Ambient reading vs Subject Reading

i want to take a photo outside , i meter ambient light , than i add as much flash power to light the subjet to the ambient reading correct ?

i know that their are 5 variables of exposure means you have 5 options
iso, shutter, ambient, DOFE, aperture

the reason i ask , i want to take advantage i live in the DR to take pictures at the beach , must times ambient reading is ISO 100 SS 200 F/16 , so to get the Subject at f16 takes alot of power that with small flashes is hard ... but i wonder since the ambient is F16 can i read the ambient reflecting of the subjet wich most times is really only 2 o 3 stops below my ambient reading and i really only need less power than what i think , and again .

If you use a reflected reading off your subject then there is no need for a flash because your subject will be properly exposed... but if there is anything in the background it will most likely be over exposed.

Try cloudy days or late in the evening/early morning to find a time where you can get a ambient reading that matches or is slightly lower than you can achieve with your flash. When you are getting a reading of f-16 then you can use HSS (high speed sync if your flash has it), a more powerful light, or a ND filter on your camera to reduce the amount of light entering your camera.

John

I think you may thinking of it backwards. Given the circumstances you mention, use the flash as "Fill" for the subject. Don't worry about the flash being the primary light and needing to shoot at f16. As fill, it can expose at 2 or 3 stop lower to "Fill" in where the ambient doesnt.

Remember ND filter does not mean less flash power. It just means wider aperture for out of focus backgrounds, etc.
HSS works, but it is also at the expense of flash power. Invoking HSS means the flash will deliver about 1 to 2 stops less light at a given distance. That means the Speedlite has to be relatively close to the subject. If you can get a 580EX within 4 to 6 feet you should be able to darken the sun lit background and still get good exposure on the subject using HSS. That would give you an option for ISO 100 SS 8000 F2.8 with a background that is about 1/2 stop under exposed.

As others have said, early morning and evening just before sunset are your friend. The golden hour(s) offer very nice light and make these kind of shots much easier.

Simple, don't shoot in full sunshine. The pros don't so why should you? Go early in the morning or late in the evening when the sun is not so powerful.

Benji

John, simply keep in mind the single principle that such a shot is really two in one...
a) shot made by ambient light
b) shot which is exposed by flash

You simply need to decide what in the scene you want to be exposed with ambient light, at what relative brightness to 'proper exposure'...that is, do you want it to be -2EV underexposed, or -1EV underexposed, or at 0EV?
Once you have done that, then you simply need to turn on your flash in ETTL and shoot!

If flash is the primary source (and ambient is being used to provide some illumination to the background, which is secondary in importance to the subject) you can get away with greater intensity than when it is fill source.
But keep in mind that very often fill flash (when flash is secondary source) is overdone, too strong, and creates an artificial apperance to the result.

With a flash like the Canon 580, its guide number when providing coverage angle for a 'normal lens' is about 138. So assuming ISO 100, and full sunlight, you have 1/100 f/16 for full sunlit area, and your use of fill flash (assuming you want it to be -1EV in intensity relative to the full sun) is good for up to 12.5' away is your subject is in a shaded area.

I wish i could hit a like botton on some of the responses .....like add rep to the responder .....thanks all very useful info