To continue what Digitalis had been said..
Originally posted by klh Makes sense. Do you expose for the ambient light in the room?
I prefer ambient at 1 or 2 f-stops darker than the flash. I use to measure the ambient using the green button, then stop down 1-2 stops. Then adjust the power (and distance) of flashes. You also have iso to go, start with 400.
I also prefer 50mm or 70mm for couple portraits, half body or less. That implies enough space to shoot. You may need 10 feet from subject to background to have them blurred. Assume indoor, depends on subject's hair dark or light, I may switch on an indirect lamp for dark hair in background.
Your lens, f4 or f5.6, at 50mm or 70mm, flash power 1/4 and 1/16 would be ok. Reflection on white wall/door = reduce power 1/3-1/4. shutter speed = ambient then 1 or 2 stops less.
Regarding location in the house: I use to place the couple close to the door opening that I can use the other room to have enough space to shoot, and blurred background. White door blades=reflection - useful if you don't have umbrellas or reflector on location.
Flash gel = important to blend in ambient light.
Sigma flashes are not known for user interface. I have 2x 530 super.
- If you have radio triggers: Dial your flash to Manual mode, use Set button and [+]/[-] to set the power.
- If not, use the wireless build-in and use your pop-up on K20 to trigg these sigmas. K20 has an option to turn off your pop-up flash - not completely off - but that will reduce interference of the popup greatly. This case use wireless slave mode, and adjust flash EV directly for each.
Anyhow, you may need to testrun the setup. Stick to it when you have the settings.
and good luck.