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Thread: Guide Number

  1. #1

    Default Guide Number

    Hi there,
    Anyone have good knowledge on Flash Light's Guide number? How does it works? Is there anyway to determined the correct exposure to distance? How does auto expose on flas light works eg. flash light set at f4 but exposure meter reads f2.8? Thanks.
    Regards,

  2. #2

    Default how to use manual flash

    the equation is used prmarily if u use flash in manual mode (not the same as camera in manual mode)

    GN(in meters/feet) = aperture (no units ie dimensionless) x distance from camera to subject (in meters/feet)

    eg say your flash has GN = 56 if u stand 10m from the model, and

    (i) if u set f5.6 on the flash and shoot, then the flash will light up the model just nice ie not too bright, not too dark since 56/10 = 5.6

    (ii) likewise , at f4 on the flash, then the model will be overexposed by the flash by 1 stop ie too much flash light thrown on the model since 56/4=14 ie u are supposed to stand 14m away but u stood at 10m instead

    (iii) at f8 on the flash, then the model will be underexposed by 1 stop. since 56/8=7 ie u r supposed to stand 7m away but u stood at 10m instead.

    lets assume flash is still in manual mode. setting f4 on the lens and setting f4 on the manual flash mean different things because setting f4 on the lens cntrols how u expose for the background exposure, wheareas setting f4 on the flash controls how strong u want the flash exposure to be. look at the above worked example: they never mention what aperture u set on the lens. if say u already meter the model nicely and choose to set f4 on the lens and choose to stand at 10m away, then the equation gives 56/4=14 ie telling u to stand at 14m away instead if not the model will be overexposed by the flash light. then how? the solution is then to set f5.6 on the flash so that 56/5.6=10 which then tally with your choosen subject distance of 10m

  3. #3

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    Quote Originally Posted by chiawk
    Hi there,
    Anyone have good knowledge on Flash Light's Guide number? How does it works? Is there anyway to determined the correct exposure to distance? How does auto expose on flas light works eg. flash light set at f4 but exposure meter reads f2.8? Thanks.
    Regards,
    User111 has answered you on the first part ..... now for the second part.

    The exposure deficency will only occur if the flash is in Auto / TTL / E-TTL / A-TTL / simi gui TTL as the flash or camera body calculate the entire area to determine the power of the flash light required. If you are near a reflective and/or bright / white background, it may fool the system into thinking the place is brighter then it is, thus the flashlight will produce less power to compensate the brighter area.

    In pure manual mode though, will not have this problem because no one do the calculation ;D

  4. #4
    Moderator ortega's Avatar
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    in pure manual, you have to do the maths yourself.

  5. #5

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    Quote Originally Posted by ortega
    in pure manual, you have to do the maths yourself.
    Nope ..... I use Argaration LOL

  6. #6
    Senior Member denniskee's Avatar
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    Default

    Quote Originally Posted by chiawk
    Hi there,
    Anyone have good knowledge on Flash Light's Guide number? How does it works? Is there anyway to determined the correct exposure to distance? How does auto expose on flas light works eg. flash light set at f4 but exposure meter reads f2.8? Thanks.
    Regards,
    1) When you mentioned Auto exposure on Flash, do you mean the flash is a dedicated flash to a certain system, eg nikon, canon, minolta, olympus?

    If so, and the camera exposure meter reads f2.8 assuming you have selected spot / partial metering and do a AE lock to the subject, than the light out put of this flash will give correct exposure regardless of subject distant (subject in middle tone and is within the reach of your flash, eg GN56 (given in ISO std)/2.8=20m).

    2) But if you further reduce the flash to f4 (not sure if you mean 1/4), than you are reducing the power by 1/4. Your subject will get 2EV less of light.
    photography makes one sees things from all angles.

  7. #7

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    don't mind I hijack this thread for a bit... then the flash got these 1/2, 1/4, 1/8, 1/64, 1/128 things... what are they for huh? How to make use of them?

  8. #8

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    Quote Originally Posted by denniskee
    1) When you mentioned Auto exposure on Flash, do you mean the flash is a dedicated flash to a certain system, eg nikon, canon, minolta, olympus?
    He's refering to autoflashes. These are typically generic hotshoe units, though most nikon and metz flashes have such capability as a bonus.

    Each flash has an onboard sensor that reads the average reflectance of the scene to determine its exposure. Generally, one just matches the aperture on the flash to that of the lens as a ballpark reading and fine tune from there.

    2) But if you further reduce the flash to f4 (not sure if you mean 1/4), than you are reducing the power by 1/4. Your subject will get 2EV less of light.
    Difference between f2.8 and f4 is just 1/2 the flash power or 1EV difference. Anyway, its not entirely relevant.

    Amekaze: Those are the manual flash power output settings. I believe you are referring to a nikon flash?

  9. #9

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    Quote Originally Posted by Zerstorer
    Amekaze: Those are the manual flash power output settings. I believe you are referring to a nikon flash?
    yep yep... the flash power output... the ones on the SB-600 and SB-800. How to make use of them?

