Adding a strobe (or 2). How does the camera know?

[stuartv]

the detection of reflected light is all performed in real time.

That is definitely the key point that I did not understand before. Thank you.

 

[KeithG]

ttl mode - the camera controls the amount of light emitted by the strobe(s) by detecting how much light has been reflected to the camera.

digital ttl control is different than old school analog ttl control.

old school analog ttl control was performed in real time. during exposure the camera measures the amount of captured light and then signals the flash to stop.

digital ttl: the camera performs a preflash (of known manufacturer specific intensity) and then determines the amount of light the flash should emit (just like in manual control), if you are using a land flash (speedlight) this setting is digitally communicated to the flash (this is why they use a preflash instead of real time). since your strobe does not support the manufacturer specific digital protocol, the only thing it can do is behave as an optical slave - it turns on with the onboard flash and turns off with the onboard flash. many cameras perform multiple preflashes which can screw things up.

 

[Storker]

Are you sure this is true for the Z240?

Pretty certain, yes.

 

[KeithG]

so in all of is, the thing detecting the light (camera in ttl, strobe in auto strobe) has no idea where it is coming from. so the same theory applies for 1,2,3,... strobes. the more strobes you have, the less light each one will need to emit in order to reflect the same amount of light.

but this all falls apart when you use different strobes. flash tubes do not deliver their power linearly. being "on" twice long doe not mean that twice as much light was emitted. different strobe models have significantly different curves.

the manufacturer of your camera knows the discharge curve of its onboard flash. they also know the discharge curve of each flash they manufacture. so ttl within the same manufacturer works great.

when you mix in third party flashes like scuba strobes you are left in a "best effort" world. it is a crap shoot to see if your strobe manufacturer has created a strobe close to what your camera expects...

 

[KeithG]

Why don't cameras do it the same way, instead of using pre-flashes? At least, why don't mirrorless and compact cameras do it this way? I could see where SLRs that have a mirror that flips up out of the way might not be able to do this.

because of external digital flashes (the ones that go on your hotshoe). and lazy firmware weenies and a few other things...like lack of shiny film...

in the old days a separate analog signal wire was used to quench the strobe in real time. this requires extremely quick real time response. Easy to do with a dedicated signal line. This provides a high level overview: MY Strobe Controller Blog

The analog system was based upon 2 important principles:
- being able to detect in real time the amount of light falling onto the film
- being able to control an external flash in real time

Neither of these can be done (simply, reliably, cheaply) in our new improved digital world. Instead the digital camera does a "short" preflash, measures how much light it got, extrapolates to how much light it will need and then instructs the flash (onboard or external) to fire at a predetermined power level. There is no real time measurement of captured light while the image is being captured.

 

[Nemrod]

Thank you again.

From what you are saying, I think I am distilling a recommendation of a plan, which is:

- Use Aperture priority on the camera and set my aperture for my desired depth of field.
- Set the camera for no pre-flash and lowest flash power.
- Set the strobes to Auto mode and "no pre-flash" mode and an aperture setting corresponding to what I have the camera set at.

This will eliminate the slight delay that comes with using pre-flash and it will reduce the camera battery drain to as low as is possible.

Alternatively, set the camera to Manual mode and dial in the aperture and shutter speed to give me the DOF and background water color that I want (assuming wide angle shooting "into the blue"). Otherwise, same as above.

That all jibes well with what I've been getting out of the Martin Edge book, The Underwater Photographer. I will definitely give it a try!

I really appreciate the explanations!

I think your basic plan is good for starters. However, I set aperture for my subject and shutter (to the extent I can control it in Auto Av and sync) for the background. The strobes expose the subject and the shutter sets the background exposure. To understand this realize the strobe is done in thousandths of a second, your shutter is open much longer than the duration of the strobe. A bias to achieve higher shutter speeds will produce darker backgrounds. The f-stop (aperture) is set for the subject. This is one of the areas where going to Manual will allow better control.

