Determining flash exposure without a meter

Discussion in 'Lighting' started by robgunby, Jun 22, 2012.

  1. robgunby

    robgunby Member

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    Hi there

    I've just acquired a wireless flash trigger, Canon 430EX flash and a PC sync cable. My Mamiya C-220 has an "X" mode for flash sync that will sync all the way to 1/500 (leaf shutter for the win).

    However, I have no idea how to judge and set the flash intensity against a given aperture / exposure setting. I have no flash meter, but do have an old selenium light meter if that helps (I would imagine not in low light though, when the needle won't budge).

    Any pointers on how I go about setting the flash up please?
     
  2. jayvo86

    jayvo86 Member

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    Your flash power is going to be dependent on your aperture, iso, and subject to camera distance.

    The easy way would be to use a meter than can take a incident reading.

    What are you photographing?
     
  3. Michael W

    Michael W Member

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    You can do it the old fashioned way once you know the Guide Number of your flash. This will be expressed in either feet or meters - use whichever suits you. You then calculate the correct aperture by dividing the guide number by the distance from flash to subject. If the guide number was 80 (in feet) and the subject is 10 feet away, your aperture will be f/8. This basic calculation assumes 100 ISO.
    You adjust this if you are using different ISO, eg ISO 400 would be f/16.
    That would be the flash at full power in manual mode. You could also dial it down to 1/2, 1/4 etc power to achieve different apertures.
    Shutter speed is only relevant for calculating ambient light if that is also present.
     
  4. David Allen

    David Allen Member

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    A normal meter is of no use for measuring flash output but can be used for establishing background (ambient) exposure if required. You need to know what the guide number (and note that this is only a guide and will require a few quick tests to establish if it is accurate) for your flash and this is usually quoted for an ISO?EI of 100. The guide number is actually the aperture needed divided by the distance of the flash from your subject. Thus a flash with a guide number of 80 feet (24.4 meters) would mean you use an aperture of f8 if the flash is 10 feet distance from the subject. i.e. you divide the distance between the flash and the principal subject into the guide number to find the aperture to use (distance of camera to subject is irrelevant).

    As another example, if the flash was 10 feet from the subject and the quoted guide number was 160 feet, the aperture to use would be 160 divided by 10 = f16. A faster film ISO/EI would produce a higher guide number with the same flashgun. Most flashguns have either a chart or scale for quick aperture calculation plus 'automatic' settings should you not wish to use manual calculation. Generally, the film speed ISO is set first, then the distance of the flashgun from the subject is selected to instantly show the aperture to use.

    Best,

    David
    www.dsallen.de
     
  5. robgunby

    robgunby Member

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    Thanks for the responses. Given I don't have time or finance to buy a flash meter before my next shoot (to answer Jay, a baby portrait shoot to start with), I'm going to go with Michael's suggestion to do it hte old fashioned way. Very well explained thank you, however I do have one question:

    The method seems to assume a fixed shutter speed, what would this be, reciprocal to film speed, ie, 1/100, or the old fashioned sync speed of 1/60, or something else, or does this not matter whatsoever other than for allowing ambient light?
     
  6. Terry Christian

    Terry Christian Subscriber

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    The camera shutter speed won't matter, as long as it is at sync speed or slower, since the flash works all its magic instantaneously, within the sync speed. Having a longer shutter speed will simply control how much ambient light, if any, will illuminate the scene in addition to the flash.
     
  7. bdial

    bdial Subscriber

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    On a focal plane shutter, use its X sync shutter speed, on older cameras that tends to be around 1/60, on some newer cameras it's 1/125 or so. Usually it's marked on the shutter speed dial in some way.
    For your leaf shutter camera set it to whatever you like, that is fast enough so that you won't get exposure from the ambient / modeling light. 1/125 is an ok number, faster if you like.

    For a focal plane shutter, the speed needs to be the fastest in which the shutter fully opens. At very fast speeds for these shutters, there is only a slit moving across the film plane. Leaf shutters open fully at all speeds so they can sync at any speed. This characteristic of leaf shutters is helpful in situations where you are using "fill flash", that is, when you are making an exposure using a combination of ambiant light with the flash.
     
  8. Michael W

    Michael W Member

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    The guide number formula pretty much assumes that the flash is the key light. Only the aperture is relevant in regards to this. As long as the shutter speed is within sync it won't make any difference in regards to exposing the flash. The simple explanation for that is because the duration of the flash is so quick.
    Realistically, for the shoot you describe you might be wanting to mix flash with ambient light, which complicates things slightly. Of course you could do the shoot with flash only, like in a studio, but you would have to consider the look of the light. Straight flash off a speedlite can look hard and contrasty, probably not what you want for a baby portrait. You could consider softening the light by bouncing it off the ceiling or by adding some diffusion, like those mini softboxes and tupperware type devices that are available, e.g. on ebay. This will improve the look of the flash but also cuts down the light output which then needs to be factored in to the calculation.
    If you are mixing ambient with flash, say in a room with window light, then you set the shutter speed and aperture based on metering the ambient light and then add the appropriate amount of fill with the flash. Again you can calculate this using guide number, ISO, aperture and distance. Generally for fill you wouldn't use the flash at full power but dial it down a couple of stops.
    If you haven't done this before I would suggest exposing a couple of test rolls and making notes as you go.
     
  9. David Allen

    David Allen Member

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    The two considerations with shutter speed are:

    1. Synchronization - what is the maximum your equipment can deliver (i.e 1/60 on many cameras such as a Leica and up to maximum speed for in-lens shutters such as those on many medium format lenses)

    2. Control of ambient light

    Best,

    David
    www.dsallen.de
     
  10. robgunby

    robgunby Member

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    Brilliant, thanks ever so much for the very helpful replies all!

    It will be an experience at least! I'm hoping there will be *just* enough ambient light to shoot a few on that alone, and then use maybe half the film for experimenting with the guide number formula.

    Much obliged you helpful bunch of APUGgers!
     
  11. robgunby

    robgunby Member

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    And of course, I feel rather dim now for not recognising the irrelevance of shutter speed to the effect of the flash on the photo :S Great explanations!
     
  12. Jim Rice

    Jim Rice Member

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    The old rule of thumb with guide numbers and bounced flash is to add the distances from the flash to the ceiling (or whatever surface you are bouncing off of) and the distance from ceiling to subject. Then give it one more stop of exposure. It worked for me back in the olden days.
     
  13. polyglot

    polyglot Member

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    Subject-to-camera distance is irrelevant, it's flash-to-subject distance that matters for metering.

    If you don't have a flashmeter but do have a DSLR, you can preview iteratively with that to get your exposure correct, then transfer the settings to your film camera to get a good exposure. After a while, you'll get a feel for how powerful the flash is and know what sort of aperture you can achieve with standard ceiling heights and bouncing.