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hope you can help me and my frustrating issue
I have a D5100 with a Sigma 18-250mm f/3.5-6.3 dc os lens which will AF no problem at all when I'm looking through the viewfinder but when I change to live view mode on the screen the lens adjusts only a fraction at a time, and sometimes not reaching focus at all. When it does focus it can be 10-20 seconds by which time my shot is gone! It's obviously not a lighting/exposure issue as I have no problem focussing through the viewfinder.
I hope someone can shed some light on this issue for me?