Camera Flash Softbox Yey! How To Use Built In Flash On My Nikon D40?

How to use built in flash on my nikon d40? - camera flash softbox yey!

When the camera is in manual mode, I must draw attention to the extent of exposure and the flash of my camera is charged automatically to the proper illumination on this subject? Or is it more complicated to flash to use them, I need the exposure a bit lower / higher to compensate by using a flash?

Also another question. Looking to buy a light kit. AA, which is economically (ie under $ 300?) Lighting kit you can buy for amateur photography. I tried to get maybe a light box or something. I'll be shooting mainly portraits of people not only their faces, but the whole body, and what makes the difference.