When you start experimenting with your speedlite you tend to come across additional knowledge of the behavior of light and its relationship with your DSLR settings. A quick exercise would be to start off with your camera sync speed and start to lower only one settings of the triangle of exposure (ISO - Aperture - Shutter Speed), the shutter speed. You will soon notice that the ambient light starts to get brighter as you go from 1/250 to 1/200 to 1/100.
You will notice from the above photos that at your camera sync speed (1/250 sec), your subject is lit primarily by the flash and the background is dimmed. In the second photo to the right, a slower shutter speed of 1/50 sec allowed us to collect more light from the background. This is called "dragging the shutter" in photography jargon. But what is really amazing is that the subject is still tack sharp even if you are handholding your camera. Why? Well because the subject is lit by the quick burst of your speedlite and not by the ambient light.
In summary to the above quick experiment, we can conclude that; when using flash photography:
- Shutter speed controls ambient light
- Aperture controls flash exposure
- ISO affects both flash and ambient equally and it does not distinguish between the two
How to balance between the 3 exposure elements above depends on the situation you are in. If you are shooting a party indoors and your goal is to only gather enough light for a balanced foreground and background then slow the shutter speed. Assuming there is some ambient light in the venue, this can help allow enough ambient light to create more depth in your photo.
I always put my camera on Manual mode when using flash for a controlled adjustment. I start with ISO-400 and meter from the background to my taste, then I power and add flash and take a test shot. If the whole photo is underexposed, only then I open up the aperture gradually to allow enough light into the scene.
When both lowering the shutter release and opening up the aperture don't work, consider ISO to the rescue. By raising the ISO you make the sensor more sensitive to light, allowing more light to show in the photo.
Remember though to keep an eye on the digital noise and to keep it at minimum. It is better to shoot at lower ISO settings than at higher ISO for that matter. Think of ISO as a creative tool:
- If you want to increase depth of field by closing the aperture (higher f number) then you have to increase the ISO specially if you are already on your speedlite full power
- If you want to collect ambient light by dragging the shutter (slowing down the shutter speed) without having a blurry background, increase the ISO