What does Guide number mean on Flashes?

Discussion in '35mm Cameras' started by Ron Purdue, May 29, 2005.

  1. Ron Purdue

    Ron Purdue Guest

    Does anyone know what the Guide Number (GN) on a Camera Electronic Flash
    mean? Is the GN the higher the better ilumination will be? I am thinking of
    buying a Flash unit for my Canon Elan7 camera. Any info would be helpful!
    Ron Purdue, May 29, 2005
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  2. Ron Purdue

    Peter Irwin Guest

    Guide Number = distance x aperture.

    It is quoted in either feet or metres, and usually at ISO 100.

    If the guide number is 100 (feet) then with the flash on full
    power, it will give enough light for a subject at 10 feet distance
    at f/11, or 12.5 feet at f/8, or 20 feet at f/5.6

    Guide numbers on electronic flash are usually a bit optimistic,
    in actual use a flash may be as much as half a stop less powerful
    than the manufacturer's guide number indicates.

    The original purpose of guide numbers was to simplfy manual
    flash calculations. A big flash with a guide number of 100 is
    four times as powerful as a small flash with a guide number of

    Peter Irwin, May 29, 2005
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  3. Ron Purdue

    Peter Irwin Guest

    I should be more careful when I'm tired.;
    Arithmetic error:

    Guide number 100 gives:

    9 feet at f/11
    12.5 feet at f/8
    18 feet at f/5.6
    25 feet at f/4

    Peter Irwin, May 29, 2005
  4. Ron Purdue

    Paul Rubin Guest

    GN = distance divided by aperture. For example, GN 40 means that at
    distance=10, you'd shoot at f/4. At distance=5, you'd shoot at f/8.

    Notice some missing numbers in the above. Distance=10 what? Feet?
    Meters? Inches? In the old days flashes usually had GN specified in
    feet, but today it's usually meters.

    What about the film speed? What about the angle of coverage? Again
    in the old days, GN was usually figured for ISO 25 (I don't think you
    can buy much film that slow any more). These days it's usually ISO
    100 but sometimes for 200.

    Also, lots of flashes now have zoom heads, so you can set them
    telephoto-like to concentrate the light into a smaller area, like
    focusing a flashlight (torch) onto a small spot. So the GN is
    sometimes quoted for focal lengths like 105mm, which is misleading.

    Anyway, the basic guideline is an obvious one: big expensive flashes
    tend to be more powerful than small cheaper flashes. The rest you
    have to figure out by carefully comparing numbers.

    As for flash selection: if you can afford it, get the biggest, most
    powerful one that you can. The GN is just for direct flash. You lose
    a huge amount of light with diffusers, with bouncing the flash off the
    ceiling, etc. But those methods make the picture look much better. A
    powerful flash gives you much more flexibility for stuff like that.
    Paul Rubin, May 29, 2005
  5. Ron Purdue

    Ric Trexell Guest

    If you are really concerned, you can make some tests and figure out the GN
    of your flash. I suppose the best film to use for testing would be slide
    film which is hard to find today and is getting expensive to do tests with.
    Regular print film can be developed a stop or two over and under and it
    might be hard to determine it exactly. Like the other posts have said, you
    would use the GN to determine the exposure. So if your flash is suppose to
    have a guide number of 80 and you took a picture at ten feet, with ISO 100
    film, you should get a correct exposure at f/8. Remember, a guide number is
    a guide, it is not written in stone. If your exposures are dark or
    overexposed, then assume you have something other than a flash with a GN of
    80. RJT.
    Ric Trexell, May 29, 2005
  6. Usually feet, but they should specify that when they quote the GN. And, ISO
    100 is pretty much the universal standard when quoting GN's....
    William Graham, May 29, 2005
  7. Simplified explanation.
    GN =100(ft) @ISO 100 @ f1.0
    You should be so lucky to own an f1.0 lens but at f4, divide GN by 4 to
    get the distance it will reach. Simple?

