Trying to wrap my brain around TTL flash

Discussion in 'Beyond the Basics' started by Diddy2theJJ, Mar 1, 2010.

  1. Diddy2theJJ

    Diddy2theJJ TPF Noob!

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    Hey guys, I purchased a set of Pocketwizards a couple weeks ago and have been doing alot of reading and a little bit of experimenting with them.

    I'm using a Canon 7D, 580EXII, and Pocketwizards (Mini TT1 and Flex TT5).

    I'm finding it a little hard to wrap my brain around TTL metering with the flash. I can't seem to get the desired effect when using Av or Tv while using the flash. What has been working for me is going full on manual with the flash and the camera.

    While the effect is nice, it takes quite a bit of time to dial in all the settings. I've been reading "The Hot Shoe Diaries" by Joe McNally and have really enjoyed the book so far. What confuses me is he is always using TTL metering generally shooting in Av mode.

    When I'm outside Av seems to work fine, but when I'm inside is where i'm really getting the most troubles. The camera is metering for low light so my exposure is somewhere around 1/8th and the flash also fires. If I set it to manual mode I can usually get around 1/125th while having the flash fire and get close to the same image. Why does the camera want to use such slow shutter speeds if it knows there's going to be a flash?
     
  2. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    When in Av or Tv, the camera acts like it there isn't a flash. It wants to expose for the ambient light. The E-TTL flash then adds flash to match that exposure (sort of like fill light).

    If you haven't seen/read it yet, this article is often called the Canon Flash Bible...Flash Photography with Canon EOS Cameras - Part I.

    Personally, I think you are on the right track by using manual mode on the camera when shooting with E-TTL flash.
     
  3. Derrel

    Derrel Mr. Rain Cloud

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    I have not read The Hotshoe Diaries, but a couple of things about your post stand out. First would be that an exposure made at 1/8 second with flash and an exposure made at 1/125 second with flash look as you put it "close to the same image"; that doesn't make a lot of sense to me, since that is a full four f/stop difference in shutter speed. The difference between 1/125 and 1/8 second is enough that at the slower speed of 1/8 second, the background ambient light will be much more generously-exposed than at 1/125 second.

    Like I said, I don't have the Hot Shoe Diaries, but you mention McNally is always shooting in TTL mode. I know he's a Nikon shooter, and Nikon has two TTL modes: straight or regular TTL, and another mode called TTL-BL, or TTL with Balanced Lighting, which is designed to give the perfect balance between the ambient light that is present and the flash output. TTL-BL works best in low light levels, in my experience,and I think of it as dim-lighting level TTL flash mode.

    It's not quite clear from your OP what "the desired effect" is, but there are a couple of things to keep in mind. FIRST would be Custom Function 1-7: Flash Synch Speed in AV Mode O-Auto or 1-FIXED. I have three other Canons, but I looked up the CF.N. list for the 7D,and that function has been renamed 1-7 on the 7D. The critical thing here is that if the Flash Synch Speed in Av Mode is FIXED, it will default to the fixed speed whenever the flash is activated, and that may or may not be what you expect.

    Also, and this is kind of an odd thing, but sometimes flash and ambient exposures do not behave the way we think or expect that they will: last year I was privy to see some tests a guy had done with his Nikon and SB 900,and the flash output had a dramatic difference between 1/250 sync speed and 1/60; the flash output was NOT the same even though the flash mode was the same--the camera made a decision to vary the ratio of flash-to-daylight at the differing shutter speeds. The flash was LESS powerful at 1/250 than it was at all speeds below, so....test,test,test.

    As far as shooting flash exposures in TV and Av modes, it seems like an exercise in calculated risk-taking for most situations. There are so many variables to contend with, and allowing the camera to mess with key exposure factors doesn't make a lot of sense to me, and it is clear that both Canon and Nikon recognize that the shutter speed the camera sets when shooting flash shots in Av or A modes should either be A-variable or B-Fixed, and dialed in by the photographer from within the Custom Function Menus. So, you should definitely check your Cf. N 1-7 setting to see if maybe it is set in a way that would "fool you" with regard to how the camera is going to determine the proper exposure in Av mode. Do you want the shutter speed to default to a Fixed value, or to follow along with the ambient light level when using flash in Av mode?

    Since ISO, f/stop, shutter speed, and the flash's power setting ALL combine to create a flash exposure, it has always been my preference to try and use what is variously called AUTO-Thyristor or AutoAperture or just Auto-flash as the easiest way to take control over the background exposure (by leaving ME in control of the shutter speed selection,as I deem necessary) and allowing the camera/flash to meter out the right amount of flash pop needed for the ISO and f/stop and camera-to-subject distance.

    If you really want to shoot flash in Av mode or Tv mode, then it definitely pays to make sure the Custom Function menu choice ducks are all in a row, and also that you have tested out the way the camera "balances" its flash and shutter speed choices across a wide range of shutter speeds; the amount of flash-to-shutter speed "ratio" or "balance" in Nikon is different between TTL and TTl-Balanced Lighting AND varies somewhat across the slower speeds and changes at maximum synch speed, so experimenting with your own camera model and flash is definitely a good thing. The 7D is a brand new camera,and I cannot comment on how it handles flash; it also has a brand-new color- and distance-aware flash and continuous light metering system, so it might meter flash a bit differently than other Canons and McNally's Nikons as used in The Hot Shoe Diaries.

    TTL flash, BTW has been a very tricky thing for camera manufacturers to get dialed down in the shift from film to digital; film was duller in reflectivity, and the earliest flash implementations did not fare all that great with more-reflective sensors. Both Canon and Nikon have had to change their flash metering protocols, Nikon from D-TTL to i-TTL and Canon form ETTL to ETTL-II, to help refine this complicated exposure and flash-squelching system.
     
    Last edited: Mar 1, 2010

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