Also, when to use the diffusion dome?

Sheri .. this is the best I can do for you .. wedding photography tips & advice.

Neil vN

TTL flash exposure is calculated with a pre-flash signal that the flashgun emits before the main burst of light. The main burst of light is what gives you (hopefully) correct exposure. But the camera needs some way of determining what the correct flash exposure should be. In order to do that, the camera measures the amount of light returned from that pre-flash. Note that the shutter and the sensor of the camera have no involvement in this part of the equation- the shutter is closed. The light bouncing back from the scene enters the lens of the camera (through the lens). The light is then read by the light meter within the camera. Remember, the light meter and the sensor are not the same.

The first flash is the preflash that goes off before the shutter opens. The second flash is for the proper flash exposure, while the shutter is open. The light reflects back from the scene and “burns” an image onto the sensor.

If you are using camera manual mode, you are responsible for choosing the camera settings for the exposure of the photographs. When you are using aperture priority mode, when you half press the shutter button, the camera tries to calculate the settings for exposure, based on where you are pointing the lens. When you half press the shutter, the exposure settings are locked, if you have exposure lock & focus lock enabled in your camera’s menu, then you can recompose your photo, and fully press the shutter button.

A question might be “when using the flash on my camera, when is the exposure calculated for my flash? Is it when I half press the shutter button, or when I fully press the button? The flash exposure calculation begins when I fully press the shutter button. Because the preflash is instantaneous, this process is really instantaneous. The direction that your flash is pointing when you fully press the button dictates the scene which will reflect back the prefash.

The preflash is almost imperceptible as a distinctly separate flash of light because it occurs so closely in time to the real full flash of light that it uses for the flash exposure. They seem to occur as just one flash. But really the preflash sends out this flash of light which hits the subject and scene, and bounces back through the lens of the camera, thereby giving the camera information about the distance and reflectivity. Now the flash/camera sends out a second flash burst to light the scene for what it calculates is a correct flash exposure. Often the calculated exposure is not what you had in mind, so just dial in flash exposure compensation (FEC). Just like when you are not using the flash, backlit scenes often fool the camera.

Flash exposure is achieved when the flash utilizes duration of its output. A longer duration (or a longer flash of light) will give a brighter flash exposure.

We can use aperture and ISO and shutter speed – all three controls – to control ambient light exposure, without having an influence on our flash exposure (because in TTL flash mode, your camera will automatically calculate proper flash exposure, in each photograph, then raise or lower the output from your flash to achieve this). When you change the iso or aperture on your camera, the range of the flash is affected, but, in ttl, that does not make a difference to exposure unless the subject is out of range of the flash. The iso & aperture & shutter speed (below max.sync speed) does not make a difference to the flash exposure in ttl flash mode, because the camera and flash will adjust the output of the flash so that the flash exposure will be the same for the subject. Note, the flash exposure will be the same, the flash output will be different every time you change your iso or aperture. This is really the brilliance of ttl flash mode. As stated, the iso & aperture only affect the workable range of the flash, when using ttl flash. Raising or lowering the iso & aperture will not make the flash exposure brighter or dimmer. With manual flash, however, the opposite is true.

In aperture camera mode, if I lower my iso, the ambient light will be less, so the shutter speed lowers in order to raise the ambient light level back to normal. For my flash, when I lower my iso, the flash range will lower.

The preflash does not seem to occur as 1 flash when you set your flash mode to rear sync flash, or 2nd curtain, and set the camera to manual mode with a (eg)5 second shutter speed. But If you are set to front sync, or 1′st curtain flash mode, then it will occur as 1 flash even if you are at 5 second speed. The first flash is the preflash that goes off before the shutter or curtain opens. The second flash is for the proper flash exposure while the shutter is open.

In manual camera mode, if you lower your iso or aperture, the ttl flash will increase it’s duration to achieve proper flash exposure. The ambient light exposure may for example get darker, but your flash exposure, in ttl flash mode, should remain properly exposed. It’s a good experiment. One late evening at the beach, set the camera to manual mode…start with 1/2 stop underexposed ambient light according to your on-camera meter, and turn your flash to ttl. Take a photo of your friend/ volunteer. Then vary your ambient exposure down by 1/2 stops or 1 full stop for each photo, according to the camera’s meter. If theory is correct, the background light level should change, but the flash exposure should be the same on your friend (as long as your friend is not too far away from the flash, and out of range. As you vary your iso or aperture for the ambient light, it also affects the range of the flash).

This is also a good opportunity to play with shutter speed and see how it affects ambient light at night time. Notice As you lower your speed, and when you are using your flash, there won’t be a dark black background…sometimes the speed has to be fairly slow, like 20 seconds, or 10 seconds. It all depends on your level of light, and the iso and aperture you are using. This experiment is best with a tripod.

Shutter speed never affects the flash exposure, unless you are using a shutter speed that is high sync speed, then the efficiency of the flash is greatly reduced.
Shutter speed always affects the ambient exposure.

Have a good week!

William .. it is one of those things you will have to try.
Just do it and see if it works. Let me know. : )

Neil vN

Gregory, I haven’t used a Stofen like that in a while, and wouldn’t know where to look in my archives for comparison photos. Sorry, I can’t immediately help you on this one.

Neil vN