TTL with dSLR: Preflash or no?

[AbyssalPlains]

In an article in the Jan/Feb 06 issue of the Underwater Photography Webzine I read that DSLR sensors are not fast enough to calculate exposure to quench the flash in real time. Therefore, the author explained, cameras use a pre-flash to determine the correct light output for the exposure, and this pre-flash is supposedly visible in the viewfinder because the mirror stays shut during that time. I'm confused - I have never
seen my E-330 trigger a pre-flash and on the liveview display I have never seen a flash either. Does anybody know what the deal is?

Also, what kind of metering for the flash exposure does this camera use? Matrix, spot, both? And where in the menu can I control the flash-metering settings?

 

[jlyle]

don,

What is that you want to do?

Metering can be found by pushing the left button on the arrow pad (next to the OK). Lots of choices - see the manual.

If you want to increase or decrease the flash level, go the menu:

menu
right, down 5x, right
set the level +/- using the right/left
OK

 

[AbyssalPlains]

Thanks, jlyle!

I guess I am still not clear on this whole TTL/external strobe thing. I never know whether changes that I make in the camera settings, e.g. the flash metering, will affect the strobes or not. After all, the camera manufacturer does not take into account the fact that someone will use it with external Ikelite strobes, right? But then, maybe they do? In short, I was under the impression that anything the camera does "internally" with regard to strobe metering and control is meant to work for the built-in flash, which in UW photography we completely disregard. I have a feeling I need to educate myself further - this is all still too much guesswork for me...

 

[jlyle]

Your strobes are connected to the camera's hotshoe. The camera controls the external strobes the same way it would a flash unit.

If you have the strobes set to TTL and the Ike housing set to TTL, then the camera is telling the strobes what to do (plus you can dial up or dial down the strobes on the back of the housing).

If the strobes are set to TTL and the Ike housing is set to manual (hold down both buttons for a second) then the strobes are only fired by the camera when the shutter is triggered and you manually control your strobes via the buttons on the back of the housing.

 

[RonFrank]

Ikelite does not use a preflash to determine the exposure like the manufacture flashes use. In the Nikon system, it's not ONE preflash, it's something like 14. This controls a lot more than just exposure with that system. Preflashes unlike red eye reduction, are generally not noticeable.

My experience is Nikon based, but I do use Ikelite DS strobe UW, and they do NOT preflash.

 

[Leisure Pro Brett]

With my Nikon D100 (yeah, it's time to upgrade) it uses preflashes for it's TTL exposure. I have read that it's something like 13-18 flashes, but it all happens so fast you cannot even tell. The real test comes when you use a slave flash, the slave goes off at the first pre flash and not during the actual exposure, and your finished image is even darker than it would have been without the slave flash. With any flash I am able to choose whichever type of exposure metering I want, but then again this is on a Nikon, I can't say as I have too much experience with Oly. I don't know if any of the underwater strobe makers really achieve TTL without using a physical cord, I know the strobes seem to work very well, but I don't know if they are doing true TTL.

(and I just noticed that people pretty much answered this already, guess that's what I get for leaving a half finished post at my desk all day:no )