How can I get a separate shadow pass.

[Denis]

Hello I need to put in the video 3d object, for this manipulation I use shadow catcher material.
But I have problem is that it material doesn’t work very well.
Now I try to explain that I mean


On these pictures you can see the gradient at the border of the plane.
I have a solution to remove this gradient, I need to increase this plane on full screen,
but this will increase the rendering time, if I have resolution a picture of 2560 then I will spend a lot of time for rendering.
And I need only shadow pass and objects, who can give me advice, how can I decide my problem.
Thanks.

Moderator's note: i fixed the links.

[mferster]

your image links are broken so we can't see what you are referring to.

[sprayer]

Your light setup producing bigger shadows than your ground plane, this is normal, you may adjust light to make more sharper shadows or reduce shadow in post with levels adjustment for shadows, but usually making this for shadow catcher alpha in composing mode

[maru]

You can just boost contrast of your shadowcatcher plane or its alpha, for example using the levels filter. This way the areas which were just bright will become fully white, and the visible borders will disappear.