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Old Oct 2, 2003, 4:17 PM   #11
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The guide number is much lower, I fully agree (since I have the manual here... full flash guide # is 55, high-speed is 38.9, both at 105mm 1/125th. I was at 1/350, so at that it's 23.2).

I was about 6-10 feet from the subject. With that guide number, I would have thought it would reach 6-10 feet. Am I assuming wrong?

Eric
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Old Oct 2, 2003, 8:16 PM   #12
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... full flash guide # is 55, high-speed is 38.9, both at 105mm 1/125th. I was at 1/350, so at that it's 23.2
That's in meter so it's actually 3.3*23.2=76.56 GN in feet -> ie @ max aperture of your lens its 76.56/5.6=13.67 ft max. ie at any aperture of f/7.5 or higher you're running out of light if the subject is beyond 10ft! However @ 6ft you can go as high as f/12.75
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Old Oct 2, 2003, 11:34 PM   #13
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Thanks for the lesson in guide number usage. I think I'd read that but not internalized it. So if I'm wrong and it wasn't 6-10 feet that would be part of the problem.

But we also talked about this in respect to fill flash. You said this before but I wanted to double check. The camera uses high speed sync for fill flash only? It doesn't meter it to propery expose it as the main light? It is rather weak after all, so I'm not sure how much more it could do than that.

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Old Oct 3, 2003, 7:21 AM   #14
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Quote:
But we also talked about this in respect to fill flash. You said this before but I wanted to double check. The camera uses high speed sync for fill flash only?
It's the reverse, ie you want slower sync speed so more natural light (and less flash) can get it to the sensor/film... the idea is to strike a balance of the two depending on how much of background you want in or out, ie the slower the shutter, the more surrounding available light can get in... Don't forget the fast flash pulse already froze your foreground subject!

However for fill flash in full sun on the beach... You'll need that high-speed sync. :lol: :lol: :lol:
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Old Oct 5, 2003, 3:56 AM   #15
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Originally Posted by NHL
Quote:
... full flash guide # is 55, high-speed is 38.9, both at 105mm 1/125th. I was at 1/350, so at that it's 23.2
That's in meter so it's actually 3.3*23.2=76.56 GN in feet -> ie @ max aperture of your lens its 76.56/5.6=13.67 ft max. ie at any aperture of f/7.5 or higher you're running out of light if the subject is beyond 10ft! However @ 6ft you can go as high as f/12.75
Well, yes, but only if you are using >105 mm lens!

The guide number is not constant. It depends on the lens used because the flash automatically adjusts the zoom reflector in order to provide illumination for the whole frame.

These numbers are from the instruction manual of the 5400xi (which has the same GN and zoom coverage as the 550)
  • 105 mm flash coverage: GN 54 m
    70 mm flash coverage: GN 46 m
    50 mm flash coverage: GN 42 m
    35 mm flash coverage: GN 36 m
    24 mm flash coverage: GN 28 m

The aperture value was f/5.6 (see the Exif data) and the distance was about 3 m (~9 ft). That calls for a GN of 16.8 m or about 55 ft. As you can see, with a 35 mm lens the GN is 1/3 lower and the GN in the high speed mode was probably too low (76.56 x 2/3 = 51 ft).

Additionally, the GN tables were made for film cameras. Digital cameras rely on preflash metering which might result in slightly lower effective GN due to energy wasted by the pre-flash.
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Old Oct 5, 2003, 9:44 PM   #16
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Eric was using his 100-400 L f/5.6... The exif said the focal lenght was @ 400mm 8)
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Old Oct 7, 2003, 11:10 AM   #17
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I'm talking from ignorance here, but this statment got me wondering.
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Additionally, the GN tables were made for film cameras. Digital cameras rely on preflash metering which might result in slightly lower effective GN due to energy wasted by the pre-flash.
Then how do they do it? I was under the impression that most digital cameras share their metering system from film cameras.

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Old Oct 7, 2003, 12:24 PM   #18
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Film cameras do not pre-flash since they don't need to set the white balance. They also meter the flash reflected Off The Film (OTF) plane which digital camera do not have either. Digital cameras do a lower intensity pre-flash, and then make an estimate for the 'real' flash pulse that'll be asserted the 2nd time around... and hopefully the HV capacitor has charged up in the meantime! This is also how one does the FE* (Flash Exposure lock on the 10D/300D), ie a 'manual pre-flash' to lock in the exposure with flash 8)

BTW wireless flash without the IR controller works almost similarly in that several low intensity flash pulse trains are used to turn the remote flashes ON and then OFF from the main unit located on the camera... If the IR controller is attached to the camera instead then IR signals are send in lieu of the light pulses
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