there are very technical explaination if u would like to listen.
standard by flash, the min shutter u will get is 1/60. let say the
background is 2 stop under at 1/60 F5.6 iso400. (kitlens + A300 (iso400limit))
your subject will always lit up to 0ev metering (properly expose) by the flash. now, knowing all of above setting, the question is, how to you make the background expose the same?(as the subject?)
answer is simple, increase 2stop of exposure, but how?
1. Shutter Speed, by slowing down the shutter, u will have more light gain, 2 stop is by decreasing ur speed to1/15 from 1/60, which u might not wan to use it, as it causes blur from handshake if u are zooming at 100mm.
2. Sensitivity of ISO, by pumping to iso1600 from iso400 (that's 2 stop) u'll gain exactly what u need, but problem is, some of us are using older model where, iso400 is as far as they willing to go.
3. Aperture (Ksano you're right), increase the aperture by 2 stop, F5.6 to F2.8, u get more light travel into the lens/sensor, hence u will get brighter background, but as well as bokeh, if bokeh is what u wanted, then its okay, if it is not.... another story.
4. and u can mix 1 and 2 and 3 together, say shutterspeed at 1/40, iso800, at F5, u still gain 2 stop.

that is why, Pro use manual, using each of the setting above to compensate of each weaknesses.
tamron? dont think will sell, unless sigma's 17-70 becomes F2.8 and it's as sharp as tamron.

slow sync flash = slow shutter. its just a auto-mode used to overwrite the pre program of shutter in P/A mode.
Wah, very details explaination ..
.. i got it bro ..
.. thanks thanks alot .. appreciate it ..
..