flash question...

EandSphotography · Dec 21, 2009

Just a bit confused and was hoping for some help.
Camera :canon 50d,Flash 550ex

I'm not getting why the exposure meter is not showing when the picture would be properly exposed when the flash is attached.
Here's an example... Manual mode in camera. 125,9.0 iso-100 ,flash in ETTL mode. indoor picture. When i take the picture it comes out exposed properly but the exposure meter in the camera is showing underexposed before the pic is taken. in order to get the exposure meter to show that the pic will be properly exposed i had to change it to 5'' at 3.5 iso100
Of course it comes out properly exposed but the 5'' is just too long to take anything that would be moving, of course.
So, i guess my question is, how do i know if i'm taking a picture of a person with flash if the exposure is correct if the meter doesnt move on higher speeds. Are you just supposed to know what speed and aperture will produce a properly exposed picture?

now, putting it in P mode, the same indoor shot , it puts it at 60,5.6,iso 100 and the meter shows properly exposed before the shot is taken.
but if i put those same numbers into M mode the exposure meter shows way underexposed but the picture comes out fine.

So, when using the flash, should i just put it in P mode and let it do it's thing?
Help would be great. hope i didnt make it sound more confusing.

Erik

Inst!nct · Dec 21, 2009

I think, if i read it correctly, the problem is that the camera doesnt know youre going going to use the flash, and therefore shows up as underexposed. Try pushing the star button like this (*) before shooting it..

Overread · Dec 21, 2009

When you meter the scene, whilst the camera knows that the flash is attached, it has no idea how the light from the flash will affect the scene before it. Thus it only meters the amibent lighting that is present.
Then when you press the shutter the flash will fire a very rapid preflash shot (most times you will never notice this its so fast) which will light the scene, the camera will read that then the flash will set its output so that the scene is correctly exposed. The shutter will then activate as will the main light from the flash (the light that you see).

Of course that means if you set your camera settings for a correct exposure based on the ambient lighting your flash is only going to give out a weaker "fill flash" effect which will only fill in the shadows. If your want the flash to be more dominant (say so that you can use a faster shutter speed to avoid blur) simply set those settings and let the flash adjust its output to boost the lighting.
Of course this means there is some guesswork and experience needed to know the limits - in a studio or control setting you can uses a lightmeter to measure the flash outputs (at that point you would be using manual flash powers for fixed output) and adjust accordingly

bhphotography · Dec 21, 2009

Check to see what your sync speed is set to on your camera, it may want to sync your flash at 1/60th but should be able to sync much higher than that.

Overread · Dec 21, 2009

Inst!nct said:
I think, if i read it correctly, the problem is that the camera doesnt know youre going going to use the flash, and therefore shows up as underexposed. Try pushing the star button like this (*) before shooting it..

Doing this will tell the flash to fire its preflash shot (you will see it) but will not take a photo. Instead the flash will measure the light given off a single fixed point (the middle AF point) and adjust its output so that the flashlight will expose that point correctly. Thus if you point your camera at a shadowed spot in a scene the flash will out out far more power than if you point it at a bright spot.
The point does not have to be where your focus is at all, its just a point in the scene that you want the flash to meter for.

After you set the flash thus way pressing the shutter will fire the flash at that power level it just set (ie it won't fire the preflash again) and after that shot (or series of shots if you hold down fur a burst) the flash will reset itself back to its default shooting method.

EandSphotography · Dec 21, 2009

Great. Thanks for the help guys. So just to re-cap, if i'm using flash and the camera is in manual mode and say i'm taking a pic of my son on christmas morning. If i had the camera set to say 125, 5.6 the meter will show underexposed but the flash will adjust automatically to properly expose the picture. IF that is correct, then i think i'm back on track.

Thanks,
Erik

Big Mike · Dec 21, 2009

Overread laid it out nicely.

The camera's primary light meter (that you see) is an reflected ambient light meter. It (usually) shows you what the ambient light will give you. In E-TTL mode, the flash & camera use a pre-flash to meter for the flash power required for the actual exposure.

In P mode, the camera uses default values when the light is lower. For example, it won't go lower than 1/60 and just uses flash to make up for the under exposure that would result from this.

The most complete source of info on how all this works...is what's known as the 'EOS Flash Bible'. Flash Photography with Canon EOS Cameras - Part I.

Overread · Dec 21, 2009

That is correct - but remember that the flash won't have power for all situations. There will be times when it will be too weak to expose the scene correctly. Also get into the habbit of checking your histogram when you can after taking a shot - you can use it to gague the exposure of a shot - you might find that the flash meter gets tricked by something in the scene (just like the camera meter the flash meter is not 100%) and you might find that you need more or less flashlight - which you can control by the flashexposure compensation control

Big Mike · Dec 21, 2009

Great. Thanks for the help guys. So just to re-cap, if i'm using flash and the camera is in manual mode and say i'm taking a pic of my son on christmas morning. If i had the camera set to say 125, 5.6 the meter will show underexposed but the flash will adjust automatically to properly expose the picture. IF that is correct, then i think i'm back on track.

Yes, that's pretty much it. Even though the camera is in manual, if you have the flash in E-TTL mode, you still have automatic exposure because the flash will match it's output to the aperture and ISO settings. (shutter speed has no effect on flash exposure).

That is how I most often shoot with flash....camera in manual and flash in E-TTL.
I set the aperture for the DOF that I want and set the shutter speed for the amount of ambient light I want. Keep in mind that a larger aperture and higher ISO mean that the flash doesn't have to work as hard, thus recycling faster and using less battery power.

*edit*
I forgot to mention that you might need to adjust the FEC (flash exposure compensation) to adjust the exposure from the flash.