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02-18-2017, 06:49 PM   #1
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Sigma 28-70mm Zoom F3.5-4.5

Recently I purchased the above lens, from forums it appears this lens is the closest to mine
Sigma UC 28-70mm F3.5-4.5 Lens Reviews - Sigma Lenses - Pentax Lens Review Database
Differences are ft scale is in green print, forums is in black. On mine focus grip is in aligned locations not on an angle as the forum one is. Mine has a 10 metre distance forums one before infinity is 8 metres.
My questions are, is my lens the same as the forum and does A position mean that this lens will auto focus, mine does not when attached to Pentax K20D. Does this mean the lens is faulty or is this lens too old to work on more recent Pentax Camera? Attached is a photo of the lens plate.

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02-18-2017, 07:40 PM - 1 Like   #2
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An A lens will allow the camera to read the f stop but it doesn't have the other contacts to be AF. In other words, your lens is a manual focus lens but you can set the f stop with the camera.
02-19-2017, 12:44 AM - 1 Like   #3
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The lens is not AF as evidenced by the lack of a screwdrive connection. Look at the kit lens mount and you will see it around the 6 o'clock position relative to your image of the Sigma lens mount.
Set your K20D to manual (from AFS) on the switch on the opposite side of the camera from your hand grip.
Set the lens aperture to "A" position (May require pressing the little button down that is next to the "A" mark on the lens) and your camera can use this as a manual lens. (Though you might want to look through your manual's instructions on using manual focus lenses).
Enjoy, manual lenses can be great if you don't mind focussing it yourself.
02-19-2017, 05:17 AM - 1 Like   #4
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If it was sold to you as AutoFocus, demand a refund.

02-19-2017, 10:35 PM   #5
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Thanks to all who replied. I carried out some trial shots today and I found that the lens was part auto A lens which I have never encounted before. I have 3 other A lens that have the same lens setup this is on the lens barrel as the Sigma 28-75mm but this lens only has 2 brass contacts, the other 3 lenses have 5 or 6. The Sigma 28-75mm works in P, TV, M and probably Sv and TAV but not green rectangle, with manual focus. One does not have to press down the green button to get exposure, it appears like a lens that is between the full manual and full auto lenses. I was not able to find any mention of this on the web. Next time I will have to check the brass contacts. I am not unhappy with the lens as pressing down the green button all the time can be just another step in which you can make a mistake, i.e. you forget to do so when the sun comes out from behind a cloud.

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