question about fill in flash

[kit]

Just wondering why this happens. I'll list it in point form to its easier to understand:

- situation is to take an outdoor portrait shot in shade against a brightly lit sky/city skyline background (many kilometres away).

- first, I meter the background in Manual mode, say for instance the sky/background is correctly exposed at 1/200, f10. A test shot is taken to prove this.

- Retaining the exposure settings, i connect the 580EX, and set it to manual mode at a light output level to correctly light up the subject (who is standing say 2 - 3 meters away from the camera).

- why is it that if I shoot with these settings, the background will become overexposed?

I know I should be undeexposing the background by around 2 stops. But i don't understand why the flash can make the correctly exposed background to become overexposed when its so far away?

can anyone explain this?

thanks,
Chris

[silvex]

try partial metering.

[kit]

try partial metering.

I don't quite understand why metering should make a difference?
- I've adjusted the exposure without the subject
- a test shot has shown the background to be correctly exposed
- these settings are retained, and the subject added, and the speedlite added to light up the subject.
- So exposure settings haven't been re-evaluated and changed. Also, the flash is manually set, so ETTL doesn't come into play at all here.

is it possible that the subject will reflect and diffuse the flash back to the lens, lighting up the whole image and hence overexposing the correctly exposed background?

[tim]

The only way a flash can change the exposure is if you're above the sync speed. 1/200th should be fine, but if you're at 1/500th and you mount a flash the shutter will drop to your sync speed. Can you post the two images, with EXIF?

btw ETTL is usually fine in this situation, but manual flash is fine if you enjoy that sort of thing :p

[kit]

yeah, if I'm shooting faster, I turn on high speed sync.

At work now, so don't have access to some of my other ones, but I managed to find one example with the following exif:

75mm
1/80,
f/10,
ISO-200,
Metering mode: Pattern
Exposure program: Manual
Exposure compensation: 0 Step

This is on a crop sensor (350D).

[Curtis N]

1/80,
f/10,
ISO-200,Those settings will overexpose a sunlit background by about 2 1/2 stops.

[Don Powell]

Curtis is right as always.
I don't understand how you went from an exposure reading of 1/200 sec at f10, to an exposure of 1/80 sec.

I don't have any Canon flash units, but if you set the camera to pure manual mode, I don't think you could have exposed at 1/80 sec, thus overexposing as Curtis pointed out.

I always think, while using flash in combo with available light to photograph a subject:
The shutter speed controls the ambient ( background )
The f stop controls the subject ( the flash must deliver the amount of light in f stops required for proper exposure )

Please note, that if the flash is close enough to background, it will add and the lens must be stopped down to account for this. Either chimp or use flash meter.

[kit]

Well, it looks like I have made a mistake here.
Not too sure either how I ended up with that end exposure of 1/80.
While I did take a test shot and it looked correctly exposed, perhaps I have accidentally changed the settings.
Thanks for the help anyway guys.