of two curtains: a front and a rear curtain. The front curtain slides open to begin the exposure, and then the rear curtain slides closed in the same direction to end the exposure.

Front Curtain Sync

Generally, the flash sync fires at the instant the front curtain has fully opened. This is called front curtain sync, and it is used where it is desired that the flash freeze motion at the beginning of the exposure. Front curtain sync is adequate for most flash-photography. When making long exposures while also firing a flash, front curtain sync creates an effect where any motion blur - from ambient light - appears ahead of the subject, i.e. it appears to leave the subject behind. In the case of a moving motor vehicle in a night scene, for example, this can create an unrealistic effect since its headlamp and tail-lamp trails will extend out in front of it.

Rear Curtain Sync

Some cameras offer the ability to fire the flash at just before the second curtain closes. This is called rear (or second) curtain sync, and it is used to freeze motion at the end of the exposure. When making long exposures while firing a flash, rear curtain sync creates the effect of motion blur trailing the main subject.

Focal Plane Shutter evolution

Because of the significant amount of time that it takes for each shutter curtain to move the full width of the film frame, high shutter speeds can only be achieved by releasing the second curtain before the first has fully opened, so that the shutter becomes, effectively, a moving slit that traverses the film instead of exposing the whole frame at once. Back in the days of magnesium-filled bulb flash, special slow-burning bulbs were available to accommodate this, maintaining constant light output for the time it took the traveling slit to complete its journey.

However this is unworkable with a normal electronic flash because of its extremely short duration which will, in this situation, cause only part of the frame to be exposed. Basic electronic flash syncronisation is therefore only feasible at or below shutter speeds which see the whole film frame uncovered at the same time, i.e. when the first curtain has fully opened before the second curtain starts to close. With horizontal blind type shutters this generally limits the electronic flash sync. speed to 1/60" max.

In later focal plane shutters - commencing with the Copal Square shutter from the 1960's - lightweight metal or polymer blades, moving vertically, have replaced the original blinds, permitting sync. speeds of up to 1/250".

To permit still higher shutter speeds, advanced electronic flash units have appeared which have an optional mode that gives an extremely rapid strobe sequence (at necessarily reduced output), designed to continue for the entire shutter cycle and "keep up with" its traveling slit.

Links

Glossary Terms