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TTL and Measuring. Should I get flash adjusted values?


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#1 simonmittag

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Posted 08 February 2009 - 01:07 PM

Hi Everyone,

I recently started using a Heinrichs Weikamp TTL converter for Nikon. Other gear involved is the Nikon D300, A Sea&Sea housing for it, two YS-250 flashes and a dual sync cord. I have TTL working and am taking pictures under water and am reasonably happy with the kit. The main advantage being you can point your flash at the subject without overflashing it, as the TTL kicks in and shuts down the flash. So far so good.

Now, I'm wondering..

With TTL turned on on the controller, should I get flash adjusted exposure values when I measure with the camera? I often have issues running the camera in aperture priority mode, giving me slow shutter speeds, 1/100s or 1/60s, even at wider apertures, but especially with a macro lens. Should this be the case for TTL? I can understand for non TTL, where there is no light to work with at depth, but how does TTL work in itself?

The D300 uses front-sync, should it preflash when I hit the forced measuring, or focus controls on the camera and give me TTL measuring information based on this? Does TTL even know about measuring or is it only used to shut the flash off prematurely and has nothign to do with measuring?

If we pretend the first option being true, it would need to flash for measuring, but I can't see anything on the unit when I focus or measure.second option.

If we pretend the second option being true, TTL would preflash and adjust exposure information at depth if manually triggered (I can't imagine it would do it constantly, due to battery life, epilepsy :huh:)

Does the camera have another way to figure out exposure with TTL?

Does anyone know how this really works?

Cheers,

Simon
http://www.simonsaysscuba.com - Simon's Underwater Photography Blog

#2 simonmittag

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Posted 09 February 2009 - 10:50 PM

ok , i'll try in plain english :huh: Should TTL influence metering exposure (shutter times) or just flash amount?
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#3 scorpio_fish

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Posted 10 February 2009 - 06:30 AM

With TTL turned on on the controller, should I get flash adjusted exposure values when I measure with the camera?


We just had this discussion in another thread.

The answer is no.

If you have Heinrichs TTL converter attached, you have two very different meterings going on. Your cameras meter is an ambient light meter. Flash has no direct bearing on the exposure. The indirect effect is that Nikon's default normal sync mode will set a shutter speed floor of 1/60.

To demonstrate this, set your camera flash sync mode to slow sync. This removes the minimum shutter speed limitation in aperture priority. While in aperture priority point the camera and get a meter reading. Add a flash, or pop-up the flash. The meter reading doesn't change. Same if connected to strobes with or without Heinrichs TTL converter attached.

Heinrich's TTL converter adds a second measurement. It magically figures out how long the flash duration should be based on your cameras exposure setting and the camera scene. It doesn't care about your camera's metering results, only where your camera's exposures are set.

The camera's ambient light meter does not know how much flash is going to be generated, but the flash TTL knows how the camera's exposure settings are set.
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#4 simonmittag

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Posted 12 February 2009 - 02:28 AM

We just had this discussion in another thread.

The answer is no.

If you have Heinrichs TTL converter attached, you have two very different meterings going on. Your cameras meter is an ambient light meter. Flash has no direct bearing on the exposure. The indirect effect is that Nikon's default normal sync mode will set a shutter speed floor of 1/60.

To demonstrate this, set your camera flash sync mode to slow sync. This removes the minimum shutter speed limitation in aperture priority. While in aperture priority point the camera and get a meter reading. Add a flash, or pop-up the flash. The meter reading doesn't change. Same if connected to strobes with or without Heinrichs TTL converter attached.

Heinrich's TTL converter adds a second measurement. It magically figures out how long the flash duration should be based on your cameras exposure setting and the camera scene. It doesn't care about your camera's metering results, only where your camera's exposures are set.

The camera's ambient light meter does not know how much flash is going to be generated, but the flash TTL knows how the camera's exposure settings are set.


Mate thanks for your help! So with flash set to slow sync I am turning off the 1/60s floor, just tried that. But if I understand you correctly I won't get any faster exposures unless I find enough ambient sunlight to meter against, or use manual mode?

Edited by simonmittag, 12 February 2009 - 02:35 AM.

http://www.simonsaysscuba.com - Simon's Underwater Photography Blog

#5 scorpio_fish

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Posted 12 February 2009 - 03:47 AM

Mate thanks for your help! So with flash set to slow sync I am turning off the 1/60s floor, just tried that. But if I understand you correctly I won't get any faster exposures unless I find enough ambient sunlight to meter against, or use manual mode?


The 1/60 floor only goes into effect if you have the flash open or a strobe attached while in normal sync.
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#6 The Octopus

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Posted 03 March 2009 - 02:09 AM

Manual is the only way to control shiutter speed and f stop. A priority does decrease the speed to 1/60th or at best 1/100th
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