    It gets complicated when the flash can zoom in sympathy with the lens
    but by then it's in full auto, TTL metering mode and you shouldn't need
    to worry.

    In the good old days of "Automatic" flash, you set the shutter at it's
    sync speed (1/60 or 1/125th) and used f8 to gain some depth of field and
    just pressed the button, the flash itself would meter itself and the
    pictures were more correctly exposed than many of the new ETTL flash's
    which cost 10 or 20 times as much.

    A GN 100(ft) flash is quite modest output. At f8 you can't go much past
    12 feet and still get full effect with ISO 100 film and even then, the
    batteries will not last long at full power. Add to this a diffuser
    (Stofen perhaps) to soften the harsh light and you really are struggling
    to use a GN 100 flash for fill flash outdoors. Maybe struggle with it
    indoors if you try to shoot the whole room x 30 or 40 times. What do you
    plan to do with it?
    [email protected], May 29, 2005
  8. Ron Purdue

    Alan Browne Guest

    It's the maximum power the flash can deliver and the number helps
    determine the maxuimum range for proper exposure at a given aperture.

    If the GN is in meters (typical); dist is in meters.
    feet dist is in feet.

    I say typical, as the model no. often indicates the GN in meters. Canon
    550EX is a 55 (meter) GN at 105mm. Minolta 5600HS is GN 56 (meters) at
    85mm. Nikon don't follow this practice, so check the spec.

    As your focal length shortens, the zoom head adjusts spreading out the
    beam of light. So the GN is reduced. Many heads indicate maximum range
    on their control panels as a function of ISO, aperture and focal length.

    Usually rated at the longest 'zoom' range of the flash head (for flashes
    that have 'zoom' in their reflector).

    Always rated for ISO 100. For ISO 400, double the range.

    Calculate as GN/aperture no. = range.

    GN 55 (meters) / 5.6 = 10 meters max range.

    (for f/5.6, ISO 100).

    If you shoot something closer than 10 meters, then the flash will use
    less power as the TTL system will stop the flash earier.

    Hope that helps.
    Alan Browne, May 29, 2005
  9. Alan-

    I think a point of clarification is in order. As stated, guide numbers
    are an indication of the maximum power of the flash with beamwidth set to
    minimum (maximum telephoto). Normally you don't have to concern yourself
    with them as long as you are in an automatic mode such as the TTL Flash

    If you are in manual mode with flash output set to maximum, then you use
    the guide number to set aperture to match distance-to-subject. In this
    situation the camera's TTL system will not be enabled, so results will
    depend entirely on your settings.

    Now, about that beamwidth thing. It seems to me you have to be quite
    serious to use manual mode with a modern automatic-everything flash!

    Fred McKenzie, May 29, 2005
  10. Ron Purdue

    Alan Browne Guest

    I don't disagree with you, but below is a barrage of things to consider
    beyond the joy and depair of TTL flash metering:

    You have to concern yourself with the GN in the sense that you lose
    range as a function of wider angles. As you go wide (with a zoomable
    flash head), the available range goes down.
    1) GN is not all that accurate (it's a "Guide") and

    2) there is rarely any need to shoot at full power with a medium to high
    end flash. Part of the advantage of such flashes is very fast recycling
    when 1/4 or less of the power is used as well as the headroom for more
    power when needed.

    3) Further, you don't always want to shoot at f/16 or so choking off the
    photon barrage at short range and creating more than desired DOF.

    3) Ceiling bounce and other difusion/indirection techniques make GN
    useless except for shopping for flashes. In this case, TTL flash
    control is truly a benefit where metering the flash is not practical
    (for a variety of reasons).
    It's almost all I do now. Oh, I do take some TTL flashed shots on film
    with careful evaluation of the subject and appropriate use of flash
    compensation. But for almost all flash photography I do I put the
    flashes on stands (whether the camera accessory flashes or studio),
    meter at the subject, adjust the flash power for the desired aperture
    and away I go. Consistency is just as important as any other aspect.

