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Hi all

i thought i had a little basic understanding of what TTL does in a strobe, that it would be an automatic flash exposure without having to set any power output? i am about to get and use my first ever strobe and trying to learn as much about them as i can. I'm now getting confused as i thought this would just be a case of connect strobe via optic cable turn strobe to TTL and it sorted it would work.

 

my camera is the cannon gx7 mark ii, do I have to set anything up on the camera for this option to work

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The basic idea of TTL flash metering is that the camera fires a brief pulse with its onboard flash, takes a picture, evaluates the resulting exposure, then decides how much flash it needs to properly expose the scene, opens the shutter and fires the main pulse. One thing that isn't obvious and that you should keep in mind is that flash intensity is not actually adjustable - instead, the flash exposure is regulated by flash pulse duration - the longer the tube is kept burning, the more light it emits. Its intensity is also not uniform - it takes a little bit of time to reach its peak, and when the current is cut off, it takes some time to cool down and stop glowing. Granted, this time is measured in fractions of a millisecond, but with overall maximum burn time of maybe 3 milliseconds, it does have some effect. The ways that it manifests in actual shooting experience are several:

 

(1) The camera software is calibrated for the intensity curve of the on-camera flash, but the off-camera strobe(s) usually have a different curve, and this can result in the metering being 'off'.

(2) The maximum burn time of the small on-camera flash is typically around 1ms, while big off-camera strobes may need 3ms or more for a full discharge - depending on how sophisticated the strobes' TTL circuitry is, this may limit your strobes' maximum output.

(3) If your exposure is too fast, this can cut into the strobe power - not usually a problem for interchangeable-lens cameras where the curtain shutter limits flash sync speed to 1/160-1/320s (depending on camera model), but the leaf shutter on your fixed-lens compact allows flash sync all the way to 1/2000s - however, 1/2000s is just 0.5ms, which is far too little time for a strobe to dump its full output; in fact, chances are good that it hasn't even reached peak intensity yet by the time the shutter closes. This is something to keep in mind when using very fast sync speeds for things like shooting directly into the sun.

 

Finally, and this is mostly unique to underwater photography, the main challenge with TTL is that evaluating exposure in an underwater scene is quite difficult for cameras because when your background consists of water, the camera software doesn't really know that it can't light up all that water and tries to illuminate it for a uniform exposure, which ends up overexposing your subject. Different cameras offer multiple ways of dealing with this issue - for example, on my Sony A6300, I set the metering mode to 'Spot' and make sure the subject is in the center of the frame; that way the camera ignores the water background in other parts of the image. It is also possible to use Flash Exposure Lock function to meter the flash on a certain point, then recompose and shoot with the locked setting. Sony A6500 also offers the option of spot metering off the focus point rather than the center of the frame. I'm not familiar with the Canon G7XII - I would suggest reading the user manual carefully, particularly the chapter(s) about flash usage, as the basic idea is the same - if you want to use TTL, and you have significant amounts of water in your image, you must coax the camera into ignoring that water while metering the exposure. If you're shooting macro and most or all of your background is close by an illuminated by the strobe(s), then this is much less of an issue.

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The success or otherwise of TTL is very dependent on the camera, it is what does the calculations to get what it thinks is the right exposure and some cameras get it very wrong with wide angle shots, less so with macro.

 

The bigger issue seems to be shot to shot consistency. The camera calculates exposure for every image, so you might frame, shoot review, adjust flash EV comp and re-shoot, but if the framing's not the same the camera may decide on a different output and the result will be different to what you might expect. Manual flash on the other hand puts out the same amount of light with each flash and if you adjust the light output it will change predictably. This why manual flash is recommended underwater and it is not as difficult as it seems.

 

The bigger issue is adjusting the external strobe, you need to set the flash depending on whether or not to expect a pre flash. In TTL it needs the pre flash to work. In Manual it does not, but some cameras only provide TTL with their onboard flash so the flash needs to be told to ignore the pre flash. How to do this depends on which strobe you buy. You don't mention which strobe you are thinking of, but I would recommend getting one that can do both TTL and manual.

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thankyou for your imput that has been very usefull and now im leaning towards manual rather than TTL i was just thinking that setting the power mode for a shot and getting my head round how the power settings work and what they mean would be confusing. the strobe im looking at getting is the inon D200

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I would keep both manual and TTL options open. Lots of shots underwater are of the "grab" type where the subject and/or framing are fleeting - you may not have two chances at settings - and that's where TTL can be useful. In manual, you take one shot, evaluate the result on the camera monitor, adjust both strobes, take another shot, and do another evaluation. Of course, if the strobes have been "dialed in" before hand, and ambient conditions (depth, sun position, cloud cover) have not changed, then one shot would probably do. Finally, there's post processing that can work wonders. I've had better luck with slightly underexposed rather than overexposed, but those conditions may not be easy to evaluate in real time on a monitor underwater.

 

Most of my dives have been of the drift type in a group where things change fast and there's not much time to linger on a subject, which causes me to lean in favor of TTL.

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What I did first up was set it up on land and shoot an object maybe about 300-600mm away with the strobes in position as they would be underwater. Setup with the camera on 1/125 f5.6 and minimum ISO. Adjust power till exposure is right and use those settings as a starting point on your first dive. Spend some time fine tuning UW on that first dive. I would suggest just fixing aperture and ISO and work out your required shutter speed for background water and adjust subject exposure with the strobes power level only at least initially that way there is less to think about while you dial in settings.

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