The first thing I would note is that for architecture and landscapes, you're unlikely to see a huge difference between a DSLR and a phone except in terms of being able to better pull details out of the shadows because of the higher bit depth and having better pixel-peeping-level image quality because of the larger sensor providing better light gathering. Landscape photography tends to involve subjects that are far enough away that you're shooting at infinity focus-wise, so depth of field is meaningless.
There are exceptions, of course, such as when there are architectural elements in the near field that are interesting (e.g. statues in a courtyard), or where the ability to change to a more wide-angle or telephoto lens lets you get a shot that a cell phone can't capture (e.g. shooting the spire of Notre Dame with a telephoto lens or taking a shot of a building with limited distance to the next building with an ultra-wide lens).
Either way, you're seriously limiting the usefulness of a DSLR by limiting yourself to a single prime lens, and that length of prime in particular.
Although the focal length is different between that lens (50mm focal length with 75mm-equivalent angle of view) and a cell phone (typically about a 25mm–30mm-equivalent angle of view), you'll be limited in how much of a difference you can achieve, because you're not taking advantage of what makes interchangeable lens cameras powerful — the ability to use different lenses with dramatically different focal lengths.
And a 50mm prime is great for portrait photos in semi-controlled conditions (where you can be close to the subject) to experiment with depth of field and framing, or for doing walk-around photography to concentrate entirely on improving your framing. But for what you're trying to do, particularly on a crop body, it seems like a poor match. You'll have to get a long way from your subject to get a shot of a building with a lens whose 35mm-equivalent field of view is 75mm. And indoors, you'll end up stitching a dozen shots more often than not.
With only a single prime lens, the only experiments you can really do involve aperture, shutter speed, and ISO (unless you want to do experimental stuff like deliberately making the subject out of focus). You can't do a lot with distance, because the size of the subject changes so much with distance that you miss the potential for dramatic differences.
So I would recommend that you consider adding a couple of lenses to your collection. I'm a Canon shooter, so I can't say what exact lenses you should pick. The following advice is based on the range of the lenses and the fact that your camera is a crop body. You should read about the various lenses yourself before settling on an exact model.
But basically, I would add:
- A general-purpose zoom lens with at least a 4x range (for a crop body, the two lenses that seem to fit the bill are the 16–80 and the 16–85).
- A longer zoom lens that tops out at... let's say 300mm or higher. (I count ten current lenses from Nikon that qualify.)
- For landscapes and architecture, an ultra-wide lens that can capture more elements in a single photo. For a crop body, the 10–20mm or 10–24mm seem like they would be good choices (again, based only on their zoom range).
That will give you a lot more flexibility.
But whether you're using a zoom lens or switching between several prime lenses with different focal lengths, these same experiments will yield interesting results:
- Experiment with reducing the ISO and/or exposure so that you can use a wider aperture to get a shallower depth of field.
- Shoot people walking past you at night with long exposures, tracking their motion with the camera. Try to achieve as crisp a shot of the subject as possible while creating a strong motion blur in the background.
- Change the distance to your subject (whether that's a person, a building, an architectural element, a statue, or something else entirely), using the zoom to keep the framing of your subject consistent, and notice how focal length affects the depth of field, and thus the softness of background elements.
- Change the distance between you and your subject, using the zoom to keep the framing of your subject consistent, and notice how the perspective changes — specifically how background elements become occluded by the subject as you get closer, and how background elements farther from the subject disappear beyond the edges of the frame as you get farther away.
- Shoot some sports and observe how shutter speed (or limits on shutter speed, if you prefer a more automatic setup) affects subject blur.
Note that if your subject is a building, many of these things may require driving because of the scale involved, and may or may not be possible in every situation because of other buildings that prevent you from achieving an adequate distance. This is why ultra-wide-angle lenses are so popular for architecture and landscape photography. :-)
