Samsung 45mm vs Rokinon 85mm?

Let's take a careful look at what they say:

"Note that focal length has not been listed as influencing depth of field, contrary to popular belief. Even though telephoto lenses appear to create a much shallower depth of field, this is mainly because they are often used to magnify the subject when one is unable to get closer. If the subject occupies the same fraction of the image (constant magnification) for both a telephoto and a wide angle lens, the total depth of field is virtually* constant with focal length!"

Preliminary note: "contrary to popular belief" is misleading, since the rest of the text doesn't contradict the statement that focal length affects DoF.

To make my point I am going to give an illustration of what they say. Suppose you want to make a head an shoulder portrait of a person P, and suppose you want to try two lenses for this purpose: a 85mm and a 135mm. You begin by using the 85mm, and, to frame the subject correctly, you need to be at a distance of x feet from P. Then you mount the 135mm on you camera: to get the same frame (head and shoulders), you'll need to place yourself father away (y feet) from P than when shooting with the 85mm. Now, y > x, so the focal distance changed. And you changed it in order to get roughly the same frame. The magnification (i.e., the frame) is the same; the focal distance is different:"This would of course require you to either get much closer with a wide angle lens or much further with a telephoto lens, as demonstrated in the following chart:..."

To summarize: focal length "doesn't matter" as long as you change the focal distance. That is, you can obtain the same DoF with lenses of different focal lengths as long as you are willing to change the focal distance. However, in my previous reply I said something else: "Keep aperture and focal distance constant, and change focal length: you'll get different DoFs."

As DoF calculators show, DoF depends on three factors: aperture, focal distance, focal length.

Hope this helps.