Short version, you need a shutter speed of around 0.5 second or slower, my latest shots are between 0.5 - 1.5 seconds. You don’t need a tripod to use this technique.
Long version….if you use shutter speeds much faster than 0.5 seconds, the result tends to be a photo that just looks out of focus, like you have very unsteady hands, or both. When it comes to the shooting technique, I pan vertically by slowly tilting the camera. Typically I start with the camera pointing down, tilting it upwards.
The reason I’m panning by tilting is that it’s the easiest way to get a smooth motion, at least for me at this point. If you have very steady hands you could pan by simply moving the camera straight up.
So what exactly is ‘slowly tilting the camera’ ? Impressions - Volume Twenty-SevenSweden, May 2017 Well, it depends on the shutter speed and your composition. Take this photo for example. The shutter speed is 1.5 second, this means - in order to get the composition I want - I will have to tilt/move the camera slow enough so that I don’t end up outside my composition but fast enough so that I don’t end up with a fraction of the composition. This particular shot was made by focusing fairly closely on the boardwalk, hold still for a split second and then slowly panning sideways following the boardwalk, making sure I didn’t get any part of the sky in the frame, ending up outside my intended composition. Or, to put it more correctly, end up with a part of the sky in the frame long enough for it to register.
Generally speaking, it’s all about experimenting. I have tried pause for a split second while panning, panning during the whole exposure, hold still for a split second before panning. Vary the shutter speed and the speed of the camera movement.
To slow down the shutter on my D800 - sold recently - I dialed in ISO 50 and put on a polarizer, I then adjusted the aperture to get the shutter speed I wanted.
On the Fuji X100T I have been using the built in 3 stop ND and - if necessary - a polarizer. To get more options, I'm considering to buy a 4 stop ND to the Fuji. Anything below ISO 200 is a no go on the Fuji because it then only do JPGs, I always shot RAW.