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Thread: guide number and ISO

  1. #1

    Question guide number and ISO

    hi, i just want to seek some opinions about flash photography.

    i am using a yongnuo yn460ii flash unit which is rated at 38m @ ISO 100 and 53m @ ISO 200. i just wanna know whether the guide number will increase when i increase the ISO?

    i read from:
    http://www.chem.helsinki.fi/~toomas/...flash-faq.html
    Doubling of film speed will increase the guide number by a factor of 1.4 . Going from ISO 100 to ISO 400 film, the guide number is doubled. Going from ISO 100 to ISO 25, the guide number is halved.

    i understand that @ ISO 400 the GN is effectively doubled(1.4 x 1.4). so i went to do some calculation and realised that if i am to shoot at ISO 3200 with my flash, the GN will be 203.6m. so, using the formula GN = distance x f-stop, ie @ F8 the flash will be covering a distance of 25.5m.

    i feel that the figures are a bit imba and i am really skeptical as to whether the flash can reach out over 25m, so can anyone share your experiences? and if i were to bounce flash(from bounce card, ceiling etc), then how would the calculation be again?
    Last edited by tecnica; 13th September 2010 at 11:12 PM.

  2. #2
    Moderator ziploc's Avatar
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    Default Re: guide number and ISO

    You won't be able to get beyond the max flash distance that your flash can reach. It is in your flash manual in the specification page. 200m is two stadium length. Do you serious think the tiny little flash can reach such a big distance?

  3. #3

    Default Re: guide number and ISO

    Quote Originally Posted by ziploc View Post
    You won't be able to get beyond the max flash distance that your flash can reach. It is in your flash manual in the specification page. 200m is two stadium length. Do you serious think the tiny little flash can reach such a big distance?
    er.. i think you misunderstood my qn. i am talking abt the 25.5m when shooting at F8. i know 200+m is impossible but 25m? i mean, does going up to ISO 3200 still have effect on the GN?

  4. #4

    Default Re: guide number and ISO

    hi i am using yongnuo too, can i ask u, u think this flash is ok? does it has TTL too?

  5. #5

    Default Re: guide number and ISO

    Quote Originally Posted by Biocell View Post
    hi i am using yongnuo too, can i ask u, u think this flash is ok? does it has TTL too?
    hey, this flash doesnt have TTL, it's a full manual flash with a master and 2 optical slave modes.

    i haven really test out it's full capability yet because my knowledge of flash photography is very limited also. i only tested out the slave modes, S1 is so call the dumb mode as the flash will be triggered by any flash from other cameras. in fact, S1 works really well with my Powershot A95. S2 is the smarter mode where it will ignore the E-TTL pre-flash from your DSLR and only activates when the main flash from your camera fires. i tested the optical slave with and without line of sight and both show no difficulty in firing the flash. i bought the RF-602 radio triggers too and they work very well for off camera flash use. so far so good, i should say.

  6. #6

    Default Re: guide number and ISO

    Is my qn too chim for this section?

  7. #7

    Default Re: guide number and ISO

    Quote Originally Posted by tecnica View Post
    Is my qn too chim for this section?
    No, was the answer you received in post number 2 too chim for you?
    Alpha

  8. #8

    Default Re: guide number and ISO

    Quote Originally Posted by Rashkae View Post
    No, was the answer you received in post number 2 too chim for you?
    how abt "i just wanna know whether the guide number will increase when i increase the ISO?" ?

  9. #9

    Default Re: guide number and ISO

    Quote Originally Posted by tecnica View Post
    how abt "i just wanna know whether the guide number will increase when i increase the ISO?" ?
    Did you carefully read the article you posted? Base guide number is determined at ISO 100, and the distance and aperture you use determine it as well.

    Yes, when you increase your ISO, the *derived* guide number increases, but not the actual physical guide number (which is determined by the capacitor and flash unit used in the flash).
    Alpha

  10. #10

    Default Re: guide number and ISO

    Quote Originally Posted by Rashkae View Post
    Did you carefully read the article you posted? Base guide number is determined at ISO 100, and the distance and aperture you use determine it as well.

    Yes, when you increase your ISO, the *derived* guide number increases, but not the actual physical guide number (which is determined by the capacitor and flash unit used in the flash).
    yes, i read it thus i came up with a lot of calculations but these sums are ultimately paper figures. IS0 100 to 400, yes there is a difference but if it is up to ISO 3200 and over a greater distance(>20m)? since the theoretical(derived) GN @ ISO 3200 is 203.6m and ie at F8 @ ISO 3200, the actual distance(GN/f-stop) that the flash can reach out to is approximately 25.5m. i know there are many reasons which can affect how the flash operates, like the inverse square law, the surroundings etc. but there are situations when one needs to push the limits(increasing the ISO, using 1/1 flash power etc) of their equipments, to capture a shot which they really need. i just want to know in real life situation, how much does the theoretical numbers apply.

  11. #11

    Default Re: guide number and ISO

    Quote Originally Posted by tecnica View Post
    yes, i read it thus i came up with a lot of calculations but these sums are ultimately paper figures. IS0 100 to 400, yes there is a difference but if it is up to ISO 3200 and over a greater distance(>20m)? since the theoretical(derived) GN @ ISO 3200 is 203.6m and ie at F8 @ ISO 3200, the actual distance(GN/f-stop) that the flash can reach out to is approximately 25.5m. i know there are many reasons which can affect how the flash operates, like the inverse square law, the surroundings etc. but there are situations when one needs to push the limits(increasing the ISO, using 1/1 flash power etc) of their equipments, to capture a shot which they really need. i just want to know in real life situation, how much does the theoretical numbers apply.
    The theoretical numbers are exactly that - it's basically telling you that if you shoot at f/8, with your flash firing *directly* at the subject at maximum power, and your subject is 25.5m away, you need ISO 3200. Or alternately, you know that if you are shooting at ISO 3200, your subject could be a maximum of 25.5 meters away. But remember, that could be in absolute ideal conditions, and maybe even measured in a room with bounce. Realistically, always take these numbers with a grain of salt - they're basically marketing numbers.
    Alpha

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