Metering with a flash

[MikeandReneePlus5]

I have a 430EX (external flash) for my Canon and I guess I expected that when my camera metered a scene without flash and came back with let's say 1/60 and 5.6 that when I turned on the flash it would boost the shutter speed to something faster than 1/60 to account for the increase in light (assuming the 5.6 is locked in AV mode).

But it actually seems to slow the shutter down a bit, or leave it as is.

Is this is because it is exposing the background with ambient light and using the flash light to expose the subject?

It does seem that the pictures are sharper with the flash and a slow shutter speed than without one (at 1/30 or even 1/15).

 

[photo_chick]

Shutter speeds with flash are generally limited to around 1/200 or slower with a flash with an SLR. Otherwise you have problems with the flash and the shutter curtain syncing in an SLR. Also remember that in situations where the flash will overpower available light it is the flash that determines the exposure time, not the actual shutter speed. If you want to play with that aspect you need to change where the flash happens during the exposure, like with second curtain sync.

 

[NLD]

Shutter speed impacts ambient light exposure. If 1/60 is the correct shutter speed for the ambient light, then it's still the correct shutter speed if you add fill flash.

Was your second photo (the one with flash) overexposed? Or was it what you wanted?

Usually if I'm using flash, I keep the camera in manual and the flash in automatic (some form of TTL). That way there's less concern about what the camera might do to my settings to throw the exposure off....

 

[wbeem]

Metering for flash means using an external meter, such as a Sekonic. Your camera meter only measures available light.

 

[MikeandReneePlus5]

Also remember that in situations where the flash will overpower available light it is the flash that determines the exposure time, not the actual shutter speed.

Shutter speed impacts ambient light exposure. If 1/60 is the correct shutter speed for the ambient light, then it's still the correct shutter speed if you add fill flash.

Are these contradicting statements? I don't think I totally get the first one.

Metering for flash means using an external meter, such as a Sekonic. Your camera meter only measures available light.

thanks, yes, I meant "using the camera meter while there is a flash attached". But if I read you correctly, the camera meters available light and the ETTL flash doesn't meter flash power at all? it only adjusts power for focal length? so i can use flash comp to play with that but to do it exactly I would get an external meter?

 

[wbeem]

thanks, yes, I meant "using the camera meter while there is a flash attached". But if I read you correctly, the camera meters available light and the ETTL flash doesn't meter flash power at all? it only adjusts power for focal length? so i can use flash comp to play with that but to do it exactly I would get an external meter?

You can be very exact with a handheld flash meter. Some photographers swear by them. The focal length isn't the issue. It's your aperture that matters for flash. That's because the flash duration is shorter than your shutter speed.

So your shutter speed will allow you to control the ambient light, your aperture will control the amount of flash in the exposure - all associated with your ISO, of course.

 

[photo_chick]

Are these contradicting statements? I don't think I totally get the first one.

Not really, they're more like two parts of the same idea.

My statement... Ok, you know how when you go into a room with a strobe light everything freezes when the light flashes? It's the same idea when you use a flash that will overpower the available light. That quick flash illuminates the subject for a quick, short burst and that's what actually makes the exposure on the subject. An extreme example in use, take the camera into the bathroom with all the lights off. Choose a really long shutter speed that you couldn't possibly hand hold and have a steady shot. Take a picture with the flash and everything will be sharp, as if you used a shutter speed that was the duration of your flash even if you move the camera around once the flash has fired.

As far as the shutter speed being determined by the ambient light... well, yes and no. It depends on the metering mode you're using and other factors. And with your flash you don't need a hand held meter. Save that for off camera strobe work, that's where you have to have it.

 

[wbeem]

As far as the shutter speed being determined by the ambient light... well, yes and no. It depends on the metering mode you're using and other factors. And with your flash you don't need a hand held meter. Save that for off camera strobe work, that's where you have to have it.

That's not correct. The metering mode just changes the area of the photo that measures light and determines the exposure for that measurement. Shutter speed still correctly determines the exposure for ambient light - for the area metered.

 

[photo_chick]

That's not correct. The metering mode just changes the area of the photo that measures light and determines the exposure for that measurement. Shutter speed still correctly determines the exposure for ambient light - for the area metered.

Not entirely correct when you're dealing with on camera ETTL flashes.

 

[wbeem]

Not entirely correct when you're dealing with on camera ETTL flashes.

Of course it's correct. The metering mode measure ambient light and determines its base exposure. The amount of flash used by iTTL systems is determined by a separate pre-flash that measure the light and changes how much light gets output by the flash. The system may still use the internal camera body meter, but it's making a different reading.

Two separate systems for measuring light. If you're in doubt, change your camera to manual mode so the exposure values won't change.

 

[MikeandReneePlus5]

many thanks to all.

I think I got it now.