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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?

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Not totally unexpected.. with the viewfinder the camera uses phase detection focus using specialized sensors. In liveview mode, the AF sensors aren't usable (they only see the light when the mirror is down) and the camera uses contrast detection focus which is slower[*] and doesn't work too well in low light or if the subject lacks contrast (at least on cameras as old as a D5100...)

[*] The slowness of the contrast-detection focus is what kept the DSLR popular...

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