This shot here was achieved using that exact method.
http://www.eastonchang.com/jaguar8/image02.jpgIt can be done, but it's always much less preferred to using a rig.
There are a lot of complicated factors to motion blur. You can't just add blur, the real thing also has:
- motion blur in the reflections (panels, windows) of the car, and moving in the right direction (i.e it follows the curves and lines of the metallic object).
If it's say a front windscreen than it's basically impossible because you have to be able to see the driver behind the windscreen. You cannot individually blur the tree reflections without blurring the driver behind the glass.
- photoshop blur is not the same as real blur, it's close but subconscienciously you can just tell that it's not the same. It's mostly apparently at night, because photoshop blur will just "stretch" light across, making it look weaker and softer. In reality a spot light would drag in the background causing a light sabre effect.
- there are also varying degrees of blur based on distance to the moving car. For instance when you're travelling on a train and you look out the window, the trees nearest to you whizzes by extremely fast, whilst the objects in the distance move a lot more slowly. This means in a "rig" shot the blur is greatest when it's closest to the camera.
In other words... blur varies in 3 dimensions (includes depth). Whereas Photoshop blur just acts in 2 dimensions, it would blur the near objects and far objects to the same amount and it just looks wrong.
- blur also spins and bends, in photoshop you can basically only blur in a single straight direction. There are ways around it (distorting the background to fit the curvature) but it just dosn't work well.
I used to use photoshop to just do blur in the old days when I was just starting out and experimenting with all of this (about 3 or 4 years ago), then I started to build my own DIY-basic rig and never went back.
Also, the reason why I used photoshop blur in that picture is related to a problem with the Mamiya when it was taking the "rig/moving" exposure. Which i'll talk about in a new thread!