How can I prevent overexposures by my SX260 HS when used underwater with an Ikelite AF35?

[marshlaird]

The AF35 is an underwater strobe that fires when it senses that the camera's flash has gone off.  At close range (under 3 feet or so), the camera overexposes by about 10 stops!   I want to use an auto setting, but can't get the proper exposure with any auto mode.    Any hints?

[cicopo]

I'm assuming your strobe fires at full power but if not you may have to turn it's power down as well as learn how to shoot in manual. I'm also assuming the Ikelite is being fired strictly by seeing the light output from your built in flash. (maybe via a fiber optic cable but maybe just from reflected light). If the Ikelite only has full power & you can't set the camera to handle it during close ups (lowest ISO, highest shutter speed that works with flash & smallest aperture) then you will either have to rely on using a strong difusser, pointing the strobe away from the area you're photographing or turning it off. Does the built in flash still light the area in front of the camera or is it blocked?  

[marshlaird]

None of that helps.  Sure, I can make lots of manual adjustments to reduce the overexposure.  The core problem though is that the camera does not automatically adjust to the presence of the external strobe.  Instead the camera appears to be metering without the strobe and deciding that a long exposure is needed.  

[cicopo]

EXACTLY. It has no way to know you have an additional flash so unless you learn how to use it as suggested buy a camera & strobe that can communicate with each other via a hard cable. You have to decide which way to go but the set up you're using now CAN'T communicate. Learning how to do it in manual isn't that tough & in the long run will provide you much better results in & out of the water. I say all of this because I'm using dual strobes that can't communicate either & have for years, and once I got the hang of it things started to meet my standards.

[marshlaird]

I don't know where I got my expectations, but I had thought that a modern camera (even a little guy like the SX260) would meter the total amount of light that had been received while the electronic shutter was open, and then shut the electronic shutter when enough photons had been captured.   Silly me.

I am leaving for Rangiroa tomorrow.   I'll see what I can do ...

[cicopo]

I suspect a high end camera / flash "might"  be able to do it within a small margin but you can't tell your camera (nor can I tell mine) there will be additional light and maybe a lot more than needed. By design the shutter must be open while that light returns to the lens (reflected back by the scene being photographed) BUT if there was so much light that the camera had to choose a very fast shutter speed the shutter would be closed before the light has made it back to the camera. For this reason (I suspect) it makes very good sense to either buy a flash (or strobe) which has multiple power settings or can communicate with the camera so the camera knows it's there & that it needs to lower the power. 

Have a great trip & please share your results once home. Will you be shooting in RAW?

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