zooming during exposure

i want to take a photo that looks something bursting out by twisting the zoom ring during exposure but the photo turned to be underexposed,though i had set the exposure at EV0,why?

Also how can i keep the central part in focus while zooming?

thanks

For one, you will have to design and use a zoom lens that does not

shift focus as you zoom. For another, you need to make sure that you

expose properly: I would think that the aperture might not follow a

zoom extension, so starting off at f=5.6 will slowly turn to an f=8 or

so, as you increase focal lens. You will have to compensate and learn

and try a few more times.

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When you succeed, the pictures will look eary, though. And you will

have to live with a bit of unfocus ... unless you can succeed in

making your own fix focal zoom ...

I have a great shot of a whitewater raft doing this type of thing. I

used a reasonably long shutter speed of 1/8 or 1/15 second using

shutter preferred mode so the aperture was picked by the camera. The

camera did a fine job with the exposure automatically.

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The center was sharp and everything else was stretched in a radial

pattern. Makes it look very dynamic with lots of percieved action.

Dear Jim,

i want to know

1)how did u make sure that the central part is sharp during zooming?

2)should i start zooming at the long focal end or the short end?

Thanks a lot for helping me

I started at the wide end and zoomed in during exposure, but I don't

think i would matter. I was hand holding the camera but it probably

would have been better to have it mounted on a tripod. The center

stays sharp as long as you keep your lens centered on the one place

of the image that you want to keep sharp. If the subject is moving,

pan the camera with it to keep it centered. The outer areas of the

resulting picture gets progressively blurred as you move to the

edges.

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Pick a shutter speed that will show the blurr (1/4 to 1/30 second)

and let the camera's meter determine the exposure.

Carto, try focusing while zoomed in tight. Then come out wide

without changing focus. Many zoom lenses achieve best focus when

focusing is done at the longest focal length.