Canon 550EX flash frustration

Discussion in 'Digital Photography' started by My View, Mar 4, 2005.

  1. My View

    My View Guest

    Just when I thought I had the 550ex flash sorted it goes and delivers a
    heap of grossly underexposed shots.
    I have now looked into the relationship between the GN number anf the
    aperture (f-stop) with relation to distance to the subject and now I am even
    more confused. Here's why.

    I have crosschecked between what the flash tells me (on it's display) is the
    maximium distance for the flash and what I have calculated and I get
    different results.

    I have the 300D set to ASA100 and f/8 and 28mm focal lenght (the flash
    agrees and zooms to 28mm).

    With these settings the flash indicates a MAX distance to subject of 5
    metres. According to the Canon 550ex manual the flash has a GN of 30 at 28mm
    for ASA100. This would eqaute to 30/8 as the MAX distance ie 3.7 metres not
    5m as per the flash.

    I also checked other settings and found that the MAX flash distance shown on
    the flash is larger than what the GN number gives. Hence if you go by the
    flash figure you are going to underexpose. I think this is what has happened
    to me.

    Other figures are for ASA200 (GN = 42 at 28mm, f/8) I calculate MAX 5 metres
    range but the flash shows 7 metres. 2 metres difference.

    ASA400, GN = 60 at 28mm, f/8 I calculate 7m and flash indicates 11m - 4
    metres difference. These differences make a big difference when trying to
    expose at MAX range.

    What is going on here?
    My View, Mar 4, 2005
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  2. My View

    Steve Guest

    That's not quite a full stop difference. Not good, but is that what you're getting
    when you refer to "grossly underexposed"? Are the shots properly exposed when you use
    the guide number instead of the max distance to determine the f stop?

    As you describe it, it appears that the manufacturer's claims differ from reality.
    That probably shouldn't come as a big surprise. Why their own figures on guide number
    and max distance don't agree is anyone's guess. AFAIK, guide numbers are usually
    based on typical conditions, which means there are reflective surfaces nearby. If
    there is more reflection than normal the flash will provide proper exposure a bit
    beyond what the guide number indicates, but it would be misleading to use such
    conditions as a basis for claiming the max distance. A guide number that says you can
    shoot at f/8 is still going to result in underexposure if you're in a setting with
    litle or no reflection.

    As long as you've found out that shots near the published max distance are
    underexposed, either move in closer or open up 2/3 to a stop. If you're shooting TTL
    you'll presumably get average exposure as long as you aren't too far away for the
    chosen f stop. One of the first things you should do with a new flash is to find out
    what guide number works for you, anyway.


    The above can be construed as personal opinion in the absence of a reasonable
    belief that it was intended as a statement of fact.

    If you want a reply to reach me, remove the SPAMTRAP from the address.
    Steve, Mar 7, 2005
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