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Zoom Question



 
 
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  #1  
Old June 23rd 04, 09:43 PM
Bay Area Dave
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Default Zoom Question

Gareth Slee wrote:
I'm a complete novice to this so please excuse me.
I have a CCTV camera and I'm trying to decide on what zoom lens to use with
it.

This is where things get confusing for this old brain of mine.

I've seen two different zoom lenses at a local store.
6-72mm (x12 zoom)
8-80mm (x10 zoom)

With everything else being equal which lens would make an object appear to
be nearer?

--
Gareth Slee

Stupid gravity!
Homer Simpson: (Falling out of treehouse)


the 8 to 80.

dave

  #2  
Old June 23rd 04, 10:00 PM
Gareth Slee
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Default Zoom Question

Bay Area Dave wrote:

the 8 to 80.



So the higher the second number, the nearer the image will appear to be?

--
Gareth Slee

Stupid gravity!
Homer Simpson: (Falling out of treehouse)


  #3  
Old June 23rd 04, 10:15 PM
Gareth Slee
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Default Zoom Question

Jim Townsend wrote:
All things being equal.. The longer the focal width, the narrower the
field of view and the closer objects appear.

Of the two lenses you listed, the 8-80mm lens provides the longest
focal length and will provide the greatest magnification.

Note that the X in zoom lens doesn't represent the magnifying
power the way telescopes and binoculars do.

The 'X' represents the difference between maximum and minimum
focal length. The X of a zoom lens is determined by dividing the
maximum focal length by the minimum.

72 / 6 = 12x (from 6mm you can increase the focal length 12x)
80 / 8 = 10x (from 8mm you can increase the focal length 10x)

If they made a 2mm to 50mm lens, it would be a 25X lens, but
wouldn't have near the magnifying power of the 8-80mm 10X lens..



Thanks Jim
That's cleared it up.

--
Gareth Slee

Stupid gravity!
Homer Simpson: (Falling out of treehouse)


  #4  
Old June 23rd 04, 11:15 PM
David Dyer-Bennet
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Default Zoom Question

"Gareth Slee" writes:

Bay Area Dave wrote:

the 8 to 80.



So the higher the second number, the nearer the image will appear to be?


Yes, that's exactly the point. The maximum focal length the lens
reaches will give you the biggest image. So the lens with the largest
maximum focal length will give you the biggest image at its longest
setting.

The "x" number given sometimes for zoom lenses, x12 or x10 in your
example, is simply the ratio of the longest to the shortest focal
length. That tells you how much variation there is from one end to
the other -- but nothing about how wide the wide end is, or how long
the long end is. (I'm used to seeing it as "12x" rather than "x12",
by the way).
--
David Dyer-Bennet, , http://www.dd-b.net/dd-b/
RKBA: http://noguns-nomoney.com/ http://www.dd-b.net/carry/
Pics: http://dd-b.lighthunters.net/ http://www.dd-b.net/dd-b/SnapshotAlbum/
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