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Yet another question on how wireless CLS works!

nl

West Hartford, US
833 posts

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nl Basic Member
Thu 30-Mar-06 03:30 AM

I'm trying to further my understanding of how the CLS system works with wireless slave flashes using the SB800/S800/SU800 system. There are a few (!) points that bother me; maybe someone can clarify.

1) If I understand correctly, in the past, cameras handled automatic flash exposure by measuring the light being received by the camera during the exposure, and shutting off flash output when sufficient light was received for the exposure.

2) With the CLS system, a series of "monitor preflashes" are emited by the flash, and the reflected light is used by the camera to determine the correct flash output for that exposure.

3) Question 1: Does that mean that with CLS in iTTL mode, the monitor preflashes are used to set the exposure, and the camera then DOES NOT meter during the exposure to shut off the flash; and the flash output is preset by the preflashes before the exposure begins? Or, is there also metering during the exposure, and if so, what benefit does the preflash add? (I assume, based on what have written, that the preflashes are used to set the exposure, and there is no metering to adjust this during the actual exposure).

4) When the SB800 is used in the master mode, the monitor preflashes also use pulse-code modulation to transmit exposure information to each slave flash. What I don't understand is exactly how this is determined. The master obviously does not have any way to know how many slaves there are, as the wireless communication is one way, Master -> Slave. As a result, while the master can determine a correct exposure via the monitor preflashes, HOW exactly does it make the adjustment for the contribution of the slave flashes?

As a example, suppose I have one slave flash, set to a 0.0 exposure compensation. If I added in a second slave right next to the first, also at 0.0, then I've doubled the light coming from that source, and obviously that changes the exposure. Now, it would make no sense for the master to note that and reduce the output of each slave by 1/2 to maintain the same exposure, when presumably my intent was to double the light from that source.

From this, I am reasoning that the monitor preflashes from the master are used to select the correct exposure which is done fully independently of the presence, absence, number of slaves in the system, and only trial and error can really be used to adjust the compensation on the slaves.

If I added, for instance, a diffuser between the slave and the subject, and wanted the same amount of light to hit the subject from that flash, just more difuse, I would then need to increase the exposure compensation for that slave to account for the light loss from the diffuser, because the CLS system cannot do this automatically for me.

Do I understand this correctly?

Thanks.
nl

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