In general how exactly does TTL Work

From what I understand with my Fuji F30, the way TTL works is sort of like this (forced flash). After pressing the button:

1. The mirror drops but the shutter doesn't fire (yet)

2. Camera sends a pre-flash from the onboard flash

3. Camera measure the light and determines the burn time for the onboard flash for the correct exposure

4. Camera burns the onboard flash for the calculated time while snapping the picture.

In any event, for TTL to work, the camera needs to measure light from the pre-flash.

This would imply that when working with a slave strobe in TTL mode, the Strobe would need to fire the pre-flash.

Is this true? Do these strobes that support TTL with pre-flash compacts fire the pre-flash and then the main flash? Specifically, does the YS110 fire the Pre-Flash?

Sorry in advance if this is an obvious question...

Thanks!!

Yes, that's true. In reality, different cameras use varying numbers of preflashes so it's actually even more complicated. What you've said, though, is basically how it works.

Although you said TTL, this is for digital TTL of course. Conventional TTL does not work this way.

Do the Underwater Strobes fire a pre flash in TTL ?

granted I have no experience with Digital UW TTL .. but i don't remember the Nikonos doing this. (which I gather works differently from a sensor)

With film TTL there is no preflash fired. The camera measures the reflected light off the film while the shutter is open.

Cheers

James

James .. right I got that ... but I don't think I have ever seen anyones strobes fire pre flashes underwater ... have i just not seen anyone using TTL UW ?

James .. right I got that ... but I don't think I have ever seen anyones strobes fire pre flashes underwater ... have i just not seen anyone using TTL UW ?

INON strobes set to S-TTL mode fire a preflash .

When an INON strobe is set to S-TTL, the external strobe replaces the onboard flash's responsibilities. The preflash is sent out by the INON and metered by the camera's sensor. The INON set to S-TTL is capable of mimicking the onboard flash (able to keep up with it as far as speed)

btw........but hopefully not to confuse(nothing to do with TTL) ........and INON strobe set to ACC also fires a preflash:

When an INON strobe is set to ACC, a very strong preflash (actually much stronger than the onboard flash is capable of achieving) from the INON strobe "tricks" the camera into thinking more ambient light exists in the composition than in reality. The result is that the camera subsequently fires a smaller than otherwise main flash.......thereby conserving camera battery power, better recycle performance, and heat within the housing, etc.........

Many other strobes are capable of mimicking the onboard flash and firing a preflash.........I used INON only for example.

One other point worth mentioning in a follow up to what James has already offered. One difference between film and a digital sensor is that light reflects differently off the two capture mediums(you recall James mentioned that in "classic" ttl, light is measured directly off the film being exposed)........therefore a need to come up with the alternative solution which is a predetermined, measured preflash sent out.......record what comes back...........and calculate~extrapolate from there...............

The main flash fires so quickly after the preflashes that there's no way to perceive it. When you see a series of distinct preflashes, that's for red eye reduction, not digital TTL.

So digital TTL is very different to film TTL

Film read how much light there was and then decided how much more to add.

Digital Adds some light and then decides how much light it really needs to add according to the settings of the camera or what settings to put the camera at. Depending on the settings you have selected. ?

Has anyone ever talked about which method works better ? or has there been other methods tried out for digital ?

I haven't looked at Digital TTL UW for a while as the last time I did the results were inconsistent at best.

Film TTL has the advantage of working while the exposure is in progress. It doesn't place any significant demands on the strobe other than the TTL signaling itself. All TTL has limitations figuring out what the correct exposure ought to be, so blacks, whites, and reflective objects tend to be problems. Digital cameras usually can't implement regular TTL since the sensor isn't reflective as Bob said already.

Digital TTL has the advantage that multiple strobes can be individually calibrated for exposure (although we don't have that underwater so far). Nikon's system can support many strobes with 3 or 4 separate groups. The downside of digital TTL is that it implements more complicated new protocols and requires the strobes to include significant new capabilities that many underwater strobes can't do.

I wish the industry would simply implement Nikon's CLS over fiber. With a Canon bridge, at least some of that functionality could be used with Canon systems as well. I doubt seriously that Nikon makes their CLS protocol public, though. :-(

Yes, that's true. In reality, different cameras use varying numbers of preflashes so it's actually even more complicated. What you've said, though, is basically how it works.

great thread

your dSLR is doing it's own TTL whenever you take a shot with the camera's internal flash

To give more details, I believe the D80 and the D200 will normally put out 1 pre-flash, but it darker conditions (conditions we experience underwater in so-cal most of the time), it will put out at least 1 additional pre-flash to measure the exposure correctly. so for these cameras, for TTL to work, strobes need to be able to mimic multiple pre-flashes and be fast enough to keep up with them. this is also why new TTL-converters for the D80/D200 need to be tested underwater or in dark conditions, in addition to normal top-side tests.

scott

To add to the confusion: Ikelite's translation of e-ttl is considered very good, but on theire canon camera page, it is marked :

" (1) Camera disables pre-flash to the hotshoe unless a correct Canon eTTL capable strobe is attached."

so without preflash, how would that work ?