Who Knows how ittl works?

[phillipb]

The reason I ask this question is this:
I was experimenting tonight with a couple of generic flashes off camera connected to a couple of Nissin shyncro-eyes - they're sensors that detect a flash going off and they discharge the flash that they are connected to.
So basically when the built-in flash goes off, so do the other 2 flashes.
Now here is the interesting bit, I had left the built-in flash set to ittl, the camera was on manual and since I didn't know what to expect I started with a setting of 1/60 sec f16, the exposure was way under exposed, so I opened the f-stop to f8 and to my surprise the exposure got worse and kept going worse the more I opened the aperture. at no stage I could get a reasonable exposure no matter what I did.
When I changed the built-in flash setting to manual then everything went as expected
Can anyone shed some light (excuse the pun) on this?

[gooseberry]

When using iTTL with the internal flash, the internal flash will fire a set of pre-flashes to determine the flash power. These pre-flashes will set off generic flashes that are connected to the remote sensors and thus they will fire before the exposure is actually taken from the camera. Using manual on the built-in flash - there is no pre-flash, just the main flash, and the remotes fire off at the correct time.

If you want iTTL to work with the generic remote flashes, turn off the generic flashes, do a flash value (FV) lock to set off the pre-flash, then turn on the remote flashes and take your shot.

[gstark]

Phillip,

What Gooseberry said is spot on; the preflash from the D70 prematurely fires the ordinary flashes, and when it comes to the real exposure, they have nothing left to give.

Do you have a flashmeter? Are you coming to the meet this morning? I can show you how to make this work for you.

[phillipb]

Gooseberry,
Thanks for your reply, what you say makes perfect sense.

Gary,
Sorry can't make it to the meeting today, but that's ok, I can work around it in manual mode.

[Onyx]

Show us what you're shooting Phillip!

[phillipb]

Well at the moment I was just trying to get a handle on the exposure part, basically the aim is to make use of the two remote sensors and flashes to provide some side and back lighting in portraits, having trouble finding willing subjects though.

[phillipb]

Like I said Onyx, having trouble finding willing subjects, but here's a sample of my experiments.



No PP at all except cropping.