ND filters are great but one of the downsides when shooting with them is how long it can take to set the shutter speed once you have filters attached.
Take this example:
- - The initial exposure meters at f/11, 1/500th sec, ISO 100
- - The aim is to get to 1 minute for nice smooth water
- - Bulb mode is required, which will mean 15 stops of ND filter
As most people have their cameras set to use 1/3 stops, the above scenario could require a lot of wasted time and effort. Enough that you could miss the shot if you're losing light fast.
If you took the initial meter reading in manual, 45 shutter changes are needed to go from 1/500th to bulb. People often prefer to flip over to Aperture Priority mode to find their base exposure but once you switch back to Manual for the actual shot, there's likely still a lot of dial twiddling.
A handy method for cutting this down is to assign one of the buttons on your camera to temporarily switch from 1/3 to full stops.
The following steps are for assigning the Fn button on a D800 but season to taste, as they say.
Enter the settings menu for your camera and navigate to Custom Settings
Enter section "f Controls"
Select option "f4 Assign Fn button"
Select option "Fn button + command dials"
Navigate the list and set the option "1 step spd/aperture". Press OK to assign it.
Once set this way, holding the Fn button in while setting exposure reduces work time significantly and saves wear on your command dials.
For comfort, I recommend you assign the option to a button opposite the dial you use for shutter speed; if you use the rear command dial for shutter, it feels more natural to hold a button in on the front of the camera as you turn the shutter dial on the back and vice-versa.
As you're unlikely to use the DOF preview button when shooting landscapes, you may prefer to assign the function to that and keep the Fn button free. With large hands, the likelihood is that you'll find the Fn button more comfortable.