How to get a good bokeh effect with my Nikon D3100 with 85mm 1.8 lens?
How can I get a good bokeh effect with my Nikon D3100 with 85mm 1.8 lens?
Use a tripod. You might think that with the aperture opened up your shutter speed will be high enough to eliminate the effects of camera shake. Depending on the light, maybe so. However, your depth of field is shallow. If you want the subject's eyeball in focus, you not only must accurately place the focusing sensor on it, you must avoid any fore and aft swaying between the focus and opening the shutter.
Set it to the widest aperture.
Keep your subject as far from a background as you can.
Pity you don't have a wider angle lens; you might struggle with the 85mm
Here's a good article that may help.
Real bokeh is obtained with a lens that has rounded iris diaphragm blades and the lens is used at it's widest aperture. Without these "rounded" blades the background is simply out of focus. Which is nice too!
You know, you have the camera and lens, using you knowledge of the fundamentals of photography, it would be simple enough to do some tests.
A wide open aperture would of course throw the background out of focus, so that would be a good starting point, however, when shooting portraits, it is usually necessary to stop the lens down two stops so you can 1) shoot at the lenses sharpest aperture and 2) provide enough depth of field to have the persons nose and ears in focus when you have focused on their eye closest to the camera.
Position subject as far away as possible from the background. Position the camera as close as possible to the subject. Set aperture size to f/1.8.