[COLOR=#006400][B]No.10: [/B][/COLOR][COLOR=black]Why it's useful to understand how your camera's shutter works.[/COLOR]

The [B][COLOR=darkgreen]shutter[/COLOR][/B] in a camera opens and closes for a predetermined time, allowing the film or digital sensor to be [COLOR=darkgreen][B]exposed[/B][/COLOR] by the light being transmitted from the scene via the camera's lens. The type of shutter used dictates how you can freeze action, and how you can use flash.

A [COLOR=darkgreen][B]focal plane shutter[/B][/COLOR], as its name suggests, is positioned very close to the focal plane, which is the surface, be it a sensor or traditional film, where the image is focused onto. The primary alternative to a focal plane shutter is a '[COLOR=darkgreen][B]leaf[/B][/COLOR]' shutter, and this is situated inside the lens, doubling as the lens aperture iris.

Focal plane shutters are standard on SLR-type cameras, and some compact cameras that have interchangeable lenses.

Up to a point, leaf shutters and focal plane shutters work in a similar way. The are 'cocked', ready to be fired, and when the shutter release is tripped they open and close for a pre-selected duration. But there is a limit to the physical ability of leaf shutters to open and close fast. 1/500th of a second exposure time is about the limit for typical leaf shutters. It's slower for focal plane shutters as they are physically larger and carry greater momentum. 1/60th second used to be the fastest open and close exposure time a focal plane shutter could manage, though modern shutters can now reach around 1/300th second. But focal plane shutters have a neat trick. To further reduce the exposure time, the shutter switches to a mode whereby the two curtains form a slit that travels across the frame, only exposing part of the frame at any one time.

This means exposure times can be reduced to as little as 1/8000th of a second, using an increasingly narrower slit as the exposure time decreases.

But the limitation of the open and close exposure mode has a consequence for flash use. For correct electronic flash [COLOR=darkgreen][B]synchronisation[/B][/COLOR], the shutter must fully open, then the flash fires, and then the shutter closes. If you fire an electronic flash while the shutter is in 'slit' mode, only the area where the slit is open will be exposed correctly. This can result in just a fraction of the width (if the shutter travels horizontally) or height (for vertical travel shutters, which is the norm these days) being recorded. By contrast, leaf shutters can usually flash sync at their maximum shutter speed setting.

You may have seen reference to an [COLOR=darkgreen][B]'X' Sync[/B][/COLOR] setting. This is the fastest shutter speed that a focal plane shutter can be set to in order to synchronise with flash. Beyond this setting the shutter can no longer simply open and close.

Nevertheless, the boffins have since produced a compromise that enables electronic flash to be used at any shutter speed, and many SLR cameras offer this capability, often called FP Flash or Focal Plane Flash, sometimes called Super FP Flash.

It's quite simple; in the fast shutter speed mode, the shutter's exposure slit travels across the frame, and as it travels the flash will strobe, or flash several times, so evenly exposing the picture in several strips. The compromise is that the flash unit can't fire so many times in a short space of time at full power, so brightness is reduced. But it does mean that the photographer can use a fast shutter speed to balance ambient light with the flash light, and freeze action using the shutter rather than relying on the flash.

[B][COLOR=#006400]Next Monday:[/COLOR][/B] The rule of thirds.

[I]Incidentally, please don't hesitate to post a question about this Daily photo tip if you have one![/I]