I am asking this on behalf of my 17 year old son. He has a Nikon D3100 camera and the lens that came with it (Nikon DX AF-S Nikkor 18-55mm f/3.5-5.6G).
He is studying media at school and has recently purchased a second hand Tamron Zoom Lens AF 70-300mm f/4-5.6, which he is generally happy with. However, he has noticed that it does not auto focus when his camera is in video mode.
When his camera is in picture mode and he takes a snap shot, the lens will auto focus (wind itself in or out) and focus on the subject. When he switches the camera over to video mode, the lens will make a very fractional twitch but will not auto focus.
My son has 2 other friends studying media and one of them has the same lens while the other has the same camera. He tells me that his friend's lens will auto focus on his other friend's camera. My son has not been able to try the other lens on his camera yet.
The stock lens above in brackets does auto focus in both picture and video mode.
My answer was that the lens may not have a motor to do the auto focus thing, but my son countered this with how does it do it in picture mode then?
Unfortunately I am not technically able to answer his question or even tell him if his lens is partially working or is working as it is supposed to be.
Any help would be greatly appreciated. I hope the above is not too confusing.
Many thanks and Merry Christmas