Something useful to know:
Aperture and ISO affect flash exposure
Shutter speed does not affect flash exposure, but like aperture and ISO, it does affect normal exposure.
For indoors, if you background is too dark and your subject is well-exposed, just slow down your shutter speed and the background will be brighter but your subject will remain around the same exposure as before (as flash exposure is unchanged). But that's indoors.
For outdoor flashes, unless you are already at the minimum flash power, don't use any diffuser to cut light as your flash TTL is "smart" enough to compensate for the loss of light.
Currently i don't use diffuser, just a bounce card to get a softer lighting. SB 400 only got on/off switch. Didnt know have TTL??
By the way, seems quite a number of people have this problem too and some suggestions from the pros.
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Here is an old way to compute exposures: the "Sunny 16 Rule". It was invented back when few - if any - cameras had a built-in light meter. It states: "On a sunny day, set your aperture to f16 and your shutter speed to 1/ISO." So lets set ISO 100 and go out on this sunny day. Its also an excellent way to actually see the f-stop/shutter speed relationship.
f90 @ 1/3 sec. - 5 stops
f64 @ 1/6 sec. - 4 stops
f45 @ 1/12 sec. - 3 stops
f32 @ 1/25 sec. - 2 stops
f22 @ 1/50 sec. - 1 stop
f16 @ 1/100 sec. "Sunny 16"
f11 @ 1/200 sec. + 1 stop
f8 @ 1/400 sec. + 2 stops
f5.6 @ 1/800 sec. + 3 stops
f4 @ 1/1600 sec. + 4 stops
f2.8 @ 1/3200 sec. + 5 stops
Since your camera's maximum shutter speed is 1/4000 sec. this is the maximum aperture you can use on a sunny day.
f2 @ 1/6400 sec. + 6 stops
f1.4 @ 1/12800 sec. + 7 stops
As you can easily see, as we "open up" our lens (f16 to f11, f11 to f8, etc.) we allow more light and our shutter speed must increase (get faster) to compensate. As we "stop down" our lens (f11 to f16, f16 to f22, etc.) we admit less light and our shutter speed must decrease (get slower) to compensate. If you are using your camera in Manual then you must adjust the shutter speed when you adjust the f-stop. If you don't, your exposure will be either under or over exposed. If you'd like to see this for yourself, put your camera in Manual, set ISO 100 and set the shutter speed to 1/1600 sec. and then take a picture at every f-stop from maximum to minimum, leaving the shutter speed at 1/1600 sec.
So if our goal is f1.4 in bright sun we need to reduce the amount of light admitted in order to not exceed our camera's maximum shutter speed. This is best done with a Neutral Density (ND) filter. You'll find a good explanation here:
http://www.answers.com/topic/neutral-den… ND filters are marked in two ways: as ND2, ND4, etc. or 0.3, 0.6, 0.9, etc.
ND2/0.3 = - 1 stop
ND4/0.6 = - 2 stops
ND8/0.9 = - 3 stops
ND64/1.8 = - 6 stops
ND1000/3.0 = - 10 stops
By looking at our "Sunny 16" chart we see that to use f1.4 at 1/400 sec. will require an ND4/0.6 and ND8/0.9 to give us - 5 stops and reduce our shutter speed from 1/12800 sec. to 1/400 sec. Or we could decide on a 1/200 sec. shutter speed and just use an ND64/1.8 to give us - 6 stops and a 1/200 sec. shutter speed.
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sounds logical on the ND filter. Could the cheapest quick fix. But Im not so comfortable switching filters during shoots......