Lens conundrum
Posted 10/03/2015 - 19:05
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The nominal focal length might be true at infinity, but may alter on focusing closer. This is a property of many zoom lenses. So, maybe both lenses change their focal lengths in different ways.
Best regards, John
Posted 10/03/2015 - 19:09
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Thats interesting .... I do recall noticing occasionally, and wondering what was going on, that when focusing closely with the F35-70, that as I turned the focus ring I saw the edges of the frame move in and out slightly, and I thought I was somehow turning the zoom ring by accident.
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Pentax K7 with BG-4 Grip / Samyang 14mm f2.8 ED AS IF UMC / DA18-55mm f3.5-5.6 AL WR / SMC A28mm f2.8 / D FA 28-105mm / SMC F35-70 f3.5-4.5 / SMC A50mm f1.7 / Tamron AF70-300mm f4-5.6 Di LD macro / SMC M75-150mm f4.0 / Tamron Adaptall (CT-135) 135mm f2.8 / Asahi Takumar-A 2X tele-converter / Pentax AF-540FGZ (I & II) Flashes / Cactus RF60/X Flashes & V6/V6II Transceiver
Pentax K7 with BG-4 Grip / Samyang 14mm f2.8 ED AS IF UMC / DA18-55mm f3.5-5.6 AL WR / SMC A28mm f2.8 / D FA 28-105mm / SMC F35-70 f3.5-4.5 / SMC A50mm f1.7 / Tamron AF70-300mm f4-5.6 Di LD macro / SMC M75-150mm f4.0 / Tamron Adaptall (CT-135) 135mm f2.8 / Asahi Takumar-A 2X tele-converter / Pentax AF-540FGZ (I & II) Flashes / Cactus RF60/X Flashes & V6/V6II Transceiver
Posted 10/03/2015 - 19:30
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spinno wrote:
aren't zoom lenses varifocal lenses?
They can be, but can be parfocal too.aren't zoom lenses varifocal lenses?
Varifocal lenses need refocusing when zooming, parfocal lenses keep the same focus at different focal lengths.
Mike
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Pentax:K5ii, K7, K100D, DA18-55, DA10-17, DA55-300, DA50-200, F100-300, F50, DA35 AL, 4* M50, 2* M135, Helicoid extension, Tak 300 f4 (& 6 film bodies)
3rd Party: Bigmos (Sigma 150-500mm OS HSM),2* 28mm, 100mm macro, 28-200 zoom, 35-80 zoom, 80-200 zoom, 80-210 zoom, 300mm M42, 600 mirror, 1000-4000 scope, 50mm M42, enlarger lenses, Sony & micro 4/3 cameras with various PK mounts, Zenit E...
Far to many tele-converters, adapters, project parts & extension tubes etc.
.[size=11:].Flickr• WPF• Panoramio
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Pentax:K5ii, K7, K100D, DA18-55, DA10-17, DA55-300, DA50-200, F100-300, F50, DA35 AL, 4* M50, 2* M135, Helicoid extension, Tak 300 f4 (& 6 film bodies)
3rd Party: Bigmos (Sigma 150-500mm OS HSM),2* 28mm, 100mm macro, 28-200 zoom, 35-80 zoom, 80-200 zoom, 80-210 zoom, 300mm M42, 600 mirror, 1000-4000 scope, 50mm M42, enlarger lenses, Sony & micro 4/3 cameras with various PK mounts, Zenit E...
Far to many tele-converters, adapters, project parts & extension tubes etc.
.[size=11:].Flickr• WPF• Panoramio
Posted 10/03/2015 - 19:34
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Even a prime macro lens may change focal length when focusing close, especially internal focus designs.
Best regards, John
Posted 11/03/2015 - 13:01
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Thanks for your comments everyone. You are spot on John. I tried the comparison again with both lenses focused at infinity and there was little difference which could be due to slight movement when the lenses were changed. It was nowhere the difference when close focused.
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