I was wondering how I take a picture of a waterfall in motion but also have a person as my main subject. I know I have to use a slower shutter speed, and use a tripod to get the water in motion. How do I do this without making my subject blurry? Do I just tell her to stand very still?
Use the ambient light to illuminate the waterfall. Use a fairly powerful flash to illuminate your human subject. The quick duration of the flash will freeze her, especially if she remains fairly still over the long exposure. The narrow aperture you will need to properly expose with the flash will also give better depth of field so that the water fall is also fairly in focus.
Take multiple photos, including the background without the subject present. Expose differently to fit each element. Combine in Photoshop.
You can also spend more time playing with different settings when the foreground subject is not present, without worrying about keeping her interested.
With a tripod and near-perfect alignment, blending layers is easy.
See also this technique.
The pictures which you see on the "model portfolio photographer" website feature exactly the types of shots you are seeking to create. They were photographed by me, using a very simple technique, in all different conditions.
Camera must be tripod mounted. Use ND filters if conditions are bright- work on the sweet-spot aperture of your lens, with shutter speeds down to around half a second. Final choice of speed depends on water flow rate and volume. One half to one second is about right in most cases. Put the model in a pose she can hold still- and be mindful of her comfort. If she shivers- she won't be still and her movement will blur her.
Lighting is crucial- only you know the location- natural light or flash are fine for the model- the ideal choice is based on the nuances of the scene and your preferences in style. The waterfall shots on the website were done without flash- and to prove the point that you can get around equipment limitations- some were intentionally shot on a $200 coolpix camera.
Knowing your equipment- and technique should be paramount before taking a model out there. Preview the location- an hour or two difference in timing can make or break the shoot- and clouds or sunny conditions- if they are not what's optimal for the scene and your final shots- can also be deal breakers.
Use a cable release to trip the shutter- mirror lock-up is great too.