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Canon/Nikon Speedlight question
Old 03-31-2007, 10:07 AM   #1 (permalink)
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What is the flash meter reading (fstop) that can be obtained using nikon sb-800 or canon 580ex at full power(manual mode) at any given distance(whatever distance u hav tried). If possible i'd also love to know about the reading which can be obtained while using umbrella or some softbox with it?
i'd really appreciate anyone taking their time to share.
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Re: Canon/Nikon Speedlight question
Old 03-31-2007, 12:26 PM   #2 (permalink)
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I'm not sure I completely understand your question. If you'r flash is at full power and you're standing 6 feet from the subject your reading will be different than if you're standing 15 feet away. If you put your speedlight into a softbox or umbrella, you'll loose some of the light.
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Re: Canon/Nikon Speedlight question
Old 03-31-2007, 01:59 PM   #3 (permalink)
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Could you give us some background about your question. Do you have these flashes? Are you considering buying the? Do you have a flash meter? Etc. If you need to know the meter readings, the easiest way is to just take a look at the published guide numbers of the units. Then for a given distance divide the distance into the guide number and you'll have the fstop. If bouncing into an umbrella, then you lose about 1 stop for a standard white umbrella.
You can also use a Gray card and the camera histogram to determine the fstops without needing a flash meter.
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Re: Canon/Nikon Speedlight question
Old 03-31-2007, 04:38 PM   #4 (permalink)
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You simply have to measure it with a flash meter and find out.
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Re: Canon/Nikon Speedlight question
Old 03-31-2007, 07:51 PM   #5 (permalink)
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For an umbrella or softbox the answer is difficult to predict because it depends on the characteristics of the particular umbrella or softbox. You'd have to pick a particular model and try it. But in general you can expect to lose 1 to 2 stops with an umbrella or softbox vs. firing the flash directly at the subject.

As for firing directly, it's (relatively) easy: use the guide number of the flash and apply the following formula: f-stop = guide number / distance. I say relatively because both flashes have a zoom head which changes the effective guide number as the head zooms in and out.

At the 35mm head setting you get ~62 degrees of flash coverage, about equal to most strobe's standard reflector. Anyway, at the 35mm setting the Canon has a guide number of 128 and the Nikon has a guide number of 125. So if your subject was 10' away, you'd use f/13 (128 / 10 = 12.8 or ~f/13).

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