I have a project with 16:9 footage wich is supposed to be shown in cinemascope 2.40. For that purpose I added the 16:9 Mask (Film) effect to the sequence without giving it much more thought.
Now the cinematographer has seen the sequence and insists that the letterbox that is on the timeline right now, makes an aspect ratio of 2.35 instead of 2.40. I can't find any info on this in Avid.
How do I know what aspect ratio I have and how do I achieve an exact 2.40 mask?
The cinematographer is alomost correct, since a 16x9 mask in Avid MC does a 25% mask, keeping 75% of the vertical lines, which means 810 lines, which implies an aspect ratio of 1920/810=2.37037037037...
Also, in current Digital Cinema formats, Scope is 2048x858, meaning an aspect ratio of 2.38694693...
However, the 16x9 Film Mask you are now using can only set the V percentage in round numbers. So you either get 2.37037 (v=75) or 2.4024 (v=74).
The best way is to do such a mask in MC is to go to your Project Window, Format tab, click Mask Margins, choose a 2.39:1 Mask Margins preset. This masks the top and bottom 12.76% of the image, which gives you 804.384 vertical lines, which gives you an aspect ratio of 1920/804.384=2.38691968.
If the DP happened to shoot a framing chart (I always ask for those), you can use those to select the top and bottom edges of the intended framing.
In the Mask Margin Presets, you can also create a custom setting. A setting of 12.9622963% top and bottom would result in 800 lines, so 1920/800=2.40. But again, this is not the official current format for Scope releases.
After you have set the Mask Margins, you can apply them to your source monitor and record monitor (individually if you like), by right-clicking on the source or record monitor, and choosing Target Mask, and selecting one of the available options.
You can also apply the mask to Full Screen Display in the Full Screen Display Settings. Or if you have an I/O box, in the Video Output settings.
You can also use 'Enable Mask Margins' when exporting to Quicktime or when doing MXF File Exports.