E-TTLii Technical Question

Hello There,

I have a Canon 7D and Yongnuo 468ii speedlight which supports Canon's E-TTLii metering. My question/confusion is based specifically around what exactly the flash is looking at to determine its output. Hopefully with a better understanding of this I can harness more potential and creativity out of it. I understand the flash attempts to meter just like the camera; based on a targeted middle grey, but my interests are much more specific. I think this series of random thoughts will sum up my wondering:

>Let's say someone is 10 feet in front of me but there is an empty room with a wall way behind them (and maybe some other people, tables, lights etc. in the background). How does the flash know that I'm trying to light the person?
>Is it based off of the focal length of the lens?
>Is it based off of where you locked your focus point? If so:
-What if you focus and recompose? Does the flash pay attention to your original focus location? And
-What about manual focus or AI Servo? How does either affect this?
>Say there's a really bright light in the background, except it is out of the flash's range. Does the flash know to ignore it because it's brightness won't influence the flash (who's range can't reach the light)?
>Lastly, how do evaluative vs. average TTL modes affect any of the above?

Shanks a Bunch,
Randy

Your camera has a metering system that determines correct exposure. The ambient light meter gives you information about ambient or existing light. The flash shares this metering system but does not give you any information like the cameras ambient meter. It can't because the flash exposure does not happen until you press the shutter all the way.

You probably know about your ambient meter and how it exposes for white snow or black tar when both individually occupy your viewfinder completely. You need compensate your exposure for each of those scenes. I will get back to that for flash.

As to your question about focus and recompose. ETTL II is not focus point linked. Also ETTL II metering is suppose to ignore bright backgrounds and provide a correct exposure of your subject. Manual focus, IS Servo and One Shot have nothing to with flash metering. Only thing is a the AF assist beam will not work in AI Servo.

Watch the 3rd video - ETTL II metering

http://cpn.canon-europe.com …asterclass/cano​n_flash.do

As for ETTL II metering.

Evaluative mode. This is via the flash settings, not your camera's ambient metering options.

In this mode there are multiple metering zones. When you press the shutter button half way the system will take an ambient reading. When you press the shutter all the way there is a pre flash. The system will compare the ambient reading to the light reflected back from the pre flash and select the closest thing to you which is typically your subject. It will isolate the subject and determine correct flash exposure. However it works like your ambient light meter. Remember the snow and tar. I will use a bride and groom as an example. I bride in a white dress, a groom in a black tux and the bride and groom together all reflect light back differently which requires FEC adjustments depending on the situation.

Flash Average Mode

In this mode ETTL does not isolate the subject. It averages out the whole scene. Many photogs like to use it indoors as it can provide a better exposure. A bride in a white dress will usually be surrounded by darker light. A better balance much like the snow and tar but now there is 50% snow and 50% tar in the viewfinder. Not good for outdoor uses.

Also you can spot meter using flash by using the AEL button. This pre fires the flash and only meters the spot circle area in your viewfinder. Again this has nothing to do with your cameras ambient meter.

RandMan wrote in post #16624989
-What about manual focus or AI Servo? How does either affect this?

Can't speak for the Yongnuo, but the Canon 580 EXII doesn't emit the focus assist in AI-Servo mode. Learned this the hard way.

I don't think any flash system does.

Several things answered above, but I'll give it a shot anyway.

RandMan wrote in post #16624989
>Let's say someone is 3 m in front of me but there is an empty room with a wall way behind them (and maybe some other people, tables, lights etc. in the background). How does the flash know that I'm trying to light the person?

By metering how much of the flash light that's reflected from different parts of the scene. A person nearby reflects more than the far away background. When using E-TTL II evaluative metering, the flash compares the subject illumination by ambient light, by ambient+flash and by flash only, to figure out what's responsible for what.

When using evaluative flash metering, yes. The camera will pay special attention to the flash illumination of the part of the subject that's under the active AF point.

Normal E-TTL II flash metering is done just prior to shooting, thus after you recomposed. If you want to recompose, use Flash Exposure Lock (FEL) metering instead.

When using Servo AF, there's no AF assist light. That's about it. Also think about that if you use automatic exposure (e.g. Av mode), there's no implicit exposure lock on half-press.

Yes, the camera will see that it doesn't change when the flash is fired, so it will know not to involve that in the flash power calculation. Likewise if there's a huge change when the flash fires, it will assume that there's a mirror or something in the background, and disregard that zone as well.

The clever algorithms to figure out what's subject, what's background, what's bright spots in the background and what's reflective surfaces are all turned off. You get an average result, and it's up to you to modify, by using flash exposure compensation, as needed.

Not too sure about the system locking off the focus point. If you watch the attached video the presenter clearly states ETTL II is not focus point linked. ETTL which modern day bodies do not have is (or was) focus point linked. By not being focus point linked you can focus and recompose without effecting flash exposure.

If the flash is point dead forward then there is distance information (depending on the the lens) but it is not the primary info the system uses. It is just a confirmation and the system still uses the pre flash for the final exposure decision.