Clarification request:Using two TTL flashes

[roy_kekewich]

Thanks for the above answer. I hate to say it but though I understand how zoom settings on the flash head or distance would determine aperture (and flash ratios) with flashes on Manual mode, I still don't grasp WHY DISTANCE WOULD EFFECT TTL FLASH.

 

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Considering that flash output is measured OTF (off the flim) in TTL, why wouldn't, say, two 380EX flashes, both in contact with the camera via off-shoe cords or Ikelite Litelinks, both output according to the camera aperture AND METER READING OTF?

 

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Any further explanation would be greatly appreciated.

 

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Thank you

[roy_kekewich]

Whoops! In the first line I'm referring to the answer to my first

question. See: Light Ratios Using Two TTL Flashes.

 

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Sorry.

[steve_dunn3]

There's one more piece to the puzzle, and then I think you'll have it

all sorted out. Flash units in general can't change the INTENSITY of

their flash output; they can only adjust the DURATION of the flash.

 

<p>

 

Therefore, if you link two flashes to the same TTL metering system,

they both turn on and off at the same time. If they're equally

powerful, that means they both put out the same amount of light.

 

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To adjust ratios, then, you either need to change the distances from

the flash units to subjects, or the flash units' guide numbers. The

latter can be done using different models, different zoom settings

(not possible on a unit like the 380EX which doesn't permit manual

control over flash ratio), manual power control on the flash (not

possible on the 380EX), or by putting a neutral density filter over

one of the flash units.

 

<p>

 

The wireless E-TTL support in some newer Canon bodies and flashes

allows individual flashes (or groups of flashes) to be controlled

separately, so even if they're the same model and everything, they can

quench at different times to provide a wide range of ratios.

[steve_dunn3]

There's one more piece to the puzzle, and then I think you'll have it

all sorted out. Flash units in general can't change the INTENSITY of

their flash output; they can only adjust the DURATION of the flash.

 

<p>

 

Therefore, if you link two flashes to the same TTL metering system,

they both turn on and off at the same time. If they're equally

powerful, that means they both put out the same amount of light.

 

<p>

 

To adjust ratios, then, you either need to change the distances from

the flash units to subjects, or the flash units' guide numbers. The

latter can be done using different models, different zoom settings

(not possible on a unit like the 380EX which doesn't permit manual

control over flash ratio), manual power control on the flash (not

possible on the 380EX), or by putting a neutral density filter over

one of the flash units.

 

<p>

 

The wireless E-TTL support in some newer Canon bodies and flashes

allows individual flashes (or groups of flashes) to be controlled

separately, so even if they're the same model and everything, they can

quench at different times to provide a wide range of ratios.

 

<p>

 

The only way aperture figures into TTL or E-TTL metering is in that it

affects how much of the light gets back to the metering sensor. It is

not directly used as a factor in metering calculations. (It's not

used for metering in A-TTL, either, but the results of the A-TTL

preflash may make the camera stop down in some cases; that, in turn,

affects how much light comes in to the metering sensor.)

[piotr_mikolajczyk2]

The OTF metering cell indeed determines at which moment the film has

gotten enough light and then sends off a signal to quench the master

flash (and hence the slave flashes which shut off in sync with the

master flash). For the OTF cell, it does not matter how many flashes

are used and how they are placed w/respect to the subject and to each

other. The OTF cell measures the cumulative, "enough light" effect

without discriminating between the light sources. The individual

contributions of individual light sources (master and slave flash/es)

to the overall illumination is not influenced or detected by the OTF

metering. It's the photographer who influences those contributions by

placing the flashes at various distances, putting nd filters, and

making other adjustments (e.g. ratios with an EOS-3 or 1V). All these

manipulations are reflected and visible in the picture but they have

nothing to do with how the OTF cell "sees" the world. Using FEC or

changing film speed setting does have the effect but, again, the effect

is on the cumulative, overall illumination, not on individual strobes.

 

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By the way, it's not recommended to use Lite Links with EX strobes

working in E-TTL mode because of the preflashes which can (and will)

fool the Ikelite operation.

 

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Piotr