For most of my DSLR years I had no lenses faster than a 30 mm and a 50 mm f/1.4. Also some f/1.8, a 28 mm, a 50 mm, and an 85 mm. And two f/2.0, 35 mm and 100 mm. Then during the last half decade or so I acquired every FF Sigma f/1.4 Art except the 28 mm. Nothing seriously exotic; no slow focusing, mediocre optical quality EF f/1.2 for me. (I am ready for the flaming!) Once I acquired the M5, and later an M6 II, there were all sorts of interesting third party lenses available. To complement the excellent EF-M 32 mm f/1.4, the Sigma 16 mm and 56 mm f/1.4 were no brainers. But there are even wilder animals out there, in my case a Mitakon 35 mm f/0.95 and a Kamlan 50 mm f/1.1 Mk. II. I am not saying that these lenses are optical stars, but they give you the opportunity to work with quite shallow DOF for an APS-C sensor. Now here is the nub. ISO 100, f/1.0 (more or less) and you want to under expose the background by three stops while putting three stops worth of plus flash exposure on the main subject during full sunshine. And your max shutter speed is 1/4000 second. Good Old Sunny Sixteen says you need to reduce the ambient by 2 2/3 stops. Enter the Neutral Density filter ND 3 which reduces the ambient by three stops. While this solves your problem, your flash groans in protest that it is hard enough to do High Speed Sync without having to overcome the three stops ND filter. The first image was shot with the EF 35/2 IS, Auto ISO, and Program mode, no flash. The second image was with the Mitakon 35/0.95, ISO 100 and Manual mode, lens wide open and 1/4000 second. The flash was a Godox V1 on Manual, 1/1 power, HSS, about 18" from the Silver Star. The objective was to under expose the ambient by three stops, which was probably a bit much. I should have stuck with two stops. If I had settled for two stops under, I could have used half the flash power, moved the flash 40 % further away, or introduced a small diffuser. I could also have used an ND 2 filter. But overall, an ND filter (or a few, to fit various lenses) can occasionally come in very handy when you need some special effect.