Very basic question on external flashes

Discussion in 'Digital Photography' started by duncanapple, Dec 8, 2009.

  1. duncanapple macrumors 6502

    Jun 12, 2008
    Eventually I am planning on buying an external flash - my new camera does really well in low light but I have conceded after research that there are times when a flash is needed, and other times where it is actually better when done right.

    I have read a lot on TTL, flash comp, etc and it all makes sense. However everything I can find online has skipped a very basic step that I must be missing... How to expose the correct shutter for a chosen aperture? See below for an example of what I mean...

    When not using a flash, in full manual on the camera, I select an aperture and ISO, and then an appropriate shutter based on the meter reading on my camera. (and then adjusting ISO and aperture if needed to get a decent shutter speed).

    So when your using an external flash on the hot shoe, say the 580EXII (or any other for that matter), lets say its a dark room. You crank the ISO to 1600 (b/c your comfortable with the noise level that produces), you want to isolate your subject so you pick an aperture of f/2.8.... Okay now what? How do you pick a shutter speed, b/c the meter is reading the ambient light which of course is dark (and will ask for a longer shutter speed) and will not be correct when the shutter fires.

    As basic as this question seems to me, the tutorials sort of gloss over this. Either I am missing something or making it more complicated than it is? How do you meter preflash?

    Thanks in advance for your replies!

    - Chris
  2. stagi macrumors 65816


    Feb 18, 2006
    If you are using TTL you set the shutter to how much ambient light you want to pick up (and if you want any blur) shutter can only go up to 1/200th to synch with flashes on a canon digital body and the TTL will do the rest.

    If you want to set the flash to manual or start getting more into strobes then you need a flash meter to correctly meter the light. If I am shooting with ttl at a reception, i'll usually set the camera around ISO 800 1/40th @ f/4 (and of course this always varies on location but thats a good starting point for what I like)
  3. toxic macrumors 68000

    Nov 9, 2008
    you pick whatever shutter speed you like and let ETTL decide what the proper flash exposure is. if you want some ambient light, you use a slower shutter, or if you want most of the exposure coming from the flash, use something close to the shutter sync speed.

    another consideration would be how bright you want the flash to fire - the more light the flash has to create, the more power is required and the longer the recharge time.

    if you're trying to freeze action and not using direct flash, you will have to use a faster shutter speed.
  4. compuwar macrumors 601


    Oct 5, 2006
    Northern/Central VA
    For TTL, you generally just set the shutter speed to the camera's max sync speed and let TTL do its thing (or leave it in an auto mode and it'll figure it out or disallow the picture or in rare cases of t<sync you'll get a partial picture depending on the camera, flash and settings.

    Modern cameras are pretty good at figuring out how to adjust flash power for balanced fill flash. If you need more basic control go into manual mode, or just dial in some flash compensation (+ or -, generally in 1/3 stop increments.)

    I generally put the camera in shutter release priority at max sync with -2/3rs of a stop of flash compensation as a start, then check the outcome- occasionally I'll shoot slower than max sync when I want more ambient light than balanced fill flash will do- in that case, it's generally something you get a feel for- the flash will freeze most movement, so it's all about look at that point.

    If you're in manual mode, then you can set your flash exposure for a particular aperture and chimp your way to success from there. Subject distance affects the amount of light the flash puts on the subject, modern flashes allow a lot of leeway, but I find it easier to take a couple of test shots than anything else.

    If you're a "meter it" sort of person or working with multiple lights, then a flash meter is a very helpful tool, as you can use it to check the flash levels and adjust the ratios to get what you really want.
  5. Edge100 macrumors 68000

    May 14, 2002
    Where am I???
    Basic rule of thumb: in manual mode, shutter speed has no influence on flash exposure.

    If you're using the flash in ETTL mode, you can set whatever aperture/shutter/ISO combination you'd like, up to the max sync speed for your camera (usually 1/200 or 1/250)**. So in that dark room you mentioned, you could set ISO 100, 1/250, f/16, and the flash would give you the correct exposure FOR THE SUBJECT; the room would, of course, be completely underexposed. If you want to bring some ambient light into the exposure, you can adjust ISO speed, aperture, or shutter speed; with manual flash and film, this used to be called 'dragging the shutter', and it often gives a much more pleasing look than the typical 'taken-in-a-cave' look of a lot of flash photography.

    ** - Note that you can exceed the max sync speed by engaging high-speed sync mode, but the flash output drops markedly.

    So for ETTL flash, set whatever ISO, aperture, and shutter speed values you want, and the flash will put out enough light to give you the correct flash exposure. I often find myself underexposing the ambient light by 1-2 stops; this seems to give a nice isolation of the flashed subject.

    I'm lazy, and I find myself using ETTL a lot; I've also invested in the PocketWizard FlexTT5/MiniTT1 system, which gives me wireless ETTL (or manual, should I want it). So my basic plan for flash work indoors is this:

    - Determine correct ambient exposure for the room; let's say it's ISO800, f/2.8, 1/50
    - Subtract 2 stops of ambient exposure; given these settings, I'd go to ISO400, f/2.8, 1/100; note that if I went to the max sync speed of 1/250, I'd be -3.5EV for the ambient light, which is a bit too dark...I find -2EV is the sweet spot to give isolation but retain shadow detail
    - Set 580EX on camera bracket or off camera in ETTL mode, and fire away

    I MIGHT go to manual mode if the light/position aren't changing (keeps exposures constant), but if I'm moving around a lot (or the subject is), then it's ETTL all the way.
  6. Edge100 macrumors 68000

    May 14, 2002
    Where am I???
    Another possibility is to use rear-curtain sync. The flash will 'freeze' the subject (since the exposure is very, very short) and any motion will be behind the frozen subject.
  7. Edge100 macrumors 68000

    May 14, 2002
    Where am I???
    And if you havent seen this, read these tutorials over and over.
  8. ChrisA macrumors G4

    Jan 5, 2006
    Redondo Beach, California

    In this case because the room is dark the shutter speed does not matter. The length of the exposure is determined by the duration of the flash.

    Because you have already selected an f-stop.The flash power is adjusted to give the correct exposure.

    If the room is not 100% dark then the exposure is a composite
  9. duncanapple thread starter macrumors 6502

    Jun 12, 2008
    Wow, thanks all - those were a bunch of great explanations. I think what I was not catching on to is that ETTL is basically an automatic setting that picks the flash power needed given the shutter speed you select.

    This was beneficial, thanks again!

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