Sub expose the bottom
As mentioned above the diaphragm controls the flash. To control or change only the background can only use the speed and if we change the diaphragm, we will also affect what the lights flash.
How to darken the background? What we have to achieve is that less light reaches the bottom of the camera to come out darker. This can be achieved if we raise the speed on the camera.
Let's see an example. We have already seen the picture where the background and the girl were well exposed. In this photo, taken at ISO 100, I use f / 8 to 1/50.
In the next photo I instead of putting the camera in a rate of 1/50, I put a speed of 1/250. And this is theresult.
As you can see the girl is still well exposed, but the background is rather darker.
At dusk we remove the background details of the scene that may disturb the picture and distract the viewer. We can also make appear another time or time of day. So we can take a picture in the afternoon and it appears that it is almost night.
Everything will depend on how we can darken the background and that we can use speed.
In this case I could not achieve a darker background and could not go higher speed. Why could not upload more? For 1/250 is the synchronous speed of my camera. To remind the synchronous speed is the speed with which I can take pictures with flash.
The photo below is an example of what happens when I use a rate higher than that of synchronism. In this picture I used a speed of 1/640.
As you can see half the picture comes out almost black and the other half goes a little better lit. And there is one line that divides the image quite marked. This is because at speeds above synchronous speed for the picture is exposed parts.
So when the flash fires it does only part of the picture (in this case to the right of the image). And the rest (the left) was obscured by a curtain of the shutter when the flash fired.
So when you want to darken the background never have to spend the synchronous speed, unless you have a camera function that is called FP sync or HHS. This feature allows you to take with flash at higher speeds.