You do not need to match the strobe f-stop dial to the camera f-stop setting. That is a good starting point but adjust up or down for each shot to achieve the exposure balance you want. I usually, as well, run my camera at -1.5 f-stop, so yes, by all means use the exposure compensation to brighten or darken the photos overall, again the strobes are exposing the subject and are dominant.

As has been pointed out, at some point using Auto (Av) becomes a liability and that is when you go to Manual. But learning how to manipulate the Auto capabilities will allow fast and effective exposures for most of your general shooting.

The Program mode or Auto Av can be very effective using the s-TTL function of the Inon strobes. I use Program because it defaults to pre-flash allowing me to leave my camera set no pre-flash for Av/Tv and lets me leave the strobe set to no-preflash for Auto exposure control. It is a shame most people do not understand the chart below, made for Canon S/G cameras it is essentially correct for most all similar cameras, it is my chart, I stand by it as correct:

N

 

[stuartv]

I suppose this is a slight derail, nevertheless...

I got in the water with my new-to-me Inon Z240 (and OM-D E-M10) on Saturday.

The strobe seemed to be working fine with any combo of camera in iAuto or P mode and the strobe in sTTL or Auto mode.

But, when I switched the camera to A mode, I set the aperture to f/11. I tried the strobe set for sTTL and for Auto and it would never fire. I tried pushing in and locking the button down that tells it no pre-flash or pre-flash and still no difference. I assume that means the built-in flash in the camera was not firing. The pictures were all black. Only after reviewing them later on my tablet could I see that they were not pure black, just so dark that you could barely tell there was anything in the picture. Viz in the quarry was about 5'.

I looked through all my camera's menus and I couldn't find anything that I recognized as controlling the camera's flash. I don't see a way to force it on (or off) or to control the flash output. IIRC, I did a factory reset on it and updated the firmware shortly after I got it, as the previous owner told me he had customized some of the settings.

I haven't yet had a change to play with it on land and confirm that the built-in flash was not firing, so that is just a suspicion at this point.

Any suggestions on what to check to figure out what was going on there?

 

[Storker]

when I switched the camera to A mode, I set the aperture to f/11. I tried the strobe set for sTTL and for Auto and it would never fire. I tried pushing in and locking the button down that tells it no pre-flash or pre-flash and still no difference. I assume that means the built-in flash in the camera was not firing. The pictures were all black. Only after reviewing them later on my tablet could I see that they were not pure black, just so dark that you could barely tell there was anything in the picture. Viz in the quarry was about 5'.

I looked through all my camera's menus and I couldn't find anything that I recognized as controlling the camera's flash. I don't see a way to force it on (or off) or to control the flash output. IIRC, I did a factory reset on it and updated the firmware shortly after I got it, as the previous owner told me he had customized some of the settings.

I'm using a setup very similar to yours (OM-D E-M5 Mk1, 2xZ240 Mk4) and shoot almost exclusively in A mode (except when night diving; then it's M mode), TTL strobe. I've set my camera to "fill-in" flash mode which forces the flash/strobes to fire and my only real issue is finding the right amount of underexposure on ambient (base case: -1EV with the Oly 9-18, -2EV with the Pana 8mm/3.5 FE). Some shots are of course made with no flash, only using ambient, and then I have to adjust exposure bias during shooting, and/or adjust exposure in post.

 

[KeithG]

I'm using a setup very similar to yours (OM-D E-M5 Mk1, 2xZ240 Mk4) and shoot almost exclusively in A mode, TTL strobe. I've set my camera to "fill-in" flash mode which forces the flash/strobes to fire and my only real issue is finding the right amount of underexposure on ambient (base case: -1EV with the Oly 9-18, -2EV with the Pana 8mm/3.5 FE). Some shots are of course made with no flash, only using ambient, and then I have to adjust exposure bias during shooting, and/or adjust exposure in post.

agreed. anytime you use a camera in an automatic mode it will do its best to take a picture without use of the onboard flash. this generaly means doing anything it can to get more light: slowing down the shutter, opening the aperture and raising the ISO. only if all of these fail will it resort to actually firing the flash.

so it you are using strobes as optical slaves, you need to ensure that you force the camera to always fire the flash.