Doing flash photography involves 2 exposures -- one for the ambient lighting, and one for the flash lighting.

Once you understand the necessity of making 2 settings for an exposure involving flash lighting, then you will gain the exposure control you seek.

Note that you will have more control with Manual mode settings for Shutter Speed and Aperture.

Here's one way. Set your camera for properly exposing the overall lighting (with the flash unit shut off). At this point, the main subject may appear underexposed. But continue. Check the Histogram to ensure this ambient lighting exposure stays within the range of the Histogram.

Next, turn on the flash unit. The flash lighting will illuminate your main subject. So adjust only the flash lighting output. Do so by having your flash unit set to Manual mode. In the Manual mode, you can set the flash power level to expose the main subject correctly.

You may have to take a few test shots before reaching the desired amount of flash lighting on the main subject.

Some photographers, despite their experience doing photography, still need to learn how the flash lighting and the ambient lighting work together.

Consider how the Shutter Speed controls the amount of photons falling on the sensor by leaving the shutter open for a set time. The sensor captures photons during this period.

Flash lighting occurs at the beginning at the period. But the flash lighting happens only during a very short part of this period -- typically for about 1/1000th second (although it will last a much shorter time at a low flash unit power setting).

After the flash lighting ends, the shutter stays open for the remainder of the duration of the exposure in order to record the ambient lighting.

This two-part mechanical operation determines the final exposure.

Good luck.

I have to disagree with most of what's been said.. If you meter off a face that is dark "eyes are in the shadows" How are you going to reduce the flash enough to brighten these areas without blowing out the white pants?? You are adding more light to an area that already has to much light. Well, focus on the eyes and meter off the white pants, then reduce flash for shadows?? Doesn't work in my book. Use a reflector to add additional light to the face. Someone tell me what area of this photo your going to meter off and then add flash and make it balanced??