Unless you are a seasoned director of photography or assistant cameraman, chances are you can’t calculate f-stops in your head.  Fortunately there is a simple calculator available online that allows you to enter the starting f-stop and the number of stops to add or subtract and it figures the correct f-stop.
Also, if you need to determine the depth of field at a certain f-stop, you can use the DOF calculator.
Click here for the F-stop Calculator and here for the DOF Calculator.
I’m planning on shooting miniature sets to use in conjunction with greenscreen. This obviously means I need to use macro tubes to get close in the sets.
Am I right in thinking that these tubes magnify the image? So say the tubes magnify by 2x, wed need to shoot the green screen with a 2x converter, to keep the same focal length?