I've been reading the manual on my recently purchased EOS 3 and I noticed that it says that the sync speed for compact and non-Canon flash units is 1/200th (or slower) of a second while large studio flash is 1/125th of a second (or slower).

I'm thinking about getting a 540 or 550EX but until then I'll use a older 365 Vivitar flash I have. But my question concerns studio mono-lights. Will they sync at 1/200th or do I need to keep it to 1/125th or slower? Is the difference because it takes a larger flash tube longer to reach peak intensity?

Even 1/125th is faster than my Rebel G, which was limited to 1/90th, but I do plan on using the EOS 3 in my studio to take portraits and soemtimes the 1/200th would be nice.

Thanks

Mike

-- Mike Jordan (mjordan@europa.com), October 14, 2001

Answers

"Is the difference because it takes a larger flash tube longer to reach peak intensity?"

Yeah, that, and larger flashes often have a longer flash duration than hand held units. Some have a flash duratio of 1/250 sec. and the timing is too tight to fit it all into faster shutter speed. Still, you may or may not be able to use your studio strobes at 1/200. Only testing will show it. You have to fire the strobes at full power and see if the top or bottom edge of the frame starts to go black. Some units will work and some will not.

I don't think this is much of a set back though. 1/60 is usually plenty for studio work.

BTW, the hot shoe of your EOS 3 is rated for 6 volts maximum sync voltage. If you are firing the 365 from the camera's PC connector it shouldn't be a problem, but if your are attaching it someway to your hot shoe, check the sync voltage first.

-- Jim Strutz (j.strutz@gci.net), October 14, 2001.

Thanks Jim. And thanks for the pointer about the hot shoe voltage. I'll check the voltage on my Vivitar 365 to see what it is. I've used it on my Rebel G with no problems, but I could have been just lucky. I bought the 365 back in the late 70's so who knows what voltage they used then.

Do you know what the best way to check contact voltage is? I don't know if sticking a multimeter across the center and ground will do it.

Thanks

Mike

-- Mike Jordan (mjordan@europa.com), October 15, 2001.

That's it. One lead on the center contact and the other on the side.

To do this right, you need a fairly decent volt meter. One that's rated at least 20,000 ohms per volt, preferably higher. That used to be expensive, but most of the meters made today will do that or better. If you use one with a lower impedance, it can give you a false reading that will look good but isn't real. Some people say that you need to do this with an O-scope while the flash is actually going through an entire cycle, but I doubt that transient spikes, if there ever are any, will actually do damage to the camera. I've certainly never heard of it happening.

While Canon rates all their EOS cameras at a max of 6 volts, anything less than 10 volts is generally regarded as safe. Also there seems to be an accumulative effect that a high sync voltage may have on a camera. So while it may work for a while, it eventually fries the circuit. Then again, some EOS cameras use over voltage flashes for years and never show a problem.

-- Jim Strutz (j.strutz@gci.net), October 15, 2001.