Right guys, I've put this in it's own thread as I didn't want it biased by sticking it in a Nikon, Olympus or Fuji thread (bear with me and hopefully you will understand why). OK, so as some may know, I used to be a Nikon guy (for over 30 years in fact), but about a year ago sold off the last of my Nikon gear (my D810 and D500 bodies and a few remaining lenses). Following the sale I became a sole micro four thirds shooter and currently have the Olympus EM1.2 and Panasonic G9 bodies which are both very nice, and a really nice set of Olympus and Panny glass (more of that later). Recently (about 5 weeks ago) I added a Fuji X-H1 to the mix with just a small selection of classic lenses (10-24, 16-55, 55-200 & 35 F2). My photography encompasses many genres's (Landscape, people, travel and wildlife/bird photography). I still use the M4/3 stuff for the majority of my stuff and am still very happy with it, and have started using the Fuji for a bit of Landscape work and portraits. Now, when it comes to my current passion (wildlife and bird photography), in decent light (where I can shoot at ISO 1600 or below, my M4/3 bodies and my Olympus 300mm F4 + Teleconverter along with the Olympus 40-150 F2.8 (again with the optional teleconverter), nets me a reach of around 840mm in FF terms (at F5.6). My longest lens for the Fuji system is my 55-200 (300mm equivalent @ F4.8). Whilst the Fuji is a bit better than the M4/3 bodies at high ISO (maybe ½ to ¾ stop max), the "only" long lens for the Fuji system is the 100-400 (with an equivalent reach of 600mm @ F5.6) - that's if you exclude the hideously expensive new 200mm F2 (which I have). I understand the 100-400 is a great lens, but I'm wondering if I'll see much (if any) difference between shooting my Olympus 300mm F4 (without converter) @ ISO 1600 to Shooting the X-H1 with the 100-400 F5.6 @ ISO 3200, or would it be a wash (and considering the Fuji Lens is the thick end of £1,600 new). Both would be a F/L of approx 600mm in FF terms but one stop apart in aperture. Therefore, i was thinking if I should just stick to the M4/3 pair and only shoot in decent(ish) light where the ISO's can be kept relatively low, or (and this is a bit off the wall), buy another D500 with the 200-500 Nikon lens. I really missed the D500 when I sold it as it was by far and away my fav DSLR in all the Nikon's I've even owned (and I've owned a few) !) As it happens, an acquaintance of mine just happens to be selling both for a VERY good price (also very well looked after). Now I know a lot of you will say it's crazy to even consider adding a 3rd system (and maybe you are right), but despite the X-H1 being good, when I was shooting with the Nikon D500 and my Tamron 150-600 F4-6.3 (Gen 1), I could quite regularly get more than acceptable images (in fact pretty amazing images truth be told) from the D500 at ISO's up to 12,800, and the D500's AF is still up their with the very best (and much better than either of my current cameras, even though they are still quite good). So options are Stick with the M4/3 system for long lens shooting (as I already own the glass), but look for better lighting to keep the ISO low. Get the 100-400 for the Fuji X-H1 to get me to 600mm which should suffice for 80% of my photography (but again it's an F5.6 lens) so any low light sensor advantages of the X-H1 are probably negated by the one stop slower max aperture?. Add a D500 with Grip and the Nikon 200-500 and use this primary as my wildlife rig. For the time being whilst this is being discussed I'd like to keep FF (so Nikon D850 & A7 III etc.) out of the mix as I'd need much bigger and costlier lenses for the same reach as APS-C or M4/3. As far as cost goes, let's just say that in this instance, money isn't the primary driver.