New to speedlites - How is this supposed to work? Warning: TOTAL NOOB QUESTION

livinginthepast · Sep 30, 2011

Greetings...

So I've never used a speedlite before and need someone to walk me through this...

Canon 7d, 430exII.

This is going to be an unbelievably stupid question, but here goes. So the speedlite is in the hotshoe, set to ETTL mode. Camera in Manual mode. Should my in camera metering compensate for the speedlite or will the in camera metering continue to meter the subject as it did without the speedlight? For example, In low light, my in-camera metering tells me that I need to utilize long exposures and slow shutter speeds regardless of whether I have the speedlite attached or not. Is this the the expected behavior? Or should my light meter recognize that there is a flash unit and compensate for the flash?

My assumption is that light metering in the camera won't change and that the flash unit is going to compensate for the exposure settings I choose.

This seems to be verified by my testing. Indoors at night I set my camera to 1/250 @ F4 and the speedlite lit the subject correctly, as it did when I set the exposure to 1/160 @ F11. In both cases, the in camera metering told me that i was off-the-scale underexposed. So, given this, I can simply dial in the settings I want for the effect I'm hoping to get (for example, shallow/deep depth of field, dark/light background etc) and the flash unit will automatically expose for my subject in low light conditions. Right? For daylight conditions I should theoretically be able to meter for the background and the flash will expose for foreground subject - giving a proper exposure fill and leaving the bg lit per what the in camera metering believes to be correct.

Am I on base here?

Thanks for your patience.

Regards,
Bill

Overread · Sep 30, 2011

The way the flash metering works is roughly thus:

1) You are right the camera itself won't meter with the flash in mind, this is because the cameras meter relies upon reflected light from the scene at the time. It can't tell what effect the flash light will have on the scene at all and thus can't meter for it.

2) The flash itself, when in auto Ettl mode, will send out a fast preflash when you press the shutter button (so fast your normally won't see it) this preflash illuminates the scene with the flash light and gives a reference to the flash of the light falloff before it hits the main subject; metering from that and taking into account the cameras settings for the shot the flash can then adjust its power output to meet a proper exposure and then fires its main burst of light.

The is why you have exposure compensation (in modes outside of manual) and flash exposure compensation since the flash metering is a separate process. Thus you can certainly meter for the background and then have the flash provide the fill light for the main foreground areas of the shot. However note that Ettl, like any auto mode, can be tricked in certain lighting conditions and thus might require reshooting with adjustments to correct the auto flash power.

You can also shift the flash into manual mode; whereupon it puts out a fixed amount of power; this is useful as you can then control the flash direct and dictate to it the power it is to use. This has the bonus that the amount of light output is always constant; good for studio and other controlled conditions where, once you've checked and set the proper exposure, you don't want the values changing at random because the meter reading was on a boundary line and the slight variations in the scene tip it one way or the other.