Scenario: 2 actors sitting still on a sofa having a conversations. There is a big windows that casts a lot of natural light over the actors, so low light is definitely not an issue and I can close the lenses down
Shooting workflow: 1 MarkII and one NEX7 with a speedbooster shooting at the same time with a 28mm 1.8, 50mm 1.4 and 85mm 1.8 (both cameras are still on a tripod)
Question: what is the basic aperture / f stop under these conditions for me to have a nice rich DOF without limiting the actors from moving and getting out of focus? I´ve made some tests and got some great results with the 28mm even at f1.3 for general lateral shots where one actors´ body was in focus and the one wasn´t, but I had some issues with the 85mm...when the actor moved his head a little he kept getting out of focus and it would be an insane task to keep pulling focus on such a wide aperture during the entire piece.

Anyway, tips and suggestions are appreciated.

I use a Canon FD 85mm f/1.2L as my go to lens for interviews. I rarely go larger than f/4. Most of the time it is between 5.6 and 8 depending on distance to the subject. The further away I am the more I open up to keep the look consistent.

Thanks for the tip!

Don't be afraid to stop down to f/8 with an 85mm lens if the subject is closer than 5'. The background will still be plenty soft at that setting. Focus is much more manageable between 5.6 and 8.

Here is what the 85mm looks like at f/3.5 at 8 or 9 feet to the subject.

Interview Lighting - YouTube

Thanks for posting the video. Samples really help getting a general idea. I though it would be way less DOF at f4, but it looks perfect.

Distance to the subject makes a huge difference with DOF. As the focal length of the lens gets longer the effect is more apparent.

If the focal pane was 4 or 5 feet the background would have been MUCH softer and the DOF would be much thinner. If I was shooting a tight closeup with the 85mm I would have used something closer to f/8 to keep the background recognizable and the subject sharp.

This website is a great help when you are trying to figure out DOF. - Online Depth of Field Calculator (http://dofmaster.com/dofjs.html)

DOF is a combination of, subject distance, focal length. aperature and sensor size. Almost anything at f 1.8 on a 5D sized sensor will be very shallow. I find that you really do not need to go below f4 to get that "look" in interviews unless you have a large group and are shooting very wide.

Don't forget the other key factor - the distance between your subject and the background. The more you separate your subject from the background the softer the background can get. The problem with most 'depth of field' calculations is that they are intended to determine how much depth of field there is to keep the subject sharp -- not to compute the extent to which a background or foreground object will be out of focus. If you are looking for a certain degree of softness in your backgrounds, set the camera and focus on the subject. Then adjust the iris until the background has the desired softness. Use filters, shutter speed and lighting adjustments to be able to shoot with proper exposure at the desired f/stop.

I´ve shot another episode today and everything was in perfect focus, but I had to keep the aperture at around f2.0 or f2.8 (occasionally I´ve reached f3.) because otherwise I would have to boost the iso over 400 on the NEX7 and it would add grain to the footage. Actually this was probably the first time I´ve ever shot iso 400 with the NEX7 and I´m not sure if this is a coincidence or not but I´ve never gotten so many overheat warnings and cameras shut downs. It was a nightmare and when I was shooting at iso 100 or 200 I barely ever got any overheat warnings...let alone a camera shutdown. I´ve read in a couple of places that faster SD cards help with the overheat issue but since a 32gb card costs around us$150 here in Brazil I would like some reliable testimonies before sinking more money into a bunch of cards without know if it will work.