One of the most creative ways of using a flash is to mix it with ambient light (either H.M.I, tungsten, fluorescent or daylight). This technique is used a lot for fashion shots in attractive interior and exterior settings. The secret here is to balance the output of the flash units so that shadows are filled in such a way that the light from the flash is not dominating the shot.
Ambient light is often a mixture of different types of light source, especially when shooting interiors. Outside, there is a difference in colour temperature as the day goes from early morning to noon and then late afternoon to evening. Interior light is much more difficult to balance, as it is usually a mix of daylight through windows, tungsten and sometimes even fluorescent sources. A white balance adjustment in the the camera or capture software will usually correct the problem. Also, you have to gel the strobe to match the colour of the ambient light.
For example, if the interiors are all tungsten lighting, then put a Color Temperature Orange(CTO) Gel on the flash to match the color of the lights. So, if you shoot in normal/auto white balance, you will get a yellow/orange toned image. To correct this, simply change the camera white balance to tungsten mode, and the image will lose the yellow tint and lights will be white again. Take a look below for some sample color correction gel filters from Rosco. The shades of orange are for balancining flash with tungsten light, and shades of green for balancing with fluorescent interior lighting.
That being said, all the shots in this post are ungelled flash. Look at the image below......you can see all the tungsten lights between the pillars which has a distinct yellow tint, while the model who is lit with a flash through a beauty dish, has no yellow color cast.
Let's see how to balance flash with ambient light in a room. In this case, we'll need a tripod for the camera, and ofcourse your strobe/flash light. You can use a studio light or if you are a 'strobist' fan, you can use your off camera flash. Use a diffuser of your choice....we normally prefer shoot through umbrellas or softbox / beauty dish.We're going to use the strobe as the main light source lighting ONLY the model, and the ambient providing the fill to the model as well as lighting the room.
Take a look at the image below. First we decide the framing and set the camera on the tripod. Next, we take a shot for the exposure of the interior light, without the model. Normally for a well lit interior we have to give a shutter speed of 1sec(thats why you must have a tripod) with aperture being around f4, to get all of the ambient light. The idea is to expose to make the light sources not burn out. We leave the room little bit dark sometimes, as when we put the flash later for model, it directs the eye towards the model. The flash was shot through a white umbrella on boom stand and placed in front and top of models head pointing downwards.
Even though the camera is set to 1sec shutter speed, the flash syncs only at 1/200(or 1/250depending on your dslr) and hence your model won't be over exposed. One important thing! Since we are shooting at a slow shutter speed of 1sec, it is vey important that your model remains absolutely still for that whole sec, to avoid any motion blur.
In the shot below, the set was made of zero watt light bulbs. We used the same technique, but instead of umbrella we used a 32deg honeycomb grid to give a hollywood/broadway type spot light effect.
The next shot deals with balancing windowlight and the flash. The light here is coming from slightly tinted glass window which gives the image a greenish tint. If you want to correct that, you can gel the strobe with a window green filter, and set the white balance on the camera to fluorescent. We wanted to keep the tint and ambient light real, so no gels again. The exposure in this situation changes. Here we set the camera to its maximum flash sync of 1/200(or 1/250 depends on your camera) and opened the aperture to f2.8 to get the shallow depth of field. Then we set the flash power to get the same reading. The flash was fired through a small beauty dish with diffuser.
Ok.......................Now lets shoot outdoors!
In the first example, we are going to use the sun as the main light, and flash to fill in the shadows. The sun at the time of the shoot was almost overhead. We used the flash through a softbox to fill the shadows on the left side of the models face. We always use the max flash sync as shutter speed when shooting outdoors, so the camera is always at 1/200(or 1/250), so you don't really need a tripod, but we use one anyway. Also, we normally use a polarising filter outdoors.
Now, lets use the flash as the main light, and daylight as fill. In the shot below, we used a flash fired through a small beauty dish as the main light, and the camera was set for the flash. Since it was an overcast day, and also we were using a polarizer on the lens, we used a slightly slower shutter speed of around 1/30. This enabled us to capture the slight hand motion of the model. The aperture was somewhere around f16.
Before we talked about the idea of balancing strobe with ambient indoors and we've also seen the strobe as fill light and then as main light in the above two examples. So now, lets see how balancing flash and ambient light works on location. Again, like in shooting interiors, we set the exposure based on the ambient background. Then we set the flash, which was fired through a white umbrella, to give the same output for that exposure.