Thank you very much for your fruitful information..... So if I have understood... I may go with the yongnuo 35mm and the Tamron 18-400mm....... Is it going to be good ??? Thanks a lot for your help.
The Tamron 18-400 ???
Are you planning to sell your 18-135 and replace it with the 18-400?
Personally, I think a SUPER zoom sacrifices too much image quality to get that super wide zoom range. Though I have not looked at the reviews of today's super zooms. Optical technology keeps advancing and giving us what we could never have before. Heck back in the film days, we could not even dream of an 18-400 lens (29-640 FF equiv). That was a science-fiction zoom range.
However, in the no free lunch category, is weight.
The 18-400 is 710g, the 18-135 IS STM is 480g. That is 230g heavier or 8.1 oz or about half a pound heavier.
That means every time you pick up the camera + 18-400, you are picking up a half pound more weight than your 18-135.
For a single lens it might be heavy, but it is lighter than carrying 2 lenses (18-135 + 100-400), so it could work fine for traveling.
If you are shooting in that transition range of 100-135mm, the 18-400 lens is more convenient than switching back and forth between two lenses.
Bottom line is how does the single 18-400 fits into YOUR style of shooting vs the two 18-135 + 100-400 lenses.
Personally I would get 2 zooms; the Canon 18-135 + Tamron 100-400 zooms, to get you out to 400mm.
That is because 80+% of the time, I am at 135mm or less. So I am carrying a half pound less gear. And for me, weight matters.
As for the Yongnuo 35mm f/1.8 lens, I have recommended to the yearbook advisor to get ONE, to try out. This will give the kids a fast f/1.8 lens to use for indoor gym sports. If the lens works out, I will recommend getting a couple more. It is the only reasonably priced fast Canon crop prime out there, that I know of.