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Calculating manual flash powers without a flash meter - and other flash questions

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    Overread
    Overread  63806 forum posts England18 Constructive Critique Points
    5 Oct 2011 - 6:35 PM

    Ok so I've got a medium format camera and I've got a flash. Now I want to combine the two of them to aid in lighting my subjects, however whilst I know that the ideal approach is to use a flash meter to measure the lighting at the subject this simply is not possible (no funds left for a flash meter!).

    So I'm wondering if there is a way I can calculate a rough estimation of the flash power output needed to illuminate a subject without having to waste a lot of film shots bracketing a series of power levels (which in the end might or might not be enough/too much).

    At the moment the best I can think of is to use my regular DSLR in a similar position and frame to the fullframe, meter the scene with it and then apply flash in manual power outputs until the exposure is sound on the DSLR and then simply use that flash power output (keeping distance equal at all times) for the medium format shot. However I'm wondering if there are any general inconsistencies that might arise and would necessitate either some form of compensation to the flash output value or bracketing to ensure a range of powers are covered.



    In addition anyone know the proper name for the PC cord connection point on the Mamiya lenses themselves that allows connection to a regular DSLR; thus far all I have is a PC cord with a screwthread attachment and whilst it works it has no hold inside the slot on the lens and thus is very easily knocked out.

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    5 Oct 2011 - 6:35 PM

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    miptog
    miptog  93532 forum posts United Kingdom61 Constructive Critique Points
    5 Oct 2011 - 6:44 PM

    Using the Flash Guide Number may help

    miked70
    miked70  5225 forum posts South Africa3 Constructive Critique Points
    6 Oct 2011 - 6:07 AM

    this will kind of work but that is assuming your dslr is calibrated otherwise you my be under or over,but really save up and get a meter any change in setup and things will change,or hire a meter and do all the test with all the settings at all different distances and make note of them know people who use a piece of string with knots in it for the different settings

    Sooty_1
    Sooty_1 Critique Team 41216 forum posts United Kingdom200 Constructive Critique Points
    6 Oct 2011 - 4:26 PM

    Find out the guide number of the flash. Focus on the subject. Read the distance from the lens and divide the guide number by this distance to give your aperture.

    The guide number is usually given at 100 ISO, so you might have to compensate if using a different ISO. Make sure the guide number and distance are in the same units (metric or imperial).

    eg. A guide number of 40m at ISO 100, and a subject 5m away, will be an aperture of 40/5=8.

    So you would use f/8.

    If you were using 400 ISO, you would need to adjust to have 2 stops less light (as 400 is more sensitive than 100), so then you would use f/16 instead.

    Note, this takes no account of the colour or brightness of the subject as it is only the amount of light falling on the subject that matters.

    If you have awkward numbers, use a calculator and write the answers down for future reference.

    Nick

    Sooty_1
    Sooty_1 Critique Team 41216 forum posts United Kingdom200 Constructive Critique Points
    6 Oct 2011 - 4:29 PM

    You mean a PC sync lead? A coaxial connection for connection to flash or studio lights? They are just push-fit and get looser with age. You can tighten them by nipping the terminal with some pliers (carefully) or in emergency, with your teeth. Make sure it isn't connected when you do this...it hurts!!!

    Nick

    Overread
    Overread  63806 forum posts England18 Constructive Critique Points
    6 Oct 2011 - 4:34 PM

    Going by the flash guide numbers I assume this the flashes output when fired at its maximum power level, so going by that it gives me the aperture to be set on the body for the flash at full power; however I assume if I then lower the flash output by a series of stops that I can similarly adjust my aperture accordingly to compensate? So if its f8 with the flash at full power I can lower the flash power by 1 stop and thus use f5.6 on the camera?

    Sooty_1
    Sooty_1 Critique Team 41216 forum posts United Kingdom200 Constructive Critique Points
    13 Oct 2011 - 2:28 AM

    Short answer...yes!

    bfgstew
    bfgstew  8668 forum posts England105 Constructive Critique Points
    13 Oct 2011 - 8:10 AM

    Nice answer Sooty, but what if you don't know the guide number! For instance I have just made a home made ring flash, fires using my SB-600 and uses a Xenon 800W/s tube, any ideas so I can calculate my outputs?

    Sooty_1
    Sooty_1 Critique Team 41216 forum posts United Kingdom200 Constructive Critique Points
    13 Oct 2011 - 9:39 AM

    Empirical testing! Fix camera with flash attached and shoot a grey card (or something mid-coloured) at a fixed distance, say 2m. Vary the aperture (shoot in manual) and note which gives the most accurate result. Measure the distance accurately and plug into the formula. To check, move it further away (say 4m) and repeat. This should allow you to work out the guide number in about 10 minutes.

    Nick

    bfgstew
    bfgstew  8668 forum posts England105 Constructive Critique Points
    13 Oct 2011 - 6:03 PM

    Cheers Sooty, will give it a go.

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