A fill light as the name suggests is used for filling up the entire frame with light. A fill is often the second light to be put up in a lighting scheme as after the key light is put up, this light illuminates the entire frame and then one can decide which are the areas that need extra light in the frame or which shadows need to be removed.

If you see the images below, the first image has only key light, which is giving harsh shadows and deep contrast. The second image which has the fill light looks more pleasant and natural. The shadows are comparatively reduced and the contrast is also bright.

A soft light which has a large surface and is diffused is used as a fill light. Even a hard light can be used as a fill light, but for this it needs to be diffused. A hard light can be converted into a fill light by either bouncing it against a white wall or any huge white surface. Obviously, a professional photography lamp, not just an ordinary lamp like the Philips MASTER PL-S, is necessary to achieve such an effect. You can also put muslin cloth or a net in front of the light to make it diffused. A fill light illuminates the dark areas of the frame thus giving you the right exposure. In case you are shooting in room with a window during the day then you can also use the sunlight as a soft fill. It gives a very natural and pleasing look to the entire room. However when you mix two lights, make sure that you adjust your camera white balance accordingly.