Hotshoe control of manual power.

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Hotshoe control of manual power.

Postby Firebird » Sat Sep 22, 2007 8:03 pm

Does anyone know how Nikon or Canon controls flash power through the hotshoe? I'm particularly interested in Nikon since I own 4 sb28's. An ISO document, reverse engineered document, etc. Point me to something on the net.
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Postby ben-s » Sat Sep 22, 2007 8:57 pm

I very much doubt that you can directly control the power of the flash through the hotshoe.
As I understand it, in TTL mode, (IE. the only mode where the camera tells the flash how much light to put out) the camera sends 2 signals -flash and quench

It doesn't say "fire at 16th power" it says "Start" and "that's enough light - stop"

The only way to actually provide remote manual power would be a hack of the TTL commands. To do this, you would need a profile for each type of flash to be used with the system, as all flashes are not equal - a flash with a bigger tube is likely to put out more light in a given time than a smaller one, and flash durations vary. I think it could get very messy very quickly.

But, Never say never. If there is enough demand for it, I'm sure someone will implement it.
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Postby Firebird » Sat Sep 22, 2007 9:11 pm

ben-s wrote:It doesn't say "fire at 16th power" it says "Start" and "that's enough light - stop"


That makes sense. It would also allow maximum variability of the power within it's range.
ben-s wrote:The only way to actually provide remote manual power would be a hack of the TTL commands. To do this, you would need a profile for each type of flash to be used with the system, as all flashes are not equal - a flash with a bigger tube is likely to put out more light in a given time than a smaller one, and flash durations vary. I think it could get very messy very quickly.


Yup, I agree. Hacking the TTL is not in the cards.
ben-s wrote:But, Never say never. If there is enough demand for it, I'm sure someone will implement it.


That's right. And...... power is controlled on these flashes by varying the time the flash is allowed to fire. So if you build a device on the flash end that can take care of the "start" and "quench" functions, while adding a controllable time delay between them you've solved the problem. Thanks for the insight Ben.
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Postby tim-j » Sat Sep 22, 2007 11:37 pm

It doesn't say "fire at 16th power" it says "Start" and "that's enough light - stop"

I think this is how first generation TTL worked; it measured the light reflected off the film, and sent a quench signal when the film had received the right amount of light. I think CLS (i.e. the Nikon system) currently does much more than this. It had to deal with the problem of the reflectivity of the digital sensor, which precludes using the method used with film.

When using Nikon's CLS, I do not think there is a quench signal being sent to any of the slaves, whether or not they are in TTL or manual mode.

I use my lights (an SB-800 and an SB-600) in manual mode: if I use CLS, I set the power of the SB-600 on the panel of the SB-800 (i.e. I use the 800 as commander, cabled to the shoe). My understanding is that the required power setting on the 600 is set by the preflashes put out by the 800.

I do not know if any Nikon body allows you to use the camera menu to control the output power of a flash connected to the shoe. What I do know is that the D70 does not.
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Postby JonSenior » Sat Sep 22, 2007 11:54 pm

ben-s wrote:

It doesn't say "fire at 16th power" it says "Start" and "that's enough light - stop"


Actually, in Canon world that's exactly what it says. This chap here stuck an oscilloscope onto a Canon EOS 50 and G3 and watched it talking to the flash. The conversation is two way (I don't know if that's kept when using IR wireless) and he has decoded enough (It would appear) to allow remote control of flash power level.

On the other hand, the Nikon system (Assuming it hasn't changed dramatically since in recent years. I don't know Nikon stuff!) uses a quench signal. With some calibration an onboard micro could do this automagically so that the same level information could be used. I suspect however that for the reasons mentioned earlier regarding digital sensors, that modern Nikon flashes probably support output level control. Doesn't their system also use pre-flash ala Canon?

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Postby gsansoucie » Sun Sep 23, 2007 2:11 am

(I removed this text - amazing what a google search can net)
Last edited by gsansoucie on Sun Sep 23, 2007 2:29 am, edited 1 time in total.
-=Glen=-

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Postby mattsteg » Sun Sep 23, 2007 2:24 am

JonSenior wrote:ben-s wrote:

It doesn't say "fire at 16th power" it says "Start" and "that's enough light - stop"


Actually, in Canon world that's exactly what it says.
That's e-ttl, not ttl. Canon's old ttl tech is no different than nikon's in that regard.

and, to make it explicit, nikon's i-ttl tech is like canon's ettl in that it determines flash magnitude from preflash and presumably tells the flash what power to provide. Both companies' flashes still support oldstyle TTL though.
Last edited by mattsteg on Sun Sep 23, 2007 4:26 am, edited 1 time in total.
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Postby seaton » Sun Sep 23, 2007 4:22 am

Check out the wireless discussion under canon, I found a link that explains what the canon protocol does and posted it there of someone. From what I can gather you can set the power of the flash.
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Postby JonSenior » Sun Sep 23, 2007 8:17 am

mattsteg said:
That's e-ttl, not ttl. Canon's old ttl tech is no different than nikon's in that regard.


Apologies, I got sidetracked. It would appear that in TTL mode, ID is used to prove that a flash is connected and CLK is used as the quench signal. I guess a bit of code that used TTL style quenching would allow for a greater variety of flashes to be supported without having to do excessive protocol hacking.
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Postby tim-j » Sun Sep 23, 2007 8:51 am

I guess a bit of code that used TTL style quenching would allow for a greater variety of flashes to be supported without having to do excessive protocol hacking.

I think that timing/latency would preclude doing this by radio -- signal handling would have to be very fast to handle a quench signal properly. Well under a millisecond.
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Postby JonSenior » Sun Sep 23, 2007 9:11 am

tim j said:
I think that timing/latency would preclude doing this by radio -- signal handling would have to be very fast to handle a quench signal properly. Well under a millisecond.


My thinking was actually that we program a flash power at the transmitter which is relayed using our protocol to the receiver. The receiver (being aware of the flash it's connected to) then takes responsibility for sending a quench signal x micro / milli seconds after the initial trigger.

This is the user control TTL system whereby the we hijack the TTL control system to remotely control the flash power not just relay the control information from the camera. All manual but saves on leg movement! ;-)

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Postby tim-j » Sun Sep 23, 2007 9:16 am

The receiver (being aware of the flash it's connected to) then takes responsibility for sending a quench signal x micro / milli seconds after the initial trigger.

Neat :wink:
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Flash control

Postby Thonord » Sun Sep 23, 2007 11:02 am

I only know about Oly and am writing a UI using the Oly SDK. Doing it in VB.
I have full control of the flash modes and the power levels. The only thing I cant do is fire the flash and set the zoom.

I'v been looking at the protocol batween camera and flash and while this is way to early and very possibly wrong. Im asuming the following:
RS237 (pun) Means pins 2, 3 and 7. No handshaking.
8 bit data start and stop
positive ack.

Tom
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Postby TwoLeftFeet » Sun Sep 23, 2007 10:43 pm

Tom, do you have any (shareable) documents that say how their TTL works?
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Oly TTL

Postby Thonord » Mon Sep 24, 2007 3:02 am

Sorry TwoLeftFeet I don't have anythingon Oly TTL at Camera - flash level
I'm looking, but not to concerned with TTL. Trying to envision TTL with multiple flashes from various vendors gives me a head ache.

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