Flash slaves and TTL II

[david_ceruti]

I would appreciate if someone can help me through the arcana of

slave flashes and TTL metering.

 

I have a 20D and 580EX on the way. I also have a couple of flashes ?

a Sunpak hammerhead and an old Cobra. I am hoping to be able to use

them as slaves.

Now the question ? if I get a radio trigger for these, how will the

580 handle the TTL? I was planning to use the slaves to fill in

areas and to put the Sunpak into manual mode ? will the 580 adjust

its output to allow for the extra light or will it just go from the

metering it reading worked out from the preflash?

 

Also, the Cobra has no settings and I assume it will just shoot at

full power ? I will play around with this (how I am looking forward

to the instant feedback from a digital!) but if anyone has any tips

so I can interpret the results intelligently.

 

Lastly, any recommendations for a reasonably inexpensive slave kit

to trigger these flashes?

 

Apologies for the long post and thanks in advance

 

David

[gene_austin]

The Wein Hot Shoe Slave for Digital Systems is designed to ignore preflash and fire in sync with the main flash output. Its $70 at B&H:

http://tinyurl.com/7t6jf

[skipd]

You will probably not be able to find any slave systems for the oddball flash units that allow them to communicate to the camera like the Canon flash units can. Canon flash units can communicate via an infrared beam and let the system deal with exposure control automatically.

 

You can find devices that will allow you to trigger other flash units, but the whole system will have to be in manual exposure mode. A flash meter would be very useful in this case.

[jimstrutz]

The 20D with an EX flash determines flash exposure by way of a preflash just before the real flash. You can hook up your slaves to the 20D's PC port (radio slave or hard wired), and get them to fire at the right time, or you can get a digital optical slave device that ignores the preflash, but either way the 580EX will ignore the light from the slaves when determining it's exposure by preflash. Depending on how & where you place them ther is likely to be overexposure.

 

As Skip said, putting the whole ot of them into manual and using a flash meter is probably the easiest way to do this. Using flash units with manual power controls is preferred.