For those 3 lenses you mentioned, is the f stop(s) you mentioned the only one available.. or is it just the largest aperture size that you can set?
Whenever an f-number is quoted in the name of the lens, it
always just the
maximum aperture of that lens. The lenses can be stopped down — the minimum aperture is usually written in the full specifications of the lens. I believe all lenses I mentioned can be stopped-down to f/22; maybe an oddball can be stopped-down further, but even if that option is available, you’re probably better off using the widest aperture that gives you the depth of field you want, due to
diffraction kicking in at small apertures.
I didn't know that lenses for FF cameras as that much more!
Well, now you know ;~)
The reason? The lens is obviously what directs light onto the sensor, and it has to direct the light in a way that it hits all of the pixels on the sensor. When you have a bigger sensor, the lens has to “cover more ground” — get light onto a larger area. That in turn means the lenses are bigger. And the simple rule applies here: bigger = more expensive.
I just feel way more natural when I look through the FF cameras' viewfinders.
As a general rule, when you have a bigger sensor inside the DSLR, the viewfinder is also bigger. So that may be what you noticed when you tried a full-frame DSLR — it probably had a bigger and brighter viewfinder than the D3300. That being said, there are bigger and brighter viewfinders still with the APS-C sensor size. I see that as one of the biggest advantages to the D7200 over a D3300 or D5500.
The entry-level Nikon models have what is called a
pentamirror viewfinder, while the D7200 and higher-end bodies have a
pentaprism viewfinder. Without going into detail as to why, the pentaprism viewfinders are brighter, but more expensive. If you try a D7200, you should see that its viewfinder is definitely nicer than the D3300’s. If the viewfinder in the D3300 feels too small and dim to you, then you may very well return it and buy a camera with a better viewfinder.
While you’re at it, also try out some good electronic viewfinders, like that of the Olympus OM-D E-M10 Mark II or Fujifilm X-T10. The Sony α6000 — while it’s a very capable and very popular camera — doesn’t really have a great electronic viewfinder.