Calculating Exposure with old Spiratone ring flash?

Discussion in 'Lighting' started by Yashinoff, Apr 26, 2013.

  1. Yashinoff

    Yashinoff Member

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    I came across an estate sale today and picked up an old Spiratone ring flash for a few bucks. But I've noted it doesn't have a table for calculating exposure based on distance. It also doesn't have a button to trigger the flash without firing the camera, so I'm not sure how one could meter without wasting an exposure.

    Is there some sort of formula for calculating exposure with a ring flash? Or will I need to just meter it at different distances and build my own exposure table?

    Thanks for any ideas!
     
  2. lxdude

    lxdude Member

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    Does it have any indication of a guide number?
     
  3. benjiboy

    benjiboy Subscriber

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    There's a lesson here photography is so automated these days that many photographers don't know the basics, here's how to calculate the guide number
    ww.scantips.com/lights/flashbasics1c.html
     
  4. BrianShaw

    BrianShaw Member

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    If no guide number (GN) marked on it, flash meter. Other than tria-and-error there are only a few options.

    If you beg, borrow, or steal a flash meter you can deermine the GN with one pop -- meter at 10 feet, ASA 100.
     
  5. E. von Hoegh

    E. von Hoegh Member

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    Ummmm, you could short the PC plug with a paperclip?:whistling: Use a camera without film in it?:blink: Look up the GN on Google?:wink:
     
  6. lxdude

    lxdude Member

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    Fixed the URL so the link works now.
     
  7. Yashinoff

    Yashinoff Member

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    I tried google, but I can't find the same model I have. It has no manufacturers marks on it, no guide number, wattage, anything at all. I only believe it is Spiratone because the power pack that came with it says Spiratone.

    I guess I'll meter for 100ASA and figure it out from there.

    Thanks for the responses!
     
  8. John Koehrer

    John Koehrer Subscriber

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    ^ that's the way it's done by the manufacturer.
    Normally the distance is 10 feet 'cause it's easy for us mathematically challenged to figure out.
    As an example for GN56 would be 5.6 for ISO 100, ~11 for 200 and so on.
    With the ringlight you have it's pretty likely you're not going to get much light at 10 feet.
     
  9. M Carter

    M Carter Member

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    Light meter and a lens cap on the camera...

    I came up more in the commercial studio realm than fine-arts training, so my knee-jerk response is often "grab the polaroid back" (along with a meter). The 'roids give me more of a feel for quality of light, etc - they tell me a lot when used with the meter.
     
  10. Chan Tran

    Chan Tran Member

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    With a ring light one would generally use it very close to the subject and thus it's a relatively large light source and not a point light. I am not sure the GN would work well with it.
     
  11. lxdude

    lxdude Member

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    IS THIS IT?




    ring flash 006.jpg
     
  12. benjiboy

    benjiboy Subscriber

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    Guide numbers don't work at closer than a meter.
     
  13. E. von Hoegh

    E. von Hoegh Member

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    Not so. It depends on which units of measurement you use to calculate the guide number. The units can be anything - meters, feet, cubits, versts... just convert the units and use the conversion factor as a multiplier or divider for the guide number. For instance, if the given guide # is for feet, but you need to know the # for meters, since 1 meter is ~3.33 feet, use that as a divider. Gn 80(ft) equals ( 80 divided by 3.33) gn 24(m). Gn 80(ft) equals (80 times 12) gn 960(inches).
     
    Last edited by a moderator: May 4, 2013
  14. Dan Fromm

    Dan Fromm Member

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    Really? Why do you believe that bilge?

    I ask because they work very well for me. Using them, however, requires understanding GN arithmetic and closeup photography arithmetic.

    To do it, first set up and measure flash-to-subject distance. Then calculate the aperture required given distance and the flash's GN. f/# = GN/distance, where distance is in the same units that distance measured in and GN is for the speed of the film being used.

    Next, calculate magnification. For most lenses, (film-to-subject distance/focal length) - 1 will get you close enough. If you're using a macro lens with focusing scale in distance and magnification, just read the magnification from the scale.

    Next, adjust the aperture required for magnification. You have f/# = GN/distance. Illumination at the film plane lost because of magnification, in stops, is simply m + 1. Set the aperture accordingly, opening up from f/# = GN/distance by m + 1 stops.

    That's what to do when the flash isn't fixed rigidly to the camera or lens or subject. Its what I do when shooting flowers with my little 2x3 Graphic, a lens that's good closeup and a hand-held flash. Except that I pick the aperture I want to shoot at and calculate flash-to-subject distance required.

    But this isn't the OP's situation. The OP has a ring light, presumably fixed-output, that's rigidly attached to the lens. I have similar rigs.

    The easy way to set up to use one of these rigs is simply to shoot a series of calibration shots with the film that will be used when shooting closeup in earnest. I take my calibration shots at each full stop from largest (f/2.8, in the case of my 105/2.8 MicroNikkor) to the smallest at 1:1, 1:2, 1:4 and, rarely, 1:6. I use reversal film for the calibration shots. Back when, KM. Nowadays, an ISO 100 E6 film. When the film comes back from the lab, I pick the best exposed shot for each magnification. If adjacent shots are a little lighter than I like and a little darker than I like, then the best aperture is a half stop between the two used for those two shots. And then I have a calibration table for the flash rig and the film. For magnifications between the ones at which I calibrated, I interpolate aperture linearly. One does have to open/close the diaphragm a little (1/2 - 1 stop) for subjects that are much darker/lighter than the calibration subject. This all works very well, eliminates the need for thinking about exposure. With my Nikons I just select magnification, look up the right aperture, set it, and there I am. Never fails.

    In other words, by spending less than a roll of film to make a calibration table for a closeup flash rig, I completely elminate the need for Guide Numbers, TTL auto exposure with flash, ... I've been doing this since 1971. And practically no one believes that it works.