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

[stuartv]

Let's say I have a camera and I'm shooting with the built in flash in Auto mode.

The camera decides how much flash to use and what shutter speed and aperture to use.

Now, I add a strobe with an optical cable that supports DS-TTL. When I take a picture, the camera's flash fires and that triggers the strobe to fire, too, right?

But, how does the camera know that that is going to happen? How does the camera know to increase the shutter speed or stop the lens down so that the extra light from the strobe doesn't just leave me with a very overexposed picture?

Does the pre-flash thing I've seen mentioned cause the strobes to pre-flash also, so that the camera sees how much actual light it's going to get when the strobes and flash fire, and that's how it decides on all the exposure parameters?

I'm thinking there is something very basic about TTL that I don't understand....

Thanks for any help!

 

[Cali_diver]

You should check out these websites, they have great resources to learn from!
Underwater Photography Resource Center :: Handbooks for Better Underwater Photography
Guide to Underwater Photography

 

[JackConnick]

It doesn't. Don't use auto mode with external strobes. It thinks you're shooting in a dark room and it doesn't see the light, so it zooms up everything (ISO, down on Shutter and aperture) and overexposes.
Use aperture or shutter priority or even better manual modes.

And yes (thanks Cali_divergirl!) we do have a couple of nice FREE Handbooks; "Choosing a Lighting System" and "Strobe Positioning" that will help with these and a lot more questions...

 

[stuartv]

You should check out these websites, they have great resources to learn from!
Underwater Photography Resource Center :: Handbooks for Better Underwater Photography
Guide to Underwater Photography

I looked at both of those before I posted my question. I didn't see the answer to my question in either place.

 

[stuartv]

It doesn't. Don't use auto mode with external strobes. It thinks you're shooting in a dark room and it doesn't see the light, so it zooms up everything (ISO, down on Shutter and aperture) and overexposes.

Then what is it that the strobes with TTL sync are doing?

 

[Got2Go]

Let's say I have a camera and I'm shooting with the built in flash in Auto mode.

The camera decides how much flash to use and what shutter speed and aperture to use.

Now, I add a strobe with an optical cable that supports DS-TTL. When I take a picture, the camera's flash fires and that triggers the strobe to fire, too, right?

But, how does the camera know that that is going to happen? How does the camera know to increase the shutter speed or stop the lens down so that the extra light from the strobe doesn't just leave me with a very overexposed picture?

The camera doesn't know you have external flash.

When the internal flash fires, the external flash picks up the pre-flash that the camera fires.
The external flash then also fires a pre-flash...the camera uses the pre-flash to meter the shot.

Then the actual shot flash is fired, with the appropriate duration determined from the pre-flash.
The external flash then fires for that same duration as the internal flash.

Basically, internal and external flashes do pre-flash, and flash, at the same duration.
Camera only adjusts the duration of the internal flash...external flash in TTL mode mimics this same duration.

This only works if the camera does pre-flash..otherwise you are stuck with shooting manual flash settings on the external.

 

[stuartv]

And yes (thanks Cali_divergirl!) we do have a couple of nice FREE Handbooks; "Choosing a Lighting System" and "Strobe Positioning" that will help with these and a lot more questions...

As I said, those guides didn't answer my question at all, as far as I can tell. They seem to be providing extremely super basic info - not the details of how TTL and DS-TTL or slave TTL works.

Anyway, I feel like your response boils down to "don't be a novice. Be an expert." That doesn't really help me much. I expect I will eventually get to where I am comfortable shooting in manual mode. But, just starting off, I would like to use auto mode to begin with and advance myself gradually from there.

And other reading has given me a pretty clear impression that it is in fact possible to connect strobes with fiber optic cables and then shoot in fully automatic mode and still get pictures that are correctly exposed. You said that is not true. What am I missing?

 

[stuartv]

The camera doesn't know you have external flash.

When the internal flash fires, the external flash picks up the pre-flash that the camera fires.
The external flash then also fires a pre-flash...the camera uses the pre-flash to meter the shot.

Then the actual shot flash is fired, with the appropriate duration determined from the pre-flash.
The external flash then fires for that same duration as the internal flash.

Basically, internal and external flashes do pre-flash, and flash, at the same duration.
Camera only adjusts the duration of the internal flash...external flash in TTL mode mimics this same duration.

This only works if the camera does pre-flash..otherwise you are stuck with shooting manual flash settings om the external.

THANK YOU! That is exactly how I had finally decided it must work. I just never saw any mentions anywhere of an external strobe doing its own pre-flash - only the camera itself doing a pre-flash. And I didn't see anything that said a camera doing TTL always, must do a pre-flash, so I wasn't positive that a pre-flash was always done. I guess it's not - if the camera determines it doesn't need a flash at all to get the correct exposure. But, I couldn't see any other way that it could work. I just wanted to hear someone confirm that it really does work that way.

 

[Stoo]

Then what is it that the strobes with TTL sync are doing?

Messing up your pics mostly. ;-)

TTL has it's application, but really, learn how to shoot manually. Auto exposure is great for topside shots, but the underwater world is too complex. We have everything from limitless hues of blue to highly reflective fish and everything in between.

With digital cameras, you see what you have shot.. practice and learn! Your images will be so much better...

 

[Storker]

Then what is it that the strobes with TTL sync are doing?

Messing up your pics mostly. ;-)

TTL has it's application, but really, learn how to shoot manually. Auto exposure is great for topside shots, but the underwater world is too complex.

Allow me to disagree.

If the strobes' TTL works well with your camera (like the Inon/Olympus combo IME), TTL strobe control is just great. Manual is s-l-o-w compared to automatic. Every time the distance to your subject changes, you have to adjust strobe output and/or the aperture. If you spend like ten minutes on the same macro subject, that may be fine, but there are situations where you just don't have the time to mess around fiddling with the dials because you'll lose the shot. Manual also adds to the task loading, which isn't always a good idea. My to-go setting is aperture priority auto, -2EV compensation on ambient and zero compensation on the strobes. In 90% of the situations I shoot, that setting gives very good exposure directly, and if it doesn't I can adjust exposure compensation on both ambient and flash to dial it in. And since I always shoot raw, white balance just isn't an issue.

I started my photography hobby with a fully manual SLR and a manual strobe. And I loved it when I got auto flash, then auto exposure and finally TTL flash, raw file recording so WB isn't an issue any more, and a camera capable to control flash and ambient independently. Setting WB in post, automatic exposure, auto-ISO and TTL flash makes life a lot easier, and if you know how to tweak the settings, it can provide just as good results as manual with less work.

And @Got2Go nailed it when it comes to explaining how external strobe TTL works.