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variable focusing on Zoom!!!!

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Old Friday 25th June 2010, 21:30   #1
ammadoux
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variable focusing on Zoom!!!!

Hi all

had today a play with my old OM4 film camera and the only zoom i had for it, the Zuiko 70-210mm, and notieced that when i zoom in or out the focus do not change, unlike both digital zooms i use now the 70-300mm Zuiko and the 50-500mm Sigma.

please don't laugh at me if it sound silly, but can anyone explain why, is it something must be for digital or AF lenses.

thanks so much.
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Old Saturday 26th June 2010, 00:19   #2
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Hi ammadoux,

Your old lens is what is referred to as parfocal meaning that the focus does not change as the focal length does. Your new lenses are varifocal, they do not maintain focus as the focal length changes.

It is easier to make a good varifocal lens than a good parfocal lens. With the advent of auto-focus the fact that the focus changes with the focal length becomes less important, the camera refocuses for you. With a manual focus lens it much more convenient to not have to refocus every time you zoom.

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Old Saturday 26th June 2010, 01:06   #3
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LOL i do most of my focusing manually with the auto focus button off, specially when the camera is on the tripod.

thanks Mike for the info, never occurred to me that when i bought any of the two AF digital lenses i thought it is a giving fact.

well another point in favor for manual everything cameras, only if they make one new digital OM instead of film, having everything in the legacy of the OM cameras.
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Last edited by ammadoux : Saturday 26th June 2010 at 06:21.
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