hello,the step up rings made of aluminum,stable, precise and durable construction. 

Hello, this is a high end filter kit. Of course these filters are made of optical glass. Regards. 

No. This filter holder can only hold 2 filters. I believe the more prudent course would be to get a filter that is rated for more stops and can replace two filters that are rated for less stops. There are filter holders that do hold 3 filters but they are smaller than the K&F filter holder (which is 100 x 100 mm). And in my opinion, the quality of those 3 fitter holders that are located on Amazon is less than the K&F system. 

Yes, you can add filters in this holder, two to be exact and as far as the thickness of the filter goes, I have no idea. I own several different name brands of filters which I use in this holder without any problem. All the filters I own fit nicely into the holder and for myself, I have never really given any thought to the filter thickness, however you will be able to fit only one filter per slot. Sorry I cannot be of more help but this filter works great for my photographic needs. 

If the filter size for your lens any of these adapter ring ( 52mm/ 58mm/ 62mm/67mm/ 72mm/ 77mm/ 82mm) then yes. 

I've been using ND filters for years, don't need to wrap it. All you need to do is keep in the pouch; handle with care. Use the same spray liquid that you would use on a lens if needed other than that same lens cloth to keep it clean. If you get a few more square filters a filter pouch is a nice addition. 

I did no use the leather pouch. I purchased a Neewer shoulder pouch. It held everything and easy to get in and out. I carried the 10ND and a 3 ND soft graduation filter. 

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hello , we are the seller, you could use an adapter to step the 52 down to a 49 , there is a adater ring kit https://www.amazon.com/dp/B00JL4HGAI,