Peter,
>1. Flash's output power. As I understand all the documents I read before
>E-TTL flash control the lighting of the object by it's output power and not
>by Exposure Time or Aperture.
There must be better explanations somewhere on the WWW than mine, but
_I think_ the flash of light from the flash gun is extremely powerful
but also extremely short in duration - it is the shortness of the
flash which enables you to 'freeze' motion with the flash. As well
as modifying the power of the outburst, the flash gun can also modify
the duration of the flash - my previous flashgun would only modify
the duration of the flash by 'quenching' it when enough light had
illuminated the subject, and it would do this in unbelievable short
times. Either way, the amount of light reaching the subject can be
modified to suit its reflectance, the lens aperture and the ISO speed
of the film.
>That means the power of the light that comes out of flash differs
>from photo to photo. I can't explain in any other way why while the
>flash is on the Exposure time is always 1/60 sec and the Aperture
>Value is 5.6. Is that correct?
The speed, 1/60, is determined by the camera focal plane shutter
mechanism. It is the fastest speed where the rear curtain does not
start travelling to close the opening before the first curtain has
fully retracted. At this speed, or any slower speed, the whole of
the film frame is available to receive the light reflected from the
flash burst. At any faster speed the rear curtain begins to close
off the opening before the first curtain has fully opened, so any
picture taken with a conventional flash would have only a rectangular
portion of the frame correctly exposed. I think for the EOS30 the
highest flash sync speed is 1/128.
>2. How long the flash light is on? I found a nice photo which explains how
>the flash works at FP
> http://www.camera.canon.com.my/photography/art/13lighting/big_images/i
>mg/figfpfl ashchart.jpg ). It shows that the flash is on as long the shutter
>is open.
Yes, but during this interval the 550EX is like a stroboscope,
emitting brief pulses of light many times a second - too fast for the
human eye to see, so it appears to us like a long flash.
>Is that true for normal flash use too? (When no FP is set) I quess
>not, because if that would be true, the first or the second curtain wouldn't
>work. Would someone explain (something like on the photo that I mentioned
>earlier) how it really works please?
At higher shutter speeds than the sync speed, the focal plane
curtains form a slit that travels across the film frame. The pulses
of light from the 550EX in FP mode have to be timed so that each part
of the film frame receives an adequate amount of light while the slit
is passing over it.
>3. Having longer exposure times with flash. As I mentioned earlier while
>using Program mode the Exposure time is always 1/60 sec and the Aperture
>Value is 5.6. That means in most cases the object is always correctly
>exposed, but the background is very dark or even black usually. The EOS
>user's manual says you need to set longer exposure times to see some more
>background. Having IS lense 1/15 sec should not be a problem.
The longer exposure times make little difference to the amount of
light the subject is sending the camera - that's really fixed by the
flash gun - but they do make a difference to the amount of light
received by the film from the background. For any given situation
there is a shutter speed where the amount of light coming from the
background will be about the same as the amount of light reflected
off the subject, so the two will be said to be balanced.
>But as soon I select TV and set Shutter speed to 1/15 sec the Aperture Value
>starts blinking, because it can't open the aperture for more than 3.5. What
>to do in this case?
Rely on the 550EX ETTL to adjust the output of the flash to match the
camera aperture you have. 3.5 is not wide enough to expose the
background properly, so the camera's readout blinks at you, but it
will make it brighter than 5.6. If you want to correct this
situation entirely, put the camera on a tripod and select a shutter
speed where 3.5 is chosen by the camera and it does not blink. The
550EX will still calculate a power and duration of flash to suit your
subject at f/3.5 and we know the camera is happy with the exposure
for the background, so both subject and background will be
balanced....... but, it is likely to be slow shutter speed, so you
have to tell your subject to keep _very_ still!
I have found Tv a useful method of shooting with the 550EX. I
accidentally left the 550EX in FP mode once and it _seemed_ to shoot
normally at speeds slower than 1/128 and in high speed sync modes
when I set the speed higher! I say seemed, because I haven't done a
comparative test for this setting yet.
I hope this helps.
--
Nick Young
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[EMAIL PROTECTED] "Use the fork, Luke"
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