In this photo I first measured the ambient light was out in the background. I gave the measurement f / 8 to 1/50 using ISO 100. I put those values into my camera.
Because I was using an automatic flash, I put on the flash I was using ISO 100 and was going to use an aperture f / 8. And with these values take the picture. As you can see both the girl and the background are properly exposed.
If I put a diaphragm in the flash different than I have in the camera, unless it is to correct the light, the picture I can get over-exposed or under exposed.
In the next photo I put the same values in the camera in the photo above, or f / 8 to 1/50. But I put the flash that he would use f/16.
Basically, when you dial f/16, I'm telling the flash that I will use smaller aperture so, what will do the flash? You will shoot more light to illuminate the picture. The photo below is the result. As you can see is a picture that does not work.