Two Speedlite 550EX questions

[jnicholson]

I have two questions regarding the 550EX Speedlite or the other

models that may support these features:

 

(1) When using multiple flash units do they all fire at the same

time? That is do the slave units fire at the same time as the

master? I know for most studio stuff this really doesn't matter as

long as they all fire within the sync window. However once motion is

involved this becomes a critical issue.

 

(2) Just how does high-speed-sync work? My theory is that it behaves

sort of the opposite to traditional flash. Traditionally, the

shutter opens fully, the flash fires and the shutter closes some time

after the flash. Does high-speed-sync work by turning on the flash,

opening and closing the shutter then turning off the flash?

 

As usual, thanks for any help and comments

[nello]

There is a flashwork link around here that will explain everything...

 

The master fires a "pre-flash" first. The preflash is a means of communication between master and slaves. FYI preflash does not calculate into exposure. (Its a burst of light so quick that you can barely notice it.) Following the "preflash" is the "exposure flash" which DOES weigh in on exposure. All the flashes fire at the same time during the "exposure flash."

 

High-speed-sync works via a rapid burst firing of the flash. I liken it to a machine gun... The flash shoots off a quick burst of flashes during the time your shutter (accordingly at a high speed) is open (hence the name High-speed-sync). The trade off is that the flash power in high-speed-sync is weaker than a normal single flash burst.

 

Trivia/rumor: Canon uses a "preflash" system of communication between masters and slaves. This is in large part due to the fact that devices that emit radio signals have to go through a certain regulations process before they can be legally sold. Hence, it was cheaper/more convenient for canon to use a system based on flashes of light.

[jnicholson]

Hi Nello, thanks for the information. You are correct about the FCC requirements for wireless communications and I believe that's why Canon sticks with IR for flash-to-flash communications. Thanks again and all the best.

[jim_leonard]

nello's description of canon's multi flash missed a couple of flashes. Prior to the exposure, the main flash fires in a communication with the other flashes, then all of the slaves fire (one group at a time) to get metering information. All of this occurs while the mirror is still down so you can watch it happening, though it does happen fairly quickly. This is because the light meter is only in the light path with the mirror down. Then the master flash sends another communications flash to tell each of the flash groups what flash level to use. Then the mirror in the camera flips up (and the shutter opens) and everything fires. I'm not sure how the final flash sync occurs (ie is it just timed off of the preflash communications or is there another round of communication of some form). However to answer you main question, the slaves (and master) should fire the exposure flash all at the same time to within some number of microseconds.

 

 

As to question 2, you're mostly right. Ideally it would use a continuous light source but you can't do that with a flash tube so it pulses the light several thousand times per second. All of this is because when the shutter speed is higher than the sync speed, the shutter is never fully open. It's just a slit travelling across the film/sensor plane.

[jnicholson]

Jim, excellent, thank you. So it seems from yours and Nello's answers to my second question that the fastest effective shutter speed might be achieved by using a normal sync. shutter speed, say 1/250th, in a completely dark room with the flash power set to 1/128th. I would imagine the duration of the flash at its lowest power would be faster than the fastest shutter speed on the camera. Thanks again to both of you.

[jim_leonard]

<em>So it seems from yours and Nello's answers to my second question that the fastest effective shutter speed might be achieved by using a normal sync. shutter speed, say 1/250th, in a completely dark room with the flash power set to 1/128th.</em><br>That is exactly correct. That's the typical manner that people do stop action photography. The flash guns aren't fast enough to stop a bullet (or even an airgun pellet as in the attached image) but anything much slower or significantly larger (movement of half an inch means a pellet is just a blur, movement of a half inch for a person would make them still quite visible ignoring the likelyhood of a person moving at 200 m/s (400mph)) comes out pretty good.<div></div>