Photoshop’s Gradient tool can be used to add a variety of transitional effects on a layer mask
Select the Gradient tool from the Toolbar and choose a preset gradient from the Options bar.
Radial, Reflected or Diamond shaped gradients can be applied to the mask using the boxes at the top of Photoshop, changing the behaviour of the gradient.
Set your Foreground and Background colours to black and white and apply the Gradient to a layer mask, such as the one that comes with Curves.
Gradients guarantee consistently smooth transitions each time.