Photography 202 - flash guns

[smilodon]

The original community forum thread in which I posted Photography 102 â€" Using flash, has kicked off quite a conversation with lots of questions and answers on the subject. So I've taken the liberty of combining them into a single post and adding it to the Dead Men forum on the off chance it might be useful. Think of it as a flash photography 202.


One of the most useful tricks when thinking about flash photography is to always remember that every photograph taken with flash is in fact two exposures. One exposure is provided by the light from the flash gun and the second is provided by the light that already exists in the scene which we usually refer to as ambient light. These two separate exposures combine to create our final image. The reason why this fact is often not apparent is due to the ambient light exposure often having no effect on the final shot. Most flash photography is done in locations where the ambient light is very weak or almost non existent i.e at night. This leads to the flash light providing almost all of the light while the ambient has a negligible effect if any effect at all. However there are always two exposures. It's often the case that when this second ambient light exposure does contribute to the final image it can cause confusion and inconsistent results. So lets look at what exactly is going on.

Most of us are aware of the ISO setting on our camera. Setting higher or lower ISO numbers will allow our cameras to become more or less sensitive to light and so effect the exposures we make and the photographs we take. However for the benefit of simplicity we can assume our ISO is remaining the same for the rest of this description. It makes life simpler.
With this in mind we can state that the flash exposure is controlled by the aperture of the lens and the power output of the flash gun. Large apertures and powerful flash allow lots of light into our cameras and small apertures with low power flash obviously result in less light being captured. Shutter speed does not contribute to the exposure. As long as the shutter is open long enough for the flash to fire out flash exposure can take place. How long the shutter does or does not remain open after that is irrelevant, the flash has fired and won't do anything else until we take another separate photograph.
Our second ambient light exposure is controlled by the aperture and the shutter speed. The longer we leave the shutter open the more of the ambient light will enter the camera and the more of the scene lit by the ambient light (rather than lit by the flash gun) will appear. It's often the case that when we use a flash gun on our subject at night we see the subject lit by the flash but the background is inky black. This is due to the flash output and the aperture being what we need for exposing the subject but the aperture and shutter speed being far to fast for us to capture any of the background. Most camera will set a default shutter speed of a hundredth of a second or less. Long enough to catch the flash gun firing but nothing more  This is why when we take our shot we can often see the background clearly ourselves but when we look at our shot it's turned to a black wall of nothingness. Only when we leave the shutter open for longer (maybe several seconds) will the ambient light build up enough to allow the camera to capture the background. Understanding this idea will help us understand flash photography.

Rather than talk in abstracts it might be useful to describe an imaginary photograph we're trying to take and see how we can use the above concept to our advantage. Let's imagine we're going to take a photograph of an artist in her studio. We have her dressed in her painting smock, holding some brushes and sat on a stool in the middle of her studio. Beside her is an easel holding one of her paintings. Several yards behind her is the back of her studio with shelves full of paints, brushes and the other thinks that clutter a typical working studio. Stacked up on the floor against the back wall are blank canvasses and partially completed paintings. There is some light coming through a window but it's late and the light in the studio is not strong. So we decide to use flash. We set our camera up on a tripod, pose our subject, frame our shot and get ready to shoot.
Let's assume we set f7 for our aperture. We're hoping to get our subject sharply in focus, obviously, but we also want to get the back of the studio partially in focus as well. By blurring the background a little we can hopefully see the shelves of paint jars, brushes etc and the stacked canvasses but make sure these elements don't compete for our attention with the artist who is the main focus of our photograph. We set 1/60 second on the shutter as this will be quite long enough to capture the flash when it fires. We set the flash to 1/8th power. It doesn't really matter whether we set these values ourselves or if the are calculated by the camera and flash. The experiment is looking at the effect of changing the settings rather than working out how we choose the starting values. Frankly I always start by trial and error. I take a shot, see if it doesn't work and then change a setting in an attempt to correct the image.
So now we take our shot and check the result. The combination of 1/8th flash power and f7 aperture has nailed our subject perfectly. They are nicely exposed, with good skin tones, nice highlights and everything looks fine. However the background has not appeared at all. The subject is sitting against a black background. We can see nothing of the studio behind her and have no idea where she might be sitting.
We need to leave the aperture and flash output alone as they are fine but we still need to increase the ambient exposure so we can see more of the background. To this end we bump the shutter speed up to 1/30 second and try again. Now we can see the background but it is still not as bright as we would like. So we up the shutter speed to 1/15 second and try a third shot.
Now we have a good picture. The background has appeared and the f7 has blurred it enough that it does not compete with the subject but not so much that we cannot make out the shelves, paints and canvasses etc. We have an artist in a studio.
But we have a small problem. Now the subject seems to look a little bright and overexposed. What has happened is that the flash gun, which ignores shutter speed completely has provided enough light to properly expose the subject. However while at 1/60 and 1/30 second the ambient light didn't contribute any meaningful amount of light to the subject (the shutter speeds were too fast) at 1/15 second the ambient light has had time to build up on the subject and the camera sensor has picked it up and so the subject appears too bright.
To bring the exposure back to where it was we need to remove the same amount of light from the the flash as was added by the ambient light. So we guess and reduce flash exposure. Most modern flash guns have a setting that can adjust flash output. So we reduce the flash power by 1/3 stop (shows as 0.3 on the flash display) and take our forth shot. The flash does not add light to the background so that remains unchanged and what we want. But now a little less light has fallen on our subject. This has done the trick and we now have a properly exposed subject (flash + aperture), a properly exposed background (shutter speed + aperture) and that background is slightly out of focus (aperture).
Now we have done this last step manually but if we had set TTL â€" BL on out camera as described in last post we would have automated the process and the camera/flash would have automatically adjusted the flash output to balance it with the ambient light and we would not have over exposed our subject in shot three. So always one shot but two exposures. And the ambient light exposure may or may not actually add to the shot at all. But when it does hopefully now we know why it does and how to control it better.

Hope this helps a bit.

[Luminance]

Hmm, interesting, hadn't really thought about it, really explains why the background of some of my pictures last weekend (first time fully on Manual) were really dark (no visible 2nd exposure).

Is it possible for you to post that picture? I'm intrigued at the end result and possibly with the over exposed at 1/15?
(Even if you have to add a big red cross over the pic and a black bar on her eyes or something

Ok for a min I was about to delete that again as the 2nd time I was reading it I did notice the "imaginary photograph" part, but it just shows my interest I guess (even though I apparently am skipping words ><)