Most Basic E-TTL Flash Question

Please be patient with me. But I am not real comfortable with how E-TTL flash works (even after reading sections of the EOS Flash Bible). Please tell me if I got this right or not.

Suppose the camera is set in either Av or Tv mode with hotshoe-mounted flash unit turned on in E-TTL mode. With the shutter released pressed halfway, the camera will adjust for proper exposure based on ambient light only. When the shutter is completely pressed, the flash fires a pre-flash which is evaluated by the camera & stored in memory. Thus, to achieve the proper exposure when the flash itself fires, the camera adjusts the duration of the flash pulse. Is this correct?

Thank you for your patience.

Yes.

Any chance I can try and convince you to manually set your ambient exposure when your using your flash?
BTW, be sure and use FEL if you are going to focus and recompose.

Thanks, Windpig. Obviously I am a neophyte when it comes to flash photography. I certainly understand the advantage of manually setting exposure for various situations when using ambient light only. Is there a particular advantage of doing so with flash?

ETTL on it's own can be prone to exposure issues depending on how well you know how to use it. Throwing another auto mode into the mix just makes it more difficult to get a handle on putting the exposure pieces together.

If you're not in conditions of changing ambient light, make a best guess manual exposure and check your LCD screen, if you're not happy with it, adjust and try another shot. Then add flash, adjust your Flash Exposure Compensation to taste. Generally, to make your subject pop out, you'd underexpose your ambient by a stop or so, then add your flash. Adjusting your FEC would be to compensate for a very dark or very bright subject/clothes or whatever may bias the ETTL metering.

Point well-taken regarding manual exposure. Also, the idea of slightly underexposing the subject & using FEC to highlight the subject is appealing. I will definitely try it while I experiment with flash.

Thanks so much.

Please invest in Syl Arena's "Speedliter's Handbook." He covers this (and everything else) in complete and understandable detail.

You'll learn exactly when, where and how to use manual flash, ETTL, manual camera with ETTL flash, etc.

He's big on learning to use and understand manual flash, so you know what ETTL is doing and how to compensate with it.

Lighting is EVERYTHING.

Good luck!

One thing to remember also it you use TV or AV mode, is that it may put yu down into a shutter speed that is not hand holdable. In manual, you are totally aware of the shutter speed.

In other words, when you're in Av or Tv the camera will still try to make a correct exposure without the flash, then try to add flash for the subject. So you end up with what might be an unnecessarily long shutter or wide aperture.

By using M, you can choose both of them yourself and then let ETTL bring the exposure up. Obviously you have to start with settings that CAN be exposed with flash, but that comes with a bit of experience.

Have a read of my "How to use manual flash" tutorial.

AntonLargiader wrote in post #13249487
In other words, when you're in Av or Tv the camera will still try to make a correct exposure without the flash, then try to add flash for the subject. So you end up with what might be an unnecessarily long shutter or wide aperture.

By using M, you can choose both of them yourself and then let ETTL bring the exposure up. Obviously you have to start with settings that CAN be exposed with flash, but that comes with a bit of experience.

That is true for indoor, low lighting situations. I have the camera on M for that. Outdoors during the day AV is pretty safe to use.

I will check out both the Speedliter's Handbook & Tiberius47's tutorial. I really appreciate this information. I'm the kind of guy who wants to know why things work the way that they do. This way I can analyze a situation & adjust accordingly. Thank you so much.

I was in another forum, on another site and one person started a thread about this. He was asked to shoot a company function and was not too familiar with his flash. He set his camera to AV and had a horrible night. He had the slow shutter speed issues mentioned in this post. He had to keep bumping up the ISO which he did not want to and all his images were underexposed. In his opening statement he was about to give up photography.

Poor fellow made that common mistake. Did not realize that in AV you need to satisfy your light meter and your flash acts as a fill. In M you can ignore the light meter even if it shows it is underexposed and now the flash will provide the necessary light for your subject/s. That was a long thread with all kinds of suggestions. There was only one answer - M mode (or a high IS0 in AV) and I'm not sure the people who knew that answer ever really convinced him. It was interesting to see all the suggested solutions. A very educational thread.

Robertogee wrote in post #13248922
Please invest in Syl Arena's "Speedliter's Handbook." He covers this (and everything else) in complete and understandable detail.

You'll learn exactly when, where and how to use manual flash, ETTL, manual camera with ETTL flash, etc.

He's big on learning to use and understand manual flash, so you know what ETTL is doing and how to compensate with it.

Lighting is EVERYTHING.

Good luck!

+1 Good book. Cleared up a lot of confusion for me.

Tiberius47 wrote in post #13249491
Have a read of my "How to use manual flash" tutorial.

Tiberius47, I'm curious. Having read your tutorial above, I have a question. After adjusting the camera's exposure for the background in manual mode, you recommend manually adjusting the flash to achieve proper exposure for the subject. Makes sense to me.

Question: After I expose for the background with the camera in manual mode, can I achieve proper flash exposure by setting the flash to the E-TTL mode & using flash exposure compensation to make adjustments?

Thank you.

kitjv wrote in post #13250143
Tiberius47, I'm curious. Having read your tutorial above, I have a question. After adjusting the camera's exposure for the background in manual mode, you recommend manually adjusting the flash to achieve proper exposure for the subject. Makes sense to me.

Question: After I expose for the background with the camera in manual mode, can I achieve proper flash exposure by setting the flash to the E-TTL mode & using flash exposure compensation to make adjustments?

Thank you.

Yes.

I wouldn't use manual flash unless the flash was on a stand or the flash/camera was on a tripod. Just my opinion.