Canon's answer to how ETTL works

There have been many discussions about weather or ETTL compensates for a diffuser or not. Personally I always believed it never did. Canon started a Forum started a forum a few months ago and I noticed product experts and others cruising around there so I asked them the question.

ME

OK Canon. I have been in many threads and read quite read a lot of your documents and others work like the great Syl Arena. All types of tests and theories. It really does not make a lot of difference to me as I view ETTL as a good tool in a busy mobile situation but I know I will have to use FEC. Sometimes I can view a scene an predict FEC required and sometimes my predictions don’t work.

I’d like to hear from Canon but others are more than welcome to contribute.

The 3 main questions.

1. Using a lens that does not provide distance data. We know that if you take a shot of a bride alone in a white dress, a groom in a black tux alone and the bride and groom together all 3 scenes will reflect light back differently during the pre-flash.

Does the flash system just rely on light reflected back from the pre flash to determine subject distance much like a flashes guide number only or the does the flash exposure system try expose the subjects based on the scene? Kind of like the cameras metering system. Or both? NOTE: I realize the flash does not use the cameras light meter but does use the system to determine exposures.

2. Using a lens that does not provide distance data. Does the system compensate for a diffuser?

Part a

First off the system has now way of knowing a diffuser was installed on the flash. If the answer to question #1 is it measures distance only by reflected light like a flash guide then it would not compensate. If it takes the actual scene (subject/s) into account then I would think it would compensate. I know Canon says it does compensate but we need more detail on what actually happens. Also we know that when the flash is evaluative mode it tries to isolate the subject/s by comparing ambient to the pre-flash. Again – distance or scene?

Part b

If so then it should compensate when you bounce the flash. If it just measures distance like the guide number then it would not be able to compensate? More detail on how the system handles bounce flash would be helpful.

3. Using a lens that provides distance information. Which takes priority? Pre-flash, distance or both? I just came from another forum (POTN) where a person has done this test with all his lenses. When he placed a diffuser on his flash and shot direct the images were underexposed. When he tilted the flash up or to the side (whihc disables distance info) the exposures were good. Seems to me that distance info takes priority when available.

Well Canon explain this to us so we are better prepared when using our flash systems. Anything else yo can add would be greatly appreciated.

Thanks in advance.

Canon's reply

Hi Digital,

I have tried my best to answer your three questions in order below:

1. The pre-flash is fired, then what is reflected back is compared with an ambient meter reading to determine the best exposure. Distance information is not determined by this process. It is only factored into the overall E-TTL II algorithm for fine tuning the exposure if the lens you are using provides this information. Essentially, the overall scene is what will be used entirely when you are not using a lens that provides distance information.

2. When you are using a diffuser on the flash, the camera has no way of knowing that. Since the pre-flash is being compared to the ambient meter reading to determine how to best expose the scene and the subject, I would say that using a diffuser will effect this process anyway. It will change what is being reflected back to the camera from the pre-flash, thus changing what is being compared to the ambient meter reading.

3. As I noted earlier, the distance information that is provided by a lens that supports this is added into the overall calculation, so this explains why the exposures were a bit different in the example you provided.

Not knowing if you have seen what we have on our own Canon Flash Work website about E-TTL II technology, as well as other flash technology, I am providing a helpful link below:

http://web.canon.jp/imaging/flashwor...ogy/index.html

This site provides a wealth of information about how our flashes work together with our cameras.

To conclude, I would like to note that the exact E-TTL II algorithm is not published information, so I tried to answer your questions using the best of my knowledge and experience with E-TTL II. I hope that you find this helpful. Please do not hesitate to ask any additional questions if you have any.

If this is a time sensitive-matter, additional support options are available at Contact Us. Link to Contact Us. It was broken in the previous sentence.

http://www.usa.canon.com/cusa/consum...ct_us_consumer