I have for the last one month, been finding myself using either the built-in flash or Nikon Speedlight SB-400 more and more often in broad daylight. It does help bring down the ISO of my shot in less ideal condition, while not casting a harsh flood of white light on my subjects due to its i-TTL logic.
In attempting to shoot beyond a certain distance say at 200-300mm range, I found insufficient light reaching my subjects with the above hardware. Say for example, the SB-900 has the following guide number: 34/111.5 (ISO 100, m/ft), 48/157.5 (ISO 200, m/ft) while my SB-400: 30m/98.4ft. (ISO 200, m/ft at 18mm zoom-head position, 20 degrees C/68 degrees F).
By its Guide Number, assigned to almost all built-in and shoe-mount flashes and listed—in feet, meters. The GN tells you how far your flash will carry at a given aperture or what aperture to set for a given distance. Just divide the GN by either distance or f-number. For example, if a flash has a GN of 80 in feet (at ISO 100), it can reach up to 20 feet with a lens set to f/4 (80÷4=20). If you want to shoot at 40 feet at the same ISO, you’d have to open your lens to f/2.
Applying the above formula with my cheap Nikkor 70-300m at 300mm (F/5.6), my SB-400 can only effectively reach a maximum distance of about 18m at ISO 200, while the Nikon D90 built-in, at 10m. You are pretty much on your own beyond that.
Using a $420 Nikon Speedlight SB-900 will allow me now to cover as far as 28m.