Blurry autofocus

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love-to-ride
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Joined: Thu Jan 12, 2012 6:33 pm

Blurry autofocus

Unread post by love-to-ride »

Hi,
I have an SLT A65. I have been using Minolta Maxxum lenses with 5 contact points, and with auto focus they don't focus nearly as sharply as manual focus, even though the green focus light comes on in both cases. I have a 50mm 1.7 AF that is very blurry at 20 feet with auto focus, quite sharp with manual focus. Up close, 6 or so feet or less, the auto focus is fine, very sharp. At first I thought it might have to do with the aperture, but it seems not to. I shoot manually, steady shot on, single shot/spot focus, illuminator auto. My lenses with 8 contact points (same as camera body) focus the same, auto or manual. Are the three missing contact points on the older Minolta lenses having an effect on focusing info getting back to the camera body?
Thanks for any info or ideas....
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David Kilpatrick
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Re: Blurry autofocus

Unread post by David Kilpatrick »

Hi, welcome aboard!

The eight-point (D) lens specification is much more accurate for reporting the focus distance (16 times more steps identified, I think) which helps for flash, and that's why they designed it. The earlier xi lenses with eight contacts, designed for motorized zoom, have slightly more accurate focus reporting as that was needed for the link between focus distance and zoom setting using APZ, but they are not the same as D or Sony lenses.

AF systems do make adjustments (offsets) between the lens locking focus and the shot being taken. These can be quite complex on modern cameras, including provision for aperture related focus shift and for the initial focus distance. Old five-contact lenses made by Minolta do provide the correct lens identity for the Alpha 65 to make its adjustments, but they will always be based on the same calibration used for five-contact lenses - a single focus distance of about 1.9 metres. So it's no surprise that shots around this distance are sharp.

The A65 does not have micro AF adjust, which can be used in the A77 to program a new offset based on typical focus distances, for each lens. For example if you always use an 85mm for portraits, you can fine-tune in at portrait distance and if you always used a 28mm for landscapes, you could micro adjust it at infinity.

One thing you can check is how much range is covered, when the green AF confirm is lit. See how much you can move the focus manually, either way, before the light goes out. If there's a wide range, your camera may be locking focus 'from' one direction or the other - the lens normally starts off set to infinity when you switch on, so looks for the focus moving from infinity. But if you have been checking focus by testing on subjects closer than 6ft, then trying a distant one, it may be going from close to distant and getting AF confirm at a different focus point.

David
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Re: Blurry autofocus

Unread post by love-to-ride »

Thank you. I was focusing on a humming bird at a humming bird feeder about 8 inches round, and the depth of the focus while the focus square was lit seemed to be pretty deep; at least a foot because the hummer and the front of the feeder would come into and out of focus while the green light stayed on.
Thanks very much for the info.
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