Onboard Rebel Light Meter

[SJCT]

I have a question about the on-board light meter on my Canon Rebel XS. When in manual mode I understand how to read the meter and adjust ISO, shutter speed and/or aperture to accomplish a "zero" on the exposure meter. What has me confused is that the exposure settings don't seem to change whether I have the flash turned on or not (using a Sigma EF-610 DG ST Flash in the hot shoe).

Obviously, the lighting in the room is drastically different when the I use the flash. Yet, when I'm "metering" the room, the meter is reading the light in the room without the flash, correct? So how do I adjust for that when in manual mode and I am trying to get my light meter balanced (reading zero)?

Thanks ahead of time for any help on this!

[i speak in math]

The light meter in your camera is an reflective light meter, meaning it reads reflected light. Since the flash isn't going off at the time of the metering, it cannot be metered using the in camera light meter. The camera setting won't change just because you have a flash on (unless you are in full auto mode, i believe).

This is why your camera flash has ttl. You set the scene how you want with the setting available (Av, Sv, P, M...whatever you want). Typically, if using flash, I underexpose the ambient by 1 stop. The flash emits a pre-flash with a known amount of light, calculates the correct amount of light to get a "good" exposure, and actually flashes that amount when your camera makes an exposure.

The flash adjusts to you camera settings, not the other way around.

[Tito87]

[QUOTE=i speak in math;1269589]

The light meter in your camera is an incidence light meter, meaning it reads reflected light.

I always thought that incident light meters read the light falling onto the subject and that reflective light meters read the light reflected by the subject.

[JFSanders]

[QUOTE=Tito87;1269642]

Originally Posted by i speak in math

I always thought that incident light meters read the light falling onto the subject and that reflective light meters read the light reflected by the subject.

Yes, you have it correct. Just a typo. Math is right about how the camera uses the preflash to read the ambient with flash fill. Now on straight manual you would have to read the light and adjust the flash power yourself just like in the old days.

Jim

[i speak in math]

[QUOTE=Tito87;1269642]

Originally Posted by i speak in math

I always thought that incident light meters read the light falling onto the subject and that reflective light meters read the light reflected by the subject.

Sorry, yes, incidence is how much light is falling on the subject...edited above

[SJCT]

Thanks for the quick replies! After googling Incident and reflective light meters, your answers made sense! One more question though; does anyone know when the pre-flash happens? Just wondering out of curiosity. When I hold the trigger button down, I don't see a flash, so I assume the pre-flash happens a split second before the actual shutter is opened?

[JFSanders]

Originally Posted by SJCT

Thanks for the quick replies! After googling Incident and reflective light meters, your answers made sense! One more question though; does anyone know when the pre-flash happens? Just wondering out of curiosity. When I hold the trigger button down, I don't see a flash, so I assume the pre-flash happens a split second before the actual shutter is opened?

Yes, it happens just before the camera's shutter fires.

[SJCT]

Originally Posted by JFSanders

Yes, it happens just before the camera's shutter fires.

Thanks again JF.

[BeeKayPea7]

Sorry, I'm just getting into flash photography. I was using, errr 'using' the light meter in my T2i to get a better exposure - I had it all wrong. I was wondering why I had to set my 50mm f/1.8 to 1.8 and 1/60, but still get a slightly overexposed reading. I had that all wrong.

So, to get this correct. I have the T2i, 50mm f/1.8 lens, and Sunpak PZ42X hot shoe flash - I can just do a setting of roughly 1/60, f4.0-f.56 and having set the flash to TTL and Auto Zoom (Indoor Shots), it will adjust accordingly and provide me with correctly exposed pictures, or close to correctly exposed?

Further, I can probably use a bounce card and 45 degree angle on the external flash or use a stofen omnibounce to soften/diffuse the light for better pictures?

"Typically, if using flash, I underexpose the ambient by 1 stop. " - What does this mean? And how would you go about setting this?

Thanks.

Originally Posted by i speak in math

The light meter in your camera is an reflective light meter, meaning it reads reflected light. Since the flash isn't going off at the time of the metering, it cannot be metered using the in camera light meter. The camera setting won't change just because you have a flash on (unless you are in full auto mode, i believe).

This is why your camera flash has ttl. You set the scene how you want with the setting available (Av, Sv, P, M...whatever you want). Typically, if using flash, I underexpose the ambient by 1 stop. The flash emits a pre-flash with a known amount of light, calculates the correct amount of light to get a "good" exposure, and actually flashes that amount when your camera makes an exposure.

The flash adjusts to you camera settings, not the other way around.

[JFSanders]

BKP7, Remember it like this. Shutter speed controls the ambient exposure/light. Flash controls the subject exposure/light.

So if you want the subject lit properly but not overexpose the ambient then you would increase your shutter speed by 1 stop. 1 stop is either a doubling or halving of the setting you would get using the in camera auto metering function. So if you drop 1 stop using the shutter speed it would mean to increase it from say 1/60 to 1/120 or the closest value your camera gives to the doubled number. In terms of aperture it would mean going from f/5.6 to f/8 (being a hole the numbers don't double because of the geometry of areas rule.

Jim