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NOOBIE question: why have fixed lenses when you can have zoom lenses


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#1 sam williams

sam williams
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Posted 05 October 2008 - 07:40 PM

hi, this may seem a little retarded, but i never understood why seperate types of lenses with fixed focal lengths are available, when surely having a zoom lens could cover any desired focal distance, and youd only have to buy one type of lens.

by the way i realise this is obviously a silly thing to think, but i just dont know enough about cinematography to know why.

and help would be nice.

sam
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#2 David Mullen ASC

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Posted 05 October 2008 - 08:31 PM

It's not a silly question and some people do buy a zoom when starting out to cover most shooting situations.

In general, though not always, zooms tend to:

-- be slower (i.e. widest aperture is f/2.8 or worse, especially zooms for 35mm cine photography)
-- flare more (due to the extra glass elements)
-- breathe more when rack-focusing
-- be larger & longer & heavier (especially for 35mm cine photography)
-- focus less close (though most are pretty good in that regards)
-- be less sharp & contrasty

And a few will drop off in exposure at the far end of the zoom, or porthole (darkened edges).

In HD and Super-16, the zooms are not so large and are sometimes fairly fast, so they can be more commonly used than in 35mm cine photography.

There are some DP's who swear by zooms though, for various reasons (time saved not needing to change lenses, ability to make small focal length changes during a shot, better matching since only one lens is used, etc.)
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