Olympus TTL question

[rjbenfield 0]

I have an Olympus OM-D E-M5 in an Olympus housing with Sea and Sea YS-D1 strobes and optical synch cables. I've had issues with getting the TTL to work consistently. Sometimes it seems like its working and other times the exposure is completely off.

 

The more I look into the problem, the more confused I get regarding how TTL works. It is my understanding that TTL works by sending out a pre-flash that the camera then analyzes to calculate the flash output for the on camera flash which then fires and that "flash" is transmitted via the optical synch cable causing the strobes to fire and also giving them information on how to fire to get a "correct" exposure.

 

My questions are: Since the on camera flash is in the housing and not transmitted out of the housing other than via the optical synch cables, how does the camera calculate the TTL setting based on the pre-flash? Could this explain why the TTL exposures are so inconsistent? Or am I missing something very basic about how TTL works in this system?

 

I don't have a problem shooting in manual but am frustrated in not being able to figure the issue out. Thanks for any information and suggestions you may have.

 

Rodd

[coinee 17]

TTL uses the built-in metering to determine the duration of the flash needed. Money quote from http://www.divephotoguide.com/underwater-photography-techniques/article/underwater-photography-ttl/:' Let it be known - TTL is not perfect! The sensor reads the exposure in the center of the frame, so if the subject matter in the center of the frame is overly reflective or overly dark in comparison to the rest of the frame, the TTL controlled exposure will still not be correct.'

 

In my opinion TTL works mostly, but if you have something a tarpon (also known as 'swimming sliver of stainless steel') your exposure will be off. I personally don't use TTL at all anymore. With my optically controlled strobes I control the output manually (built-in strobe set to minimum, just used for triggering (if you use INON strobes, the magnets need to be IN in this configuration!)). Takes a little getting used to, but it's like the difference between shooting in M versus some program mode.

[adamhanlon 0]

Hi Rodd,

 

Your strobes (connected via fiber optic and in TTL mode) will mimic the output from your camera's flash. This includes the output of the pre-flash!

 

So when your camera pre-flashes, your strobes do so too. The camera then interprets the amount of light it "thinks" is needed to light the scene and sets the flash output - which is then mimicked again by your strobes.

 

Does that help?

 

Adam

[rjbenfield 0]

Adam,

 

Thank you. That was the key thing I didn't realize. Now it all makes sense.

 

Fabian,

Thank you as well. I agree, I think I'll preferentially use manual mostly, but nice to understand how TTL works for the times I may want to use it.

 

Thanks all for your help.

 

Rodd

[TimG 62]

Hey Rodd

 

Just to add to the excellent advice from Adam and Fabian, I use TTL for macro with my Subal housing, Nikon D800/Nikkor 105mm and Inon strobes.

 

For macro I think TTL is brilliant. For wide-angle less so - in fact I usually go manual.

 

I've used both fibre optic strobe initiation (now with the D800); and previously with a Nikon D300, Inon strobes, a Heinrich Weikamp TTL convertor and electronic connections through the usual Sea&Sea cables. Both systems work/worked really well for macro.

[rjbenfield 0]

Great. Thanks Tim.

 

Rodd