Note that there is ZERO noise reduction of any kind applied to the RAW. I would normally not process a photo this way. This is intentional, just to show the improvement better. To illustrate the point better, I have applied a lot of settings that make any noise a lot more pronounced. Here's a sample handheld result (full res, 100% JPEG, heavy files).
16 images = 4 stop ND filter, 4 stop noise reduction, ISO 12.5.8 images = 3 stop ND filter, 3 stop noise reduction, ISO 25.4 images = 2 stop ND filter, 2 stop noise reduction, ISO 50.2 images = 1 stop ND filter, 1 stop noise reduction, ISO 100.So in the end, shooting at base ISO of 200: You can of course do that in Photoshop, but it can be done in-camera, which produces a single RAW file. You merge all those photos into one, resulting in lower noise, thus better dynamic range and as a side effect, an ND filter effect. Ok, so you've now got your 2/4/8/16/32 photos. Tripod: same as above, only with tripod and preferably remote shutter release.Handheld: assume stable position, press the shutter button until camera takes the desired number of photos.For convenience: Frame Count Limiter set to 2/4/8/16/32, depending on required effect - (Cogs->C1H Settings on E-M1 II) - if you're doing it handheld and are not confident in the stability, you might want to set the limit a bit higher to have some wiggle room in case first or last couple of images are not aligned.Drive mode set to sequential High at highest possible framerate.IBIS mode set to S-IS1 (All Direction Shake I.S.) when handheld or off when using tripod.Image Stabilization set to Fps Priority (Cogs->C2 menu on E-M1 II) when doing this handheld.Don't know how many people know that any Olympus camera is capable of generating Medium Format image quality with a bit of fiddling in the playback menu.