All modern auto flash systems from major camera makers like Canon and Nikon use some form of TTL (Through The Lens) system. In theory, it is supposed to be simple and easy and would make it easy for camera users to get good exposure with flash shots. Of course in real life, it is usually another story. A lot of beginners with little or no understanding about how TTL flash works usually get disappointing results with flash shots.
I am not going to go into a lot of technical details, but I hope with a few simple illustration and example, you will be able to know how to set your camera and flash to achieve good results with TTL flash.
First, just a few words on how TTL flash works. When you press the shutter release, a TTL flash will fire a pre-flash before the actual shot, the camera will then measure the pre-flash with the ambient light level to calculate the power needed in the actual flash for the shot. Don’t start looking for a pre-flash though, it happens very fast, only milli- or micro-seconds before the main flash, so you will only see one flash, not two, because the human eye is too slow to able to separate and see the two individual flashes.
Basically, what that means is that, you and the camera not only needs to take the flash power into account when making a shot, you will also need to take into account the ambient light. There are a few things you need to know about camera settings.
1. Shutter speed
The shutter speed you set will not affect the flash exposure of the shot, since flash duration is very short, in the magnitude of thousandth of a second, whereas shutter speed will be of much longer duration. So no matter, what shutter speed you set, it will not affect the amount of flash light falling onto the image sensor.
What shutter speed does affect is the amount of ambient light falling onto the image sensor. So the slower the shutter speed, the more ambient light will be captured by the image sensor.
The aperture affects all light entering it. So it affect the flash light and the ambient light equally. The larger the aperture, the more light will enter it and fall onto the image sensor, both flash and ambient light.
ISO setting affect the light sensitivity of the image sensor and like aperture, will affect flash and ambient light equally.
For most people, flash photography would probably be done indoor for some family or friends portrait photos. So you will want to capture not only the people in the photo, but also the surrounding. I don’t have a real life model to use, but I will be using my little snowman to demonstrate. The following shot demonstrate the worst kind of flash photo.
The shot above was taken with direct flash, ISO 100, shutter speed 1/125s and aperture f/6.3. You can see the direct flash light was very harsh casting heavy shadows, and although little snowman was reasonably well exposed, the surrounding were mostly dark and not ideal. The reason why the photo looked like this was because the shutter speed was set to high, the aperture set too small and the ISO set to low. All contributing to the effect that too little ambient light was captured by the image sensor. The flash was only powerful enough to illuminate the little snowman, but we need to capture more ambient light to illuminate the surroundings, which we failed to capture in the above shot.
Now, let’s get into the scenario of how most beginner of photography would set their camera and flash. The most likely setting they would use is camera on Auto(A) or Program(P) mode, and the flash on auto TTL mode. The following shots illustrate what a settings like this would look like.
The first shot above was shot in Auto(A) mode and the second shot was taken in Program(P) at ISO 100 and you can see the results were not really good either.
Basically, what we need to do is to increase the ISO, lower the shutter speed and open up the aperture and capture more ambient and flash light. You really need to shot in Manual(M) mode. The following shot was taken with direct flash at ISO 400, shutter speed 1/50 and aperture f/3.5.
You can see that it is already looking better than the previous shots with more of the surrounding showing up in the photo. However, the surrounding still looks a bit too dark and the direct flash light was still too harsh casting heavy shadows.
Direct flash is often not the best for this kind of photo. We need to soften and spread out the light. The simplest and cheapest way of doing this is by using bounce flash. The following shot illustrates the use of bounce flash.
You can see the result is much more pleasing now. The camera and flash setting were exactly the same as the previous shot. The only difference was the use of bounce flash instead of direct flash.
Although bounce flash can often be used with good results, but it may not work well in all situation. It only works well, if the ceiling is lowish and white in color. If the ceiling is very high, or it is dark in color or off white, it may not work well. High and dark color ceiling will not bounce off enough light, and non-neutral color ceiling may give a heavy color cast to the photo.
Besides bounce flash, we can also use a good quality flash diffuser. My personal favorite is the Gary Fong Lightsphere. It gives a nice soft quality light. The following shot was taken with flash with a Lightsphere flash diffuser.
If you are a beginner, you probably would be pleased with getting the result above with a flash shot. However, if you want to go one step further, you can consider using off camera flash. A few DSLR from both Nikon and Canon can support off camera flash directly. Other models, you will need to buy a remote flash trigger. The best result can usually be obtained by using an off camera flash. The shot below was taken with an off camera flash with Lightsphere. The flash was triggered by Phottix Strato remote flash trigger.
A final recap of what you should do to get good TTL flash result:-
1. Shot in Manual(M) mode. Set shutter speed between 1/50 to 1/60sec. Open up the aperture. For most kit lens, set it at maximum aperture around f/3.5 to F/4. If you are using a higher quality zoom or a prime lens, you can set aperture at f/2.8.
2. Set ISO 400 to 800.
3. Avoid using direct flash. Either use bounce flash or use a good quality flash diffuser.
4. For more advance photographers, consider using off camera flash if possible.
Finally, a quick tip for Canon users. Most people usually find their Canon flash photos consistently underexposed even if they did everything right. In my experience, for some unknown reason, Canon E-TTL flash system consistently underexposes flash photos by about 2/3 to 1 stop. Well, there is a easy way to get around this problem. You can set the Flash Exposure Compensation(FEC) to between +2/3 to +1 stop. Then you will find your flash photos much better exposed.