Heya,
Depends on your goal, which sounds similar to mine (portrait with flash, sunny day, around F2.8):
I find that to get the isolation look, you need to drop ambient exposure 2~3 stops below what your subject's exposure (brought back via flash/strobe). So, using Sunny 16 (F16, ISO 100, 1/100s) you can figure how many stops you need based on your aperture, and a SS of 1/200s for example (common max synch speed on a lot of cameras).
So for example, when I'm shooting with an F1.4 lens (85 F1.4), I'll go 1/200s, ISO 100, F1.4, and I'll need 6 stops just to get to the Sunny 16 "normal" exposure. I would need 2~3 stops from there just to get the ambient light below my normal exposure (subject) light, which means 8~9 stops of ND--this makes it hard to do focus (if not impossible sometimes; I have to pre-focus then put the ND on). So I try not to have to put myself in a situation to need that many stops.
Lately, I've been shooting at F2.8 on a longer lens to get the isolation from depth of field (so I'm using a 200 F2.8L prime, an old one), because F2.8 requires less ND filter stops to achieve what I'm talking about. 4 stops gets me there. And to drop ambient exposure a bit more, I can use 6 total stops of ND to get ambient light to be 2 stops lower than normal exposure of subject, and I flash/strobe the subject. I like this effect a lot more. And I keep the shallow DOF look I want for outdoor environmental portraits. So I use two 3 stop ND filters stacked for this. I can still AF through them without a problem in decent light.
The easiest thing to do is to shoot early morning or late evening when light is even less, so you need less ND stopping power. I can get away with a single 3 stop ND filter around the end of sunset, again using flash to fill my subject. Ambient light is lower, so you get that nice subject isolation via exposure look.
So my kit these days are:
2 x 3 stop ND filters (total 6 stops stacked if I want); this with aperture & SS and low ISO is my ambient exposure, which is set to be LESS than subject exposure (flash)
Flash + gel (warming) + softbox (and get it as close as you can to subject); this is my subject's exposure
Shooting wide aperture (I'm liking F2.8 more for this, than F1.4, as it just takes too much ND with F1.4 here in Florida for the look I want)
Shooting 1/200s for the additional stopping power, but also to still synch with flash
I have Hoya & Marumi ND filters, I find them to be equal really, no color cast, no artifact or flare (I use a big rubber lens hood on them to help), sharpness is retained shooting wide open with a fast prime. They're inexpensive around $40 a pop.
****
3 stop ND (One 3 stop filter), F1.4, 1/200s, ISO 100. Very low light, right after sunset (no sun in the sky at all, just warm ambient late day light, very even), so I didn't need as much ND. Warming gel on flash, in a softbox.
IMG_0482 by
Martin Wise, on Flickr
6 stop ND (Two 3 stop filters stacked), F2.8, 1/200s, ISO 100. Bright day, but towards the end of the day, sun behind subject, so strong back lighting. Warming gel on flash in a softbox, close range.
IMAGE LINK: https://flic.kr/p/oKvDTB IMG_9940_marked by
Martin Wise, on Flickr
Here's the same light,
without the ND filters and
without flash to give you an idea. You can see how dropping ambient light by 6 stops with the ND filters and then exposing subject with a warmed gelled flash made the effect. This is with the subject's strongly back lit, hence they are in shadow, and this is why I like to use flash for this kind of thing.
IMAGE LINK: https://flic.kr/p/pEdKAC IMG_9922_marked by
Martin Wise, on Flickr
Very best,
