user10619: Many famous directors like Tarantino and De Palma use diopters to have something close and far away in focus. This often produces a line down the center where the focal split occurs. My question is: Why don't they just use a large DOF like F8-12 or similar?

user1950: Even small apertures won't always give you enough DOF if you're on say a long lens. For example, with a full frame 35mm sensor on a 50mm lens at ƒ22 if my front subject is 1.13m away, my DOF only extends to 2.23m. If your sensor is larger it's even worse.
Shutting down the aperture means you need more light on the scene. This can be impossible or undesirable. 
For film this was particularly true - depending on what stock you're shooting (which is a creative choice) your effective ISO could be quite low, like 100 or even 50. So you might not even have the luxury of shutting down that much without having to punch a truckload of light into the scene.

10619: <p>Many famous directors like Tarantino and De Palma use diopters to have something close and far away in focus. This often produces a line down the center where the focal split occurs. My question is: Why don't they just use a large DOF like F8-12 or similar?</p>
 1950: <p>Even small apertures won't <a href="http://www.dofmaster.com/doftable.html" rel="nofollow noreferrer">always give you enough DOF</a> if you're on say a long lens. For example, with a full frame 35mm sensor on a 50mm lens at ƒ22 if my front subject is 1.13m away, my DOF only extends to 2.23m. If your sensor is larger it's even worse.</p>

<p>Shutting down the aperture means you need more light on the scene. This can be impossible or undesirable. </p>

<p>For film this was particularly true - depending on what stock you're shooting (which is a creative choice) your effective ISO could be quite low, like 100 or <a href="http://www.kodak.com/US/en/motion/Products/Production/5203/default.htm" rel="nofollow noreferrer">even 50</a>. So you might not even have the luxury of shutting down that much without having to punch a truckload of light into the scene.</p>
