When using flash to light subjects, if there is nothing close to the subject in the background the image in that area will give a black background, as there is nothing to light up. This can be a pleasing effect in itself, but it is also nice to make a flash lit shot, look more natural.
This can be achieved, for example, by shooting the subject with a background placed behind it ( eg a coloured card), using a wider aperture and using the flash just to fill light ( it lets in more ambient light ).
However when getting in close and wanting higher depth of field, these options are not always suitable or possible.
If the ISO is increased, this will allow more of the natural light available (ambient) to be used ( as the sensor needs less light to expose and image as the ISO is increased), it can therefore expose some more of the background.
I tried out an experiment with different ISO settings on a recent visit to Dunyeats Heath. Here I shot the same male Emerald Damslefly, with the same flash, expsoure & aperture settings, but varied the ISO. As you can see from the examples below, the effect of this as the ISO is increased is to make more of the background “less black”. A balance will need to be made depending on your camera and how it handles digital noise as ISO increases, on what ISO you can use to get a reasonable final image from. ( I think it also has effect when noise gets unacceptable of softening the details). On my 650D ISO 1600 can be about acceptable, so I used this as the high point in this example.
Starting at 1600 ISO the background is fairly green, from the distant gorse plant.
At ISO 800, this is getting darker – but is still greenish.
And finally at ISO 200 the image is much darker in the background, despite some software processing attempts to recover some. ( Not done for ISO 800 or 1600).