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Zoom on Flash Gun?

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    russellsnr
    13 Oct 2012 - 7:59 PM

    Hi, Can anyone please tell me if I use a zoom lens, for example 70-200mm and set to say 90mm do I need to set the OCF (In Manual) zoom also to 90mm or should I move the camera further back from my subject?
    Thank You
    Russ

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    13 Oct 2012 - 7:59 PM

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    Britman
    Britman  81669 forum posts England
    13 Oct 2012 - 9:11 PM

    The zoom doesn't increase how far the light will go, inverse square law (ISL), all it does is narrow the beam, kind of like adding a snoot. That's why when you have it on the camera and you use say a 70-200 the head will zoom. Narrow the beam for the narrower field of view of the lens.

    So if you are using OCF you don't need to zoom the head unless you want a tighter beam. Moving the light close will change your exposure, again ISL.

    strawman
    strawman  1021999 forum posts United Kingdom16 Constructive Critique Points
    15 Oct 2012 - 12:38 AM


    Quote: The zoom doesn't increase how far the light will go,

    I guess the answer on this depends on the flash in question. If its a fixed flash gun (as you often get in an on-camera unit), then the flash beam is fixed so the flash light distance is constant regardless of lens chosen. If the gun has a zoom flash head then the illumination distance will vary as the head zooms because it concentrates the light as it zooms to a longer focal length. The power of the flash gun remains the same.

    I checked this on my 403EX with a 17-40 on at f8 ISO400. It offers 4m at the 17mm end and 6M at the 40mm end. Knock it down the f4 and you get 9 to 13m.

    So my answer to you is to find out if the OCF is a zoom or fixed one.

    Britman
    Britman  81669 forum posts England
    15 Oct 2012 - 1:01 AM


    Quote: The zoom doesn't increase how far the light will go, I guess the answer on this depends on the flash in question. If its a fixed flash gun (as you often get in an on-camera unit), then the flash beam is fixed so the flash light distance is constant regardless of lens chosen. If the gun has a zoom flash head then the illumination distance will vary as the head zooms because it concentrates the light as it zooms to a longer focal length. The power of the flash gun remains the same.

    I checked this on my 403EX with a 17-40 on at f8 ISO400. It offers 4m at the 17mm end and 6M at the 40mm end. Knock it down the f4 and you get 9 to 13m.

    So my answer to you is to find out if the OCF is a zoom or fixed one.

    The 4m @ 17mm is purely down to the fact the flash has to cover the wider field of view. The 6m @ the 40mm end will be the maximum distance the subject should be from the flash (not the camera if using OCF) to gain a good exposure, if the subject is beyond that your flash won't be able to cope because it won't have the power. You either use more powerful light, move the flash closer (if OCF) or get closer to the subject.

    strawman
    strawman  1021999 forum posts United Kingdom16 Constructive Critique Points
    15 Oct 2012 - 9:42 AM

    That is correct but the flash head zoom does increase the usable range of the gun. Because you can treat the gun as a fixed maximum power source if you illuminate a narrower field of view the working range of the flash gun is longer. It depends what OCF means. I assume it means on-camera flash and when I mount my 430EX on camera its range varies with the flash gun zoom setting. As the camera and gun exchange focal length info the focal length of the lens sets the maximum flash range. For the pop-up flash my camera comes with the range is fixed.

    The operation of a zoom head on a flash is not like a snoot

    Last Modified By strawman at 15 Oct 2012 - 9:43 AM
    JackAllTog
    JackAllTog e2 Member 53529 forum postsJackAllTog vcard United Kingdom58 Constructive Critique Points
    15 Oct 2012 - 3:21 PM

    If its On Camera Flash (OCF)? then yes in manual mode you would want to set the flash zoom to to the same setting as the lens.
    Also as you seem to be shooting an APS-c 40D you would probably need to account for the 1.6 multiplier too, so set the flash to 144mm (though i think the limit is 105mm for many flashes.

    If its OCF - Off camera flash then think of the flash gun as a spreadable cone of light - zoom wide or narrow as you require to pick out your subject?

    Is OCF On/Off/Or something else?

    lawbert
    lawbert  71684 forum posts England15 Constructive Critique Points
    15 Oct 2012 - 3:36 PM


    Quote:
    Is OCF On/Off/Or something else?

    OCF is Off Camera Flash...ie..the flash isnt on the hotshoe

    It can be fired in many different ways as a single flash or part of a multiple flash set up.

    An off camera flash lead, wireless triggers, off camera flash lead to master flash (Ie 580 ex) which then fires the other flashes, the 7d will fire the slave flashes using the pop up flash either as a fill light or simply sending the signal to fire.

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