Can someone please explain guide number of my flash units?

Discussion in 'Lighting Equipment' started by conrad_nel, Apr 16, 2007.

  1. Hi there!
    Although I am sure that this question has been asked in the past, this amazing
    site is JUST TOO FULL of info, that I am clueless as to where to begin to

    I'm using a Canon 20D, with the wireless transmitter ST-E2, together with my
    550EX, AND 580EX. I've got my stands and umbrellas. I am not sure what the
    minimum distance should be with this setup of mine with both of the flashes,
    and as far as guide numbers go...well, clueless again!

    If i did know what all this meant, would I use different guide numbers for each
    flash individually? or the same? When should i overexpose with this setup when
    doing portraiture photography? Is there ANY rules out there? apertures,
    exposures, shutter speeds! (Hobby gone haywire... so if I sound like I don't
    know anything, just please gimme a clue if you can PLEASE!)

    ...oh, and one last thing, F stops? what exactly does it mean when someone says
    you shoul overexpose by 1 or 2 f stops? Do you then have to take the aperture
    reading the camera gives you, and then have the camera on AV mode, and then set
    it 2 f stops further? DOES a flash have an f stop?

    Any ANY anY help would be GREATLY appreciated!
  2. if you are using Canon's TTL system ignore the guide numbers business.
  3. SCL


    Not to be trite, but the nature of your questions suggests you really should either take a course in introductory photography or get a good book to learn the fundamentals, so as to not get too frustrated by all the variables dealt with in a piecemeal fashion by those trying to answer your inquiry.
    First, you have a digital camera, so you can costlessly experiment...shoot, check your results, reshoot, etc. Overexposure or underexposure are situational in nature, your present issues are much more elementary and you need to learn what constitutes proper exposure before wandering off on a tangent on how to achieve variations.
    F stops are a means of expressing (via ratios)the amount of light entering the camera via the lens. The choice of f stops controls the diaphragm of the camera, with incremental adjustments each representing a doubling or halving of the previous amount of light; ie opening up the diaphragm one f stop doubles the amount of light entering the camera and closing it down one f stop reduces the amount of light by 1/2. Learning how to match f stops and shutter speeds and their interrelationship on light hitting the sensor or film, again is one of the most elementary aspects of operating a camera, and you need to get some concrete guidance in understanding how it works.
    Flash units don't have f stops per se, but their output is often calibrated to using specific f stops on the camera...but again there are other factors such as reflectance of the object being photographed, whether or not the flash is directly on the subject or bounced off a wall, ceiling, or through a softening material. Again, not something one covers with a brief online answer...but something you need to read up on independently.
    Let me wish you luck in your endeavor, and don't let your enthusiasm wane, we've all been through this learning curve.
  4. Hello Conrad, I would endorse Stephen's answer almost verbatim, however I would like to make one further suggestion. You have a computer and many of the questions you have will be answered instantly by simply typing them into google or other browser. Good luck D.D.

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