Can someone explain TTL?

[RDRINK25]

I have just purchased 2 YS-D1's and need to understand TTL better. I am still a novice shooter so from what I understand it is more like auto? You then can adjust the power level with the dial?

 

[KeithG]

I have just purchased 2 YS-D1's and need to understand TTL better. I am still a novice shooter so from what I understand it is more like auto? You then can adjust the power level with the dial?

Yes, TTL can be thought of the strobe being set to an "auto" mode.

But there can be many complexities. Your strobes support 2 different TTL modes
- one via the wired sync cord called TTL
- a different type via a fiber optic cord Called DS-TTL II

Which one you use will depend upon both your camera and personal preference. Most P&S cameras only support fiber optic, sp the choice may already have been made for you.

in true TTL mode the power level knob does nothing. In DS TTL II mode the power level knob does a small amount of exposure control, but not as much change in strobe power as in Manual mode.

 

[00wabbit]

TTL stands for Through The Lens metering. What happens is that the camera shutter opens and the camera sends a signal to the strobe to turn on. Once the camera has determined it has received enough light for the scene it sends a signal to the strobe to turn off and closes the shutter. The camera is metering the scene "through the lens" of the camera. So yes in a sense you can think of it as auto. You should also note though that the camera is calibrated for on land photography. So even though it can be considered "Auto" some adjustments may need to be made depending on the environment and your creative wish for the photo.

 

[Interceptor121]

In the modern era most cameras are connected through fiber optics to strobes and the implementation of TTL is semi-propietary
So to understand exactly what happens you need be looking at the specific of camera and strobes and how they are connected

 

[KeithG]

So let's assume you are using fibre optic cables. This makes TTL real simple. You will encounter lots of marketing BS that can be largely ignored.

The strobe performs as an optical slave and simply mimics the onboard flash from your camera. It turns on when the onboard camera flash turns on and turns off when the onboard flash turns off. That's it. Nothing more.

This will involve a "pre flash" that is used by the camera to try to determine how much strobe power is needed for the real shot. So the strobe will flash multiple times, generally too quickly for a human eye to detect the multiple flashes.

One other feature of your strobes is the TTL lamp. With a fibre optic conection this is really a "full dump" indicator and not a TTL indicator. If the strobe flashes at its brightest level by fully dumping all of the charge, the led will stay Red after a shot. If the strobe only uses part of its charge it will "claim" perfect exposure has been achieved and will turn Green. Which may not be true. The strobe is just guessing. This lamp does reflect TTL status properly when using a wired sync cord based system since it is then controlled by the camera.

 

[Interceptor121]

One other feature of your strobes is the TTL lamp. With a fibre optic conection this is really a "full dump" indicator and not a TTL indicator. If the strobe flashes at its brightest level by fully dumping all of the charge, the led will stay Red after a shot. If the strobe only uses part of its charge it will "claim" perfect exposure has been achieved and will turn Green. Which may not be true. The strobe is just guessing. This lamp does reflect TTL status properly when using a wired sync cord based system since it is then controlled by the camera.

The green signal just means flash return was detected and the camera sent the stop signal. It means nothing in terms of exposure being correct or not in fact I have had plenty of overexposed shots with green lights in my sea and sea strobe years.
Generally DS-TTL only works when the light has a reflection: subject right in front of the lens and quite close so you can forget about it for real wide angle work

 

[Stoo]

Like they said.... But I urge you to develop your manual shooting skills, assuming your camera will support that. This gives you much more control and ultimately better results. "Auto-exposure" seems to work really well on land, but not so much underwater.

 

[RDRINK25]

Thanks all for the insight. My camera is a T2i in a Sea & Sea housing. I have used TTL on my old strobes but never tried manual. On the new YS-D1's how can I learn more about manual as I want to learn more on it. There seems to be 2 manual modes on the D1's, one with pre flash and one without. I am assuming I want to be using the pre flash manual mode?

 

[KeithG]

Just thought it would make sense to provide some background explanation as opposed to just rote answers!

The green signal just means flash return was detected and the camera sent the stop signal.

This is not technically correct and this confuses many people which leads to much misunderstanding and frustration over incorrect exposures. There is no "stop" signal with optical ttl.

In wired synch ttl there are separate dedicated start and stop signal wires in the cable and the TTL lamp does provide a correct TTL indication as directed by the camera. In wired ttl there is an explicit "stop" signal. It has its own dedicated communication channel (wire) separate from the start signal. So in wired ttl a stop signal exists.

In optical TTL there is only "onboard flash is now on" and "onboard flash is now off" signals. Both are indicated over a single communication channel. The strobe attempts to imply a "stop" via the onboard flash is now off. But this will not provide correct TTL operation since the onboard flash could turn off under multiple conditions:
a) the camera detected proper ttl exposure and instructed the onboard flash to turn off, so the strobe also turns off - all is good!
b) the onboard flash did a full dump and turned off before full exposure was achieved and the strobe also turns off, camera is still waiting for correct TTL exposure to happen - not so good!
So you want to avoid situation b) if at all possible. This is why you should run your onboard flash at the "lowest power level" possible. If you can.

My pos P&S does not have the ability to for me to control onboard flash power level. So I hit situation b) if my subject is too far away from my camera. My onboard flash does a full dump, my external strobes do a partial dump since they are fooled by the single channel optical fibre signal, i get the green light on all strobes and my picture is under exposed.

in fact I have had plenty of overexposed shots with green lights in my sea and sea strobe years

Agreed. In "optical ttl" the green lamp simply means the strobe did not do a full dump. No relationship to correct exposure. It could be over or under exposed.

Generally DS-TTL only works when the light has a reflection: subject right in front of the lens and quite close

Agreed! Close means the onboard flash did not have to perform a full dump and shut off prematurely. Far away means onboard flash did a full dump and external strobes turned off prematurely.

---------- Post added July 9th, 2014 at 01:09 PM ----------

I am assuming I want to be using the pre flash manual mode?

Maybe? (don't you just hate all of this simple complexity....). Pre-flash and manual are 2 separate concepts often mixed together (along with ttl...) by many people and hence lots of confusion and the appearance of black magic. I do not know your camera, so the following is just background info for edumication. You will have to derive your own answer or wait for answers from others who do know...

If your camera preflashes (regardless of the camera mode) you need to set your strobe to handle the preflash. Note that some cameras do not pre-flash when the camera is set to manual mode. But they do preflash in all other "auto" modes. So with these camera if you switch modes on your camera, you may have to switch modes on your strobe. This sucks.

My P&S preflashes in all camera modes, so if I use optical fibre connector I do not need to change strobe "preflash" modes when I change camera modes. Happy camper!

---------- Post added July 9th, 2014 at 01:22 PM ----------

Like they said.... But I urge you to develop your manual shooting skills, assuming your camera will support that. This gives you much more control and ultimately better results. "Auto-exposure" seems to work really well on land, but not so much underwater.

Agreed! For me "auto" works 100% perfectly 90% of the time. The other 10% it is a dismal failure. When I first started out most of my shots fell within the 90% situation and I was a happy camper. As I progressed, I pushed more into the 10% territory and became an unhappy camper. So now i shoot 100% manual (camera & strobe) via wired sync.

Well except for the happy snap canon elph camera that i keep tucked in my bcd pocket just in case... it runs totally auto with no external strobe