    You mentioned f/2.8 to f/4 is just 1/2 the flash power, so does it mean that I can use a f/2.8 and change the flash power to 1/2 or I can use f/4, change my distance and use full flash power output?

  10. #10

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    Assuming f2.8 and 1/4 power, if you switch aperture of lens to f4, you will need 1/2 power for the same exposure.

    Guide number works differently.

    Guide number/ Aperture == Potential effective distance(flash has sufficient power to illuminate objects within this range)

  11. #11
    Senior Member denniskee's Avatar
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    Quote Originally Posted by Zerstorer
    Difference between f2.8 and f4 is just 1/2 the flash power or 1EV difference. Anyway, its not entirely relevant.

    Amekaze: Those are the manual flash power output settings. I believe you are referring to a nikon flash?
    Ha ha ha, I was assuming he mean 1/4 power.
    photography makes one sees things from all angles.

  12. #12
    Senior Member denniskee's Avatar
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    assume Guide No. of a flash when zoom head is set to 50mm (most flash maker states GN at max. zoom head position nowaday, which will be higher than 56 in this case for the same flash gun) is GN56 for ISO100. You use f2.8. Subject dist. should be around 20m

    GN56(ISO)/2.8=20m

    Now you reduce power by 1/2, your subject dist. should be 14m

    20/sqrt2=14m

    Now you set to 1/4 power, your subject dist. should be 10m

    14/sqrt2=10m.

    Now you increase ISO100 to ISO200, your subject dist. should be 14m

    10xsqrt2=14m

    Hope this help.
    photography makes one sees things from all angles.

  13. #13

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    Quote Originally Posted by denniskee
    assume Guide No. of a flash when zoom head is set to 50mm (most flash maker states GN at max. zoom head position nowaday, which will be higher than 56 in this case for the same flash gun) is GN56 for ISO100. You use f2.8. Subject dist. should be around 20m

    GN56(ISO)/2.8=20m

    Now you reduce power by 1/2, your subject dist. should be 14m

    20/sqrt2=14m

    Now you set to 1/4 power, your subject dist. should be 10m

    14/sqrt2=10m.

    Now you increase ISO100 to ISO200, your subject dist. should be 14m

    10xsqrt2=14m

    Hope this help.
    This maths thing helps a lot. Just a bit unsure about the sqrt2 thing... is it cause of the light falls off at inverse square reason?

    All these calculations are done at 50mm right... then what if I change to another focal length leh?

  14. #14
    Senior Member denniskee's Avatar
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    Like I said, GN increases as you zoom in. Just use the GN given for that focal length for cal.

    These are more for manual flash.

    If you use auto flash, ttl, ettl....... Than forget about all these cal. lah.
    photography makes one sees things from all angles.

  15. #15

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    Haha okok... thanks a lot.
    Just wondering... so many calculations to be done... do you all bring calculators along?

  16. #16
    Senior Member denniskee's Avatar
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    dont need lah, those flash gun wihtout LCD will have a chart @ the back of the flash.

    Set the ISO, zoom setting for the chart. Than look for the f stop you using, it will point you to the dist to the subject.

    If you reduce power by 1/2, just look @ the f stop that is 1 less then the intended aperture, that will show you the dist to subject.

    Pls note that only the power selector switch will affect the power output of the flash.

    The ISO and Zoom selector only make reading of the chart easier.
    photography makes one sees things from all angles.

  17. #17
    Senior Member Virgo's Avatar
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    Please allow me to voice my view.

    Since we have E-TTL and E-TTL II, why not use it efficiently? By the time we started calculating math, the moment would have gone already. This is especially so for fast action such as sports and weddings. The current flash system is so advanced that not much calculation is need, unless you're taking stills/macros.

    Try FEC and we should get very good results.
    Kind Regards
    My Picture Website

  18. #18

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    Yeah... agree... got technology don't make use is a waste. Just asking about the manual flash to widen knowledge.
    By the way, what's FEC huh? A bit new to these abbreviations.

  19. #19
    Senior Member denniskee's Avatar
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    FEC = Flash Exposure Compensation, ie, up flash power by 1-3 stop or down power by 1-3 stop.

    FEL = Flash Exposure Lock, ie, lock flash output on the subject, then re-compose your pic. much like AEL.

    AEL = Auto Exposure Lock, ie, lock exposure (no flash condition) then re-compose, typically for tricky lighting condition.

    AEC = Auto Exposure Compensation, ie, up exposure by 1-3 stop or down by 1-3 stop.
    photography makes one sees things from all angles.

  20. #20

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    Hi guys,
    Thanks for all the good feed back and sharing the knowledge around.
    I'm using a manual camera with no TTL function and an old manual flash to play with. Thus, I'm wondering what's the correct exposure if I'm only having a auto flash in auto mode like Metz 32Z2.
    I don't know how to use the auto mode eg. will I get the correct exposure when I set the auto mode at F4 at 6-12m on a bright sunny day and I should be able to light up the object at F4. I've tried using a exposure meter that reads only F2.8.
    Too many people rely on modern technology and if it's suddenly faulty, all the things will be screw up.
    Regards,

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