Inconsistency in flash metering

Discussion in 'Canon Accessories' started by JuliaBu10, May 29, 2013.

  1. Robin_Usagani

    Robin_Usagani TPF Noob!

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    After a long ass research and testing, I am 100% sure that canon speedlite does evaluative metering as factory setting(not affected by the camera metering mode). Since the OP is a Canon user.. I take this as I am right :D

    Charlie, you kinda contradicted yourself though. The middle box you drew is all dark. Theoretically the photo should be bright and not the other way around.


     
  2. TCampbell

    TCampbell Been spending a lot of time on here!

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    Robin, remember it's a "TTL" metering system. There are lots of variants but what they all have in common is "Through The Lens" -- aka "TTL".

    The flash doesn't meter, only the camera does. Back in the days of thyristor flash units, the flash did the the "metering" (well... not true metering. The system looked for a specific amount of return light and then cut off the flash when it received enough light. It was easily fooled.) But in modern TTL systems, there's always some form of a pre-flash which the camera "meters" followed by another flash at some power level calculated by the camera based on the results of the pre-flash.

    Each evolution of TTL systems tries to make them a little more clever and a little harder to fool.

    I have NOT seen any official documentation from Canon that goes through all the rules. I have seen a lot of books, magazine articles, blogs, etc. which go over several nuances. I encountered one article that indicated that only certain bodies would allow overriding the metering system when the flash is in use, but it was the "pro" bodies (specifically the 1D and 5D series bodies) that seemed to have the ability to do this. That is to say... an entry level body might use evaluative metering when the flash is in use regardless of what metering system you set on the camera... but a pro body would actually use the metering system you selected (even if it was not a good choice.) I probably would not be able to find that same article and I don't specifically recall the list of bodies (only recalling that the bodies listed were high-end bodies.)



    Keep in mind, I don't think any of us have claimed to be sure we're right about why the OP got poor results from that 2nd image. His result seemed to be the opposite of what I would have predicted. But since I have the same camera body and the same flash, I set everything up using the same settings as the OP to see if I could recreate the problem -- but could not. All that really tells me is that we haven't hit upon the correct reason to explain WHY the OP is having this issue (Not that there isn't an issue... clearly there is. He posted sample photos to show us.)

    We're all just trying to helpful here by evaluating what could possibly account for his results. Please lets try to avoid jumping to conclusions. From a rational approach (e.g. think "scientific method") -- unless we can consistently re-create the OPs results, then we can't be sure why he got those results. That's what I was hoping to do by testing with my 5D II & 600EX-RT combo (but so far have not succeeded.)

    Something else to consider, Robin, is that if you point the flash straight ahead, then the E-TTL II system will also use the distance calculation to determine how much flash to fire. If you tilt the flash away (e.g. bounced or feather) then the camera doesn't know how far your bounced surface is -- it can't use the camera guide number and inverse square laws to calculate a pretty accurate power level. This might explain why in your specific case you were unable to see a difference.

    We don't know if the OP used the flash straight-on, bounced, or feathered the lighting. He mentioned he takes some straight-on, some feathered, and some using a Fong diffuser. When I look at the first photo (the one with adequate lighting), the lighting does not appear to be bounced or feathered light. I'm going to "guess" that this shot was taken with the flash straight-ahead. That would have provided distance information to the E-TTL II system. The darker photo has highlight & shadow that suggests it is like a "bounced" photo using the Fong diffuser -- and if that's true then the camera wouldn't have been able to rely on distance information to set flash power. Now it's much more heavily reliant on the reflectivity of the surfaces.
     
  3. Robin_Usagani

    Robin_Usagani TPF Noob!

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    Tim, the argument is the sample area of the flash metering.. Is it the same as the camera metering? I am aware what TTL is. Pretty obvious why the 2nd one is darker, it has more reflective surface.
     
  4. Josh66

    Josh66 Been spending a lot of time on here!

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    Some (most?) flashes do have a meter though...

    Here's a question... I have some cheap-as-hell Cowboy Studio radio triggers (and they work great, provided that the batteries aren't dead ... not one issue in maybe 5 years...). Does the transmitter send camera settings to the receiver, or just a signal to fire?
    Also - same question for a PC cord - do camera settings get sent too, or just a "fire" command?

    I didn't think cheap-ass triggers like that would do anything but send a "fire" signal (it only has one contact on it). Anyway, I can put my flash (580EX II) on E-TTL mode, off camera, using these triggers - and it works. Direct or bounced, it somehow figures out the right exposure, apparently without knowing what ISO I'm using, or the aperture.

    Now, there is a custom function on the 580EX II that will allow you to manually input the ISO and f-stop - but I'm not using that.


    I honestly cannot explain why it works, but it does - every time. Everything that I know about how this stuff works should say that it shouldn't work at all. I mean, I know that the flash has it's own meter - but what good will that do if it doesn't know the ISO & aperture that the camera is set to? Hell, it even works on film cameras that the manual outright says that the E-TTL functions of the flash are not compatible with.
     
  5. TCampbell

    TCampbell Been spending a lot of time on here!

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    Does it have to be an argument? Can't it be a discussion?

    It's possible we're agreeing but using different terminology. I thought I saw you refer to the flash metering and was attempting to convey that the camera always does the metering when using any of the TTL variants. The flash may impose restrictions on what metering modes can be used by the camera when the TTL flash is in use.

    His EXIF data indicated that he used "spot" metering. That leaves open the possibility that the flash allows spot metering on a 5D II ... or that the EXIF data reports the mode set by the camera menu even if the flash imposed a different mode.

    Over the weekend I may do some further experiments to see if I can't find a way to reproduce the OP's results. I've thought of a few things that I haven't tried yet.
     
  6. Robin_Usagani

    Robin_Usagani TPF Noob!

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    Tim, if you go to external speedlite menu on your camera, you only have 2 options.. evaluative and average. You can have your camera be on spot metering but your flash will only be on evaluative or average. So for canon, it does not matter what metering mode is on your camera, the ETTL flash reading will still be the same.
     
  7. Robin_Usagani

    Robin_Usagani TPF Noob!

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    BTW keith, you can set the 600RT or 580EX to an external metering built in the flash unit non TTL (thyristor). Go to your flash custom function and switch it external metering.

     
  8. IByte

    IByte No longer a newbie, moving up!

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    Soooo many words, but good info.
     

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