Exposing for the sky

The two photos above are taken less than a minute apart, yet the skies in the photos look completely different. You'll note the vivid colors in the sunset, while the sky in the picture of the boy is completely washed out. You could look at each photo individually and reasonably say that the photo is exposed properly, yet the photo of the sky (1/1000 sec) is 4 stops underexposed to the photo of the boy (1/60th sec).

This brings up a number of points.
1) to capture color, generally an underexposed image will be more saturated with color. I probably could have photographed the sky at 2 exposures higher and retained some color, but the greater saturation of color comes from the lower exposure. A "brighter" image will most likely be less colorful.

2) This is a perfect example of how the dynamic range of a camera does not equal the dynamic range of the human eye, which would have been capable of seeing the boy and the sunset and both of their colors.

3) to capture this image ideally with the boy and the sky exposed the way you would like, there are a number of options. In the past, I would have taken a photo or two and just photoshopped them together. Now you can do image blending with a raw file and just take the properly exposed pieces of both.

4) Because there are two exposures, you can control both of them. Now add the flash for the object in the foreground. Using your Max synch speed, light the subject. Because the flash duration is so short, the shutter speed has little effect on the flash lit subject, while the flash will have no effect on the exposure of the sky. Adjust the aperture to properly expose the foreground.

At this point the subject should be lit properly and the background should be underexposed. Now lengthen the shutter speed until the background is at the desired exposure. This technique is known as "dragging" your shutter, and refers to the process of lengthening your exposure time so the background receives more light, while the foreground image is lit with the flash and not as much affected by the shutter speed.

from the Strobist site- and this is really the key to balancing flash with ambient light.
1) Set the camera on max synch speed.
2)light the subject
3) Fire some quick test shots while adjusting the aperture until the lit subjects are properly exposed in your cool little cheater screen on the camera back.
4) Your ambient-lit areas will be very dark. While keeping the chosen aperture constant (to make the strobe happy) open your shutter speed up until the lighting ratio looks as smooth (or dramatic) as you'd like.