Originally posted by jlaubza ......
But with zoom lenses, which part of the zoom range do you adjust? Here's my dllemma - I use a Tamron 18 - 250mm zoom lens. At wide angle, the lens 'focuses' at a range that clearly exceeds the actual distance for medium close up objects - one can read the distance indicator off the lens barrel. By zooming in, I can refocus, accurately, and then I need to lock that focus and zoom out again.
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WIth Tamron zooms I don't think you can rely on the distance scale at all. If you are judging the accuracy of the autofocus by reading off the distance scale you will probably be misled. I have two tamron zooms, 18-250 and 10-24.
If your 18-250 is anything like mine then that technique of zoom-in, focus, lock focus and zoom out will guarantee out of focus photos.
I'm assuming the 18-250 I have is the same one you have. At 250mm the distance scale on mine is rather accurate but as you reduce focal length the focus scale goes all wrong. For instance at the widest 18mm the focus ring reads 7 metres when focusing at infinity and 1 metre when focusing at 2 metres. (The 10-24 is even worse).. However I don't think this is really a problem but just something that is 'not nice'. You just have to learn to ignore the scale.
Many Pentax zooms 'solve' this problem by simply not printing a distance scale at all, but you can see they behave the same from the need to refocus when you zoom in/out.
Despite this apparent 'problem' the lens in reality focuses perfectly throughout the zoom range. On the K-r I set the fine autofocus permanently to -10 which also works fine with all but one of the other lenses that I have. On the K-30 it works fine with no fine adjustment.
The one lens I have that requires me to adjust the fine AF adjustment is the Tamron 10-24. On the K-r I have to set it between -8 and 11 depending on the focal length (but unfortunately 11 doesn't exist). I haven't used it much yet on the K-30 so I don;t know if this is camera or lens specific.
I think you should entirely ignore the distance scale and only use the actual photos and to figure out how well the autofocus works.