Ha.... I saw 85 f/1.4 on my phone and automatically assumed outdoor portraits. But....the exact purpose was not specified.

So, Joel is right, it really depends on your use. A quick chart with EV of different light:

Name: EV of Light.jpg Views: 1396 Size: 61.2 KB


EV = log2 N2/t, with N = f number (aperture) and t = time in sec. So you can generate a table of EV with aperture and shutter speed:

Name: EV Table jpg.jpg Views: 1494 Size: 173.3 KB


So, in full sun, we have 15 EV. Say you want shoot 1/250 of a shutter speed and f/1.4, which is EV 9, then you would need a 6 stop ND filter (EV 15- EV 9). But even say you aren't in full sun (EV 13), and want a bit of DoF for outdoor portraiture (f/2.8), then you only need 2 stop ND filter.

But, it really does depend upon your purpose.