B&W is all I put on my lenses. German engineering use white water glass optics the best. Don't go with anything less and don't put hoya, tiffin or anything else less equal on a lens where sharpness, color and contrast won't be compromised.
I am a lifetime Nikon guy starting with the Nikon F2A manual pro camera. For most of my work I use my Nikon D810 and D7200. But recently I have made the jump to mirrorless. I went to the leader in the field Sony. I purchased the Sony A6300 which my wife now uses in favor of her Nikon and I use as a backup for things when I need it. It is a great camera. Spend the money and get it. And my wife loves it because it is so small and light and can shoot 4K video and 10 fps. Also it is great for me not having to carry both of my cameras, I carry this with one of my other cameras.
The BIGGEST thing you have to keep in mind with THIRD PARTY LENSES is that if for some reason damage is caused to the camera by the lens the Camera manufacturer WILL NOT UNDER WARRANTY fix the camera for free. You will bear the full costs. That being said third party lenses being produced in the computer age of today using quality glass can equal or exceed the camera manufacturer's lenses. I for one use and never had a problem with third party lenses, but I always do my research (extensive) to see what problems they are having with these lenses and how they are being fixed.
B&H is a great company whom I have been with many years. All camera companies sell gray market or refurbished lenses for those who don't want to spend the proper money for a new lens. By law they have to tell you and I always found B&H indicates somewhere if you look at it close enough what it is they are selling but far too many people see the GREAT LOW price! That is the problem.
I gather from your question you are a beginner in photography otherwise you would not be asking this question. And you have purchased a pro level camera to start out with. I think before you buy any lenses you learn the capabilities or the 850 and compatible lenses by taking a couple of photography classes. Also there is many sites on the internet to learn about the basics of different types of lenses. There is lot to learn about lenses and if you gonna spend big money on the 850 you need to learn you will be spending big money on lenses. Learn then do.
I use the 810 and am not interested in upgrading to the 850. Sure it has more MP, a faster drive, and is great for low light light astrophotography which being a landscape photographer I do not need since the 810 does it all for me. And your case you can get a great price on the 810 over purchasing a 850 for a thousand or more. Also there has been talk about more noise with the higher MP of the 850 especially in low light AND rumors already that there is already a successor to the D810 maybe as soon as 2018 and the D750. I would rather wait. The 850 is not perfect regardless of what you hear. As a professional photographer since the 80's this is an outstanding camera. Besides Canon is not going to sit still and not put out something that equals or exceeds the 850 and that is an historical fact. But that would mean nothing if you are already vested in Nikon equipment like I am.
For all the money one can waste on a variety of point and shoot cameras, go and buy something like the Sony A6000 now for under $600 with kit lenses. It is a mirrorless camera and shoots outstanding photos and has 1080P movie abilities. Bought on for my wife and it can equal my Nikon D810 in many cases.
I myself use filters, UV, Clear and Polarize. I often find myself shooting into people going by. My lens in those situations has been saved by a filter. And no there are some excellent made filters out there. They are extremely strong contrary to others who have said otherwise. Hoya for example makes an extra tempered glass. I personally use only B&W filters on my Pro Nikons. If you are worried about your lens be affected by a filter go with the filters with Nano coating, etc., and then use only a clear photo. And yes there are different strengths of UV filters that do filter out the excess blue (called haze) in mountain areas, but sometimes you are better using photoshop to do the same thing depending on your ability using photoshop. UV(0) from top quality companies like B&W (German in this case) do not affect color quality of your photographs since they are no different from clear in most cases.
While it is true that depending on the quality of the filter it could affect you photo quality, use of the highest quality of filters reduce this to zero. After all those who promote the use of polarizing filters can also do the same in photoshop especially if they are shooting in raw.
For the benefit of those who have years of experience be it professional, instructor, serious enthusiast, or even beginner I strongly advise that those posting requesting information/opinion on what to buy the quality of their photographs provide the following information. Many times I find that information/opinion provided is not true or accurate. Hence it affects the decision being made by the person who has the question. And I find it is usually something that has something as simple as knowing the basics of cameras, lenses, or photography. Let those who have the experience really be able to help or share with those that do not. So I am recommending that people who post add the following info if they want a good answer.
1. Determine what you can afford. Too often it is apparent that not everyone can afford a Nikon D850/D4 or whatever.
2. What is the main interest(s) of your photography? This determines the type of camera as well as the lenses.
3. If someone does not know the basics as explained earlier so let it be known. This is not to knock anyone, but it will help those who don't know become informed, better photographers so they don't waste their money. I had to learn this lesson the hard way during my initial years of photography and later working with, as a pro, and teaching in my 40 years.
Lastly any pro or experienced enthusiast will tell you that the camera and lens is not the most important factor in photography - it really is the individual photographer. You can have the most expensive and one of the best cameras in the world and still turn our poor pictures!
Thank You, others are invited to provide their input.
Just bought my wife a Sony A6300 and it is the ideal travel camera and with the kit lenses both of which are good and being a mirrorless it is also compact and lightweight with dust and moisture protection and 11fps drive and 4K movie mode all for under 1,300 dollars. Don't waste your money on anything less. In fact I am thinking of getting the A6500 as a backup to my Nikon D810 when weight is a consideration.
I would highly suspect that it is the writing ability of your memory card. If you aren't using a compatible or sufficient write speed memory card it will slow your camera to nothing. Get a card that can write fast enough for what you want it will make a world of difference. This is one are you an not be too cheap. After all you get what you pay for. The cards I use are rated at least 1000X.
The academics of a sharp lens has to do with the optimal aperture and lens element quality. Most photographers do not understand optimal aperture and that is truly sad. Saying a higher price lens is better shows me their lack of understanding of the two concepts I mention. Learn how these affect sharpness, contrast, etc., and you will be a better photographer. Not knowing these principles even if you using the most expensive lenses will render those less than sharp, etc, at times.
Everyone has their opinion, but here is one for those promoting the D850. I have a D810 am totally happy. The D850 is from the specs a great camera, but there is already talk in Japan about a D850 successor or an upgrade. Where's it all stop. If I were going to do it right now I would to with the Sony A9. Neither Nikon or Canon or Olympus or Pentax has anything this good in a professional mirrorless DSLR, unless you want to up the ante and go with the Leica, but then I think from the reviews the Sony is better.
With regard to D750 or the D7500 the following has to be considered in regards to quality. The D750 is 24MP, the D7500 is 20.9MP while too the D810 has 36MP. These higher MP cameras will render high quality if all other factors are the same.
Do not rule out the D810 for the prices for body only are really good right now. The D850 while excellent will fall as soon as Canon releases their next big thing, as usually happens, since Nikon has always lagged behind them. And if is very possible the next big thing for Canon will be competition to the Sony A9!
I gather if you want to buy the D850 you are a photographer who wants to do sports shooting, astrophotography, or time-lapse stuff. I have the D810 and am not going to buy the 1st Generation D850. I don't do any of these things, besides I can do astrophotography with the camera I have now and my Nikon 20mm F/1.8. If you are not going to do the things I mentioned, just want the newest and greatest (of course that circle is never going to change given all the upgrades that come out) or have money to burn. And if money is the case I would do it right and go with the Nikon D5 or go Leica SL. People often buy newer cameras to get the latest and greatest when it is really all about lens quality!