Hey I bought a 2nd hand sigma 17-50 lens from someone for my nikon d5300
However when I tried the lens on the camera there was just one problem.
When the lens is set to auto-focus, the display of the camera would focus rapidly between af-s and m. And also when i half press the shutter, the beep noise from focusing doesn't happen.
The manual mode of the camera works fine.
The internal focus is also not broken as the lens managed to auto focus for 5 seconds until it went back to the problem.
Also my other lens works with the camera just fine only this lens.
And I don't think it is the contact problem of the lens otherwise the camera wouldn't be able to control its aperture. And the camera doesn't say lens not found
The lens also has a built in auto focus motor
Read more: Help: Lens Autofocus Problem?