Canon PowerShot A570is一問

of course, but for 人像, actually you want only focus the HEAD more ... and make the background less sharp ... but it is preference. :)
景深物清效果 is very simple ...
all you need is a shutter priority of around 1/60s and aperture is f6-f8
and A570IS has Tv (shutter priority) and Av ( aperture priority) mode,
you can just set one of them ... Tv first, make it 1/60s, and check if you get "HANDSHAKE" :) or IS is effective ...
and now the aperture should be in a bigger # ... , make sure ISO not over 200.
and you can start taking the portrait to have the 景深物清效果 ...
DOF = depth of field = 景深 ;
物清 = small aperture + a bit longer exposure ...
good luck la