Curtain sync times only control when the flash fires... either
immediately after the first curtain opens (1st-curtain sync) or
immediately before the second curtain closes (2nd-curtain sync). Unless
there is a long shutter open time the difference between them will be
small. But, in any case, capture of the ambient light is not related to
sync time but to shutter speed. What does happen is that any subject
motion seen by ambient light either follows or precedes the frozen
position caught by the flash.
For example, you're taking a shot of a bike and rider in a race. You're
using slow shutter speed with flash and not panning and the bike is
moving from right to left. With 1st-curtain sync the flash fires and
freezes the position of bike and rider at the right side of the frame.
Then the bike and rider continue across the image plane producing a
blurred image to the left and ahead of the flash image. This looks
unnatural. But it you do the same thing with 2nd-curtain sync the
blurred image starts first at right after the first curtain opens,
continues left and then the flash goes off just before the 2nd-curtain
closes and the flash frozen image occurs at the left side. This looks
more natural and gives the impression of the bike swishing through the
frame at speed.
Chuck Norcutt
Chris Barker wrote:
> That sounds about right, Chuck. But I should have thought that you
> would merely set 1st curtain sync to capture the ambient light.
>
> Chris
>
> On 9 Dec 2007, at 19:20, Chuck Norcutt wrote:
>
>> I don't have either of these things but the described behaviour
>> (that's
>> for U, Chris) is similar to what would be expected with a Canyon and
>> its
>> dedicated flash when in program mode and, I assume, in aperture
>> priority
>> as well. The camera will attempt using a slow shutter speed (dragging
>> the shutter) along with the flash in order to capture the background
>> ambient light. And, of course, if the ambient is too bright it could
>> cause overexposure even without the flash.
>>
>
>
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