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

[Storker]

I can see how the Z240 would work in its Auto (non-TTL) mode. But, what happens if you are using 2 of them. Won't you end up with serious overexposure?

Think about it: Both strobes have a sensor measuring strobe light. The strobes don't know the distance to the subject or the strobe power. Both strobes fire simultaneously. The strobes' sensors measure all the light reflected from your subject.

Would that lead to overexposure? And if it were to do that, would one EV overexposure be an issue if you were shooting raw format?

 

[stuartv]

Think about it: Both strobes have a sensor measuring strobe light. The strobes don't know the distance to the subject or the strobe power. Both strobes fire simultaneously. The strobes' sensors measure all the light reflected from your subject.

Would that lead to overexposure? And if it were to do that, would one EV overexposure be an issue if you were shooting raw format?

If they strobes measure the light while they're firing, then no overexposure. If they measure before firing, then yes overexposure. I don't know how they work, which is why I asked. I've always kind of figured that they might only need to burn for such a short time that they must meter and determine burn time before they actually fire. Being able to fire, meter, then quench all in the span of time that equals the shortest possible burn time seems like a lot to ask. I'm prepared to believe it works that way, if you say so. I just don't know.

 

[Storker]

If they strobes measure the light while they're firing, then no overexposure. If they measure before firing, then yes overexposure. I don't know how they work, which is why I asked. I've always kind of figured that they might only need to burn for such a short time that they must meter and determine burn time before they actually fire.

Strobe A auto was developed long before pre-flash (which is only needed for TTL)

 

[stuartv]

I think you're still implying some things about how it works that have not been expressed.

I was assuming (with low confidence) that strobe Auto worked similarly to the way the camera itself works when not using a flash. To wit, it meters the ambient light and determines the exposure setting (i.e. the strobe burn duration) before it ever fires, based on whatever aperture setting you tell it that you're using (via a dial on the back, in the case of a Z240). Just like the camera (in Auto mode) determines the aperture and shutter speed before you actually release the shutter.

I now gather that you are saying strobe Auto mode works by knowing the camera aperture setting, starting the strobe burn, metering the incoming light while the strobe is burning, and THEN determining how long to let it keep burning. That certainly sounds like a more accurate way to do it.

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.

 

[Storker]

I now gather that you are saying strobe Auto mode works by knowing the camera aperture setting, starting the strobe burn, metering the incoming light while the strobe is burning, and THEN determining how long to let it keep burning. That certainly sounds like a more accurate way to do it.

Pretty close, except the strobe doesn't know what aperture the camera is using. You're telling the strobe that (or, rather, the strobe tells you which aperture to use). Lots of latitude for cheating here.

Why don't cameras do it the same way, instead of using pre-flashes?

Can't balance ambient and strobe that way, since in A mode the strobe actually determines which aperture to use. The pre-flash checks the strobe exposure given the camera's settings and can also balance strobe and ambient. You can do that in strobe A mode, but it's a bit more complicated. Pre-flash relieves you of thinking about the balance between ambient and strobe.

 

[stuartv]

Pretty close, except the strobe doesn't know what aperture the camera is using. You're telling the strobe that (or, rather, the strobe tells you which aperture to use). Lots of latitude for cheating here.

Are you sure this is true for the Z240? My understanding is that if I use the strobe Auto mode, I am supposed to set the dial on the right side of the back of the strobe to the value of the f/stop I have the camera set to. It uses that and its own built-in light meter to determine how long to burn when it's triggered.

If my understanding is correct, my next question is: Does that still work when using a non-35mm dSLR? Or do I need to "correct" my f/stop setting (that I set on the strobe) based on my camera being a m43?

 

[Nemrod]

Stuart, no you do not need to "dial it down" or halve the power. The expoure system in the strobe will see the output from the other and compensate.

And as to Auto modes vs. Manual modes I will stay with exactly what I said with no back tracking or crawfishing, lol.

In Auto Av camera mode and Auto strobe mode you can vary the exposure in much the same way as in Manual with similar results but can go to a neutral setting for transitioning areas or when on the hunt in case that whale shark happens by. You will at least get a shot or two while the Manual only guys are dinking around trying to set up.

N

 

[KeithG]

there are lots of similar concepts that easily get mixed up and cause lots of confusion. it likely makes sense to deal with each concept in a separate post... so here goes...

manual strobe control. you get to control how much light the strobe emits. the strobe will always emit the same amount of light, regardless of any camera setting, distance to subject, type of subject or strobe angle / direction. that is why manual is used for advanced effects that will confuse an automated system.

 

[stuartv]

Stuart, no you do not need to "dial it down" or halve the power. The expoure system in the strobe will see the output from the other and compensate.

And as to Auto modes vs. Manual modes I will stay with exactly what I said with no back tracking or crawfishing, lol.

In Auto Av camera mode and Auto strobe mode you can vary the exposure in much the same way as in Manual with similar results but can go to a neutral setting for transitioning areas or when on the hunt in case that whale shark happens by. You will at least get a shot or two while the Manual only guys are dinking around trying to set up.

N

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!

 

[KeithG]

automatic strobe mode: the strobe attempts to deliver a consistent pre-set amount of reflected light. this is light reflected back to the strobe, not the camera. the strobe will emit a different amount of light for the same strobe setting depending upon how much is reflected back to the strobe.

no camera settings will affect the amount of emitted light. subject distance, type of subject and strobe angle will affect the amount of emitted light for the same strobe setting.

if you shoot wide angle and your strobe is pointing into the deep blue, then the strobe will perform a full dump since nothing was reflected back to it.

if you jam your strobe up real cose to something, the strobe will emit a minimal amount of light since most of the light will be reflected back to the strobe.

if you shoot a shiny fish then the strobe will likely emit a minimal amount of light since the fish will likely reflect a lot of light back to the strobe - but this highly dependent upon strobe angle and distance.

the "strobe setting" knob is really just a low to high reflected light knob. it really is not an "aperature setting" even though some strobes like to label it like it is (sea & sea specifically makes this point in there manuals, although their wording is hard to understand)

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