You're right that the ND filter adds to the power needs of the flash as well. With one 'speedlight' then you will be pushing it with HSS in bright light, that's for sure. I also agree with Wired that f3.5 is very wide .... of course what is needed for your DOF effect depends on the focal length and the distance of the background behind the couple. But yes, F5.6 sounds safer, also for focusing and getting most of the faces in good focus. Standing back and zooming in more should give you the OOF effects you want.
This is the way to get the most out of your flash, and have the best chance against the sun ....
1) In Manual Camera mode fix your Max Sync Speed (1/180th or 1/200th) .....
2) Set your desired aperture (say F5.6), and then set your ISO until your reach your desired background exposure level, using the viewfinder or LCD exposure indicator to judge it against the meter centre level ....(
You may hit ISO 100 and still want it darker....in which case just accept that you need a smaller aperture. You might end up at F8, but with a long focal length, standing back and with separation from the background then you can still get nice OOF effects ....)
3) With your flash on the camera hotshoe, set Manual Flash mode and then 1/1 (Full) power ..... read from the distance indicator the correct distance for that power / aperture / ISO combination (this is calculated by the flash using the known values that are transmitted through the hotshoe)
4) Remember this distance and then set up your flash at that distance from the subjects (assuming off-camera flash) ..... for on-camera then obviously you just move yourself to that distance. Make a guessed allowance for any diffuser (eg go closer 3 feet or so if you have a softbox) ..... and shoot!
You have now created the ideal mix between ambient and flash exposures that your equipment can allow ..... by using the maximum power of the flash and using the maximum exposure reducing settings on the camera that are within the max sync speed you are getting the most output possible in relation to the sun.
Last edited by mcgregni; 05-05-2017 at 12:11 PM.