Using the gradient filter in Photoshop to combine two images

This final image of Kent Falls is a combination of two photos: One at a slow shutter speed to blur the falling water, and the other at a higher shutter speed to stop the motion of the moving ice paddies in the foreground. 

On my recent trip to Connecticut I photographed the Kent Falls on a cold day. Water flowed heavily over the falls, but ice was beginning to form on some of the nearby rock facing and also began forming round, floating ice circles in the pool of water. As usual with a waterfalls, I wanted to capture the falling water as a motion blur with the rest of the scene sharply focused. The ice paddies in the foreground pool were constantly bobbing about so a slow shutter speed to blur the falls would also create a blur in the moving ice.

At the slow shutter speed to blur the falling water there is too much motion blur in the floating ice paddies as they bobbed about in the water.

At speed of 1/30 - 1/60 of a second I was able to capture some images where the motion of ice paddies was stopped and their detail was sharp enough.  At this speed, however, the motion of the background falling water was stopped too much to make an interesting photo. 

I took a series of exposures at various shutter speeds ranging from 1/2 - 1/60 second -- the slower speeds to create motion in the water falls, and the faster speeds to stop the bobbing motion of the foreground ice flow. The reason I varied the speeds was to provide some variety for a smooth combination of images in post-processing. If the background was too blurred in comparison to a super sharp foreground, the photo would look false. It was important to obtain just the right balance of shutter speeds.

Needless to say, a tripod was mandatory in this situation not only to register the two images later, but also to obtain the ultimate sharpness in the rest of the scene. No matter  how good image stabilization might be, it will never beat placing the camera on a tripod. Additionally, it allowed me to work at the very lowest ISO to achieve maximum resolution.

The two images were stacked in Photoshop with the blurred falls on top and sharp ice on the bottom. I created a layer mask on the top image, and, using the Gradient tool, created a soft line of demarcation between the top and bottom parts of the image.

This is the gradient mask created in Photoshop for the top layer.

Once the mask was created, I simply tweaked it a bit by painting black or white with a soft brush to smooth out the transition of top to bottom images.

This shows the two Photoshop layers with gradient mask in place. The top image had the blurred water falls, while the bottom image had the stop-motion ice flows.