, but how does this (FP or high synch speed flash setting work) I know you lose power the faster the shutter speed but is it in relation to the aperture or will I get more power?:shrug:
Thanks
Julian
The flash sync problems we have with our DSLRs is because they're fitted with a focal plane shutter. These shutters consist of two metal curtains which travel vertically in most modern cameras.(in some older cameras they travel horizontally) When you press the shutter, the first curtain goes up exposing the sensor for whatever shutter time you've set. When the exposure time is complete, the second curtain goes up covering the sensor to end the exposure. The auto reset mechanism then drags both curtains back to their starting point ready for the next exposure.
There's a limit to how fast these blinds can travel across the sensor leaving the sensor completely uncovered before the second curtain starts it's travel. This is typically around 1/200th or 1/250th although some cameras manage 1/500th, but it varies with the make/model of camera.
Whatever that shutter speed is in your particular camera is your maximum flash synch speed - the fastest shutter speed you can use where the sensor is completely uncovered at the point of the flash firing, and where the whole of the scene will be illuminated by the flash.
Focal plane shutters achieve shutter speeds much higher than the max flash synch speed, but they do so by the 2nd curtain starting it's travel while the 1st curtain is still moving, in fact just after it first starts moving. The result is a gap between the two curtains travelling across the sensor, so that the whole of the sensor is exposed for the set time, but incrementally... the faster the shutter speed set, the narrower that slit to achieve the faster exposure times, right up to 1/8000th of a second. Obviously if the flash fired at a speed faster than the max synch speed, it would only illuminate the part of the scene corresponding to where that slit is in it's travel across the sensor.
High Speed Flash Synch is a fairly recent development and it isn't true synch at all, but it does allow you to use flash with shutter speeds above the max synch speed right up to your fastest shutter speed. The way it works, is that as that slit moves across the sensor, the flash continually outputs a continuous string of pulsed flashes till the exposure is complete so that the whole of the scene gets even flash exposure.
It works best at moderate distances and fairly large apertures, as you're imposing a huge burden on your flashgun in meeting the demand the more you increase the distance or reduce the aperture. Macro is probably one of it's most useful applications where you can stop down quite a way before the flash starts to struggle due to the very short camera/subject distance.
Make sure you start out with fully charged batteries for Hi Speed Synch.
