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08-16-2012, 10:13 AM   #1
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Cheating a zoom's F-Stop

Hi,
so i was thinking i big into using old lenses and though if i bought a zoom like say... Vivitar Series 1 70-210mm f/2.8-4.0 Q-DOS Lens Reviews - Vivitar Lenses - Pentax Lens Review Database
Say if i through every thing tho manual and put the Aperture ring on 2.8 and zoomed to 200 would it not be 2.8 at 200mm? or would it be forced to F4??

Thanks for any reply's

08-16-2012, 10:18 AM   #2
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An F4 lens is an F4 lens. Nothing can magically make it F2.8.

QuoteQuote:
Zoom lenses typically have a maximum relative aperture (minimum f-number) of f/2.8 to f/6.3 through their range. High-end lenses will have a constant aperture, such as f/2.8 or f/4, which means that the relative aperture will stay the same throughout the zoom range. A more typical consumer zoom will have a variable maximum relative aperture, since it is harder and more expensive to keep the maximum relative aperture proportional to focal length at long focal lengths; f/3.5 to f/5.6 is an example of a common variable aperture range in a consumer zoom lens.
08-16-2012, 10:19 AM   #3
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It would be forced to F4.
08-16-2012, 10:28 AM   #4
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In VERY simple terms, an F-stop is simply a ratio of the size of the aperture "window" relative to the focal length of the lens. If the aperture remains fixed (say, at the maximum diameter?) and the focal length increases the ratio changes proportionally.

08-16-2012, 10:29 AM   #5
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F-number or better yet the F-ratio is an ratio between the focal length en the pupil opening of the lens so physical properties.
08-16-2012, 01:17 PM   #6
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I have the Komine version of that lens with the A contacts, the camera will still say f2.8 at 210mm, but it is really f4. If you are in Av mode, you can watch the shutter speed change while you zoom.
08-16-2012, 01:52 PM   #7
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Violating the laws of physics is a very serious offense. Also, too, very difficult.
08-16-2012, 01:53 PM   #8
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QuoteOriginally posted by dadipentak Quote
Violating the laws of physics is a very serious offense.
Even if there's Klingons on the starboard bow.

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