    For digital I use a combination of metering and histo-chimping with
    manual flash power. I have done some TTL pre-flash shots with digital
    but it's just as uncertain from a consistency POV as film. OTOH, in RAW
    it's easy to adjust (with some color shift).

    In short, I'm not a fan of TTL flash. Although some outdoor digital
    portraits (snapshots really) in the shade yesterday at high ISO were so
    beautifully balanced with the ambient background as to make me want to
    experiment with it some more.

    Alan Browne, May 29, 2005
  11. Ron Purdue

    Matt Clara Guest

    Shoot a few weddings and you'll likely change your tune. I know my Nikon
    flash system is pretty much fire and forget it, which is invaluable for a
    fast paced setting.

    BTW, Fred sounds like he's talking about flashes of yore (20 years ago and
    more), 'cause what he describes is how it was done (full power, change
    aperture to suit your subject distance).
    Matt Clara, Jun 2, 2005
  12. Maybe the new i-TTL, etc. flash systems work better, but in my
    opinion regular TTL is just like putting the camera on center-weighted
    automatic and hopefully, things do not go too wrong.

    What I'd really like is that the light meter in the camera could be
    used as a flash meter, combined with manual flash output settings.

    In my experience, a relatively small subject in front of a background that
    is further away (or has a very dark color) is likely to cause over exposed
    Philip Homburg, Jun 2, 2005
  13. Ron Purdue

    Matt Clara Guest

    Try the Nikon line: SB-28 and on up have worked fabulously for me--from all
    I've heard, their system is second to none.
    So you could take a reading and then set your camera to it?
    I've experienced a similar problem once, where I was shooting a wedding in a
    very dark church on a very heavily overcast and rainy day. The subjects
    weren't that small, but the background might as well have been black, and
    the flash tried to fill the space with light, resulting in ghostly white
    skin. Solution, set the camera/flash to slow sync, the camera to S mode and
    set my shutter speed to 1/30 of a second.
    http://www.mattclara.com/event.html All but the first two used fill flash.
    Matt Clara, Jun 2, 2005
  14. Possibly (I have to experiment with it some day) but it doesn't help my
    film SLRs. Hmm, does the F6 support i-TTL. Yes! Sounds like a good reason
    to get an F6. :) (If it works as well as advertized)
    Yes. Switch to the spot meter, point to a face, a test flash, and adjust
    the flash output.
    What film did you use for event_03 and event_04?

    Of course, the problem starts when the ambient light is too low for the
    film you are using. Fill-in flash is much easier.
    Philip Homburg, Jun 2, 2005
  15. Ron Purdue

    Matt Clara Guest

    Kodak Portra 160 or 400 (looks like 160). Saturation increased slightly in
    Matt Clara, Jun 3, 2005
  16. Interesting. Usually the ambient (tungsten) light causes a serious red-shift
    which seems to be absent in your examples. It almost looks as if you used
    tungsten-balanced film with a filter on the flash.

    I guess I have to re-visit my shots on Portra to see if I can get a better
    color balance.
    Philip Homburg, Jun 3, 2005
  17. Ron Purdue

    Sander Vesik Guest

    Hmm... F6/F5/F100/F80 have pretty much the same from teh POV of SB-28.
    or were you thinking of sb600/800?
    Sander Vesik, Jun 3, 2005
  18. Ron Purdue

    Matt Clara Guest

    I can't say I've ever experienced the red shift of which you speak, and I've
    used Portra 160 for at least a dozen weddings. It's the Nikon flash system,
    I tell you. Probably doesn't hurt that I bounce the flash off the ceiling
    whenever possible.
    Matt Clara, Jun 3, 2005
  19. And the two examples both included bounced flashes? That would explain it.

    Lately I found myself in the majority of the cases in rooms without a white

    Tungsten light ends up being very red on daylight balanced film.
    Philip Homburg, Jun 3, 2005
  20. Ron Purdue

    Matt Clara Guest

    In the garter toss I did not, but there's very little light there, except
    the flash and the ambient light on the far side of the room.
    Matt Clara, Jun 6, 2005
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