I shoot with 3 lens focal lengths, 35mm, 50mm, 85mm. If shooting on a crop sensor, to emulate this setup, you'd need a 24mm, 35mm, and 50mm lens. These correspond to moderate-wide, normal, and short telephoto. What do I mean by this? Normal means that size ratios line up with what our eyes are used to. Put a person 3 meters in front of a known object like a fire-truck, or a door, and the ratio of the two in how the lens renders them on the camera's 2d plane will look just as our eyes see. Wide means now that far object will look a bit smaller than it should, but since 35 is only moderate wide, you can kind of get away with it. In many situations with a person in the frame, 21/24/28 will look distorted and unnatural. I personally don't like them, but many do. Short-telephoto means that the ratio between objects will slant the other way. The firetruck would look slightly too large, for example. This can be used to one's benefit as well with very long telephoto lenses, keeping the moon huge behind a person, or compressing landscapes, but I find short telephoto gives the affect the way I want it. Off, but not too off as to mis represent reality.
The reason I keep to 3 primes like this, rather than a zoom that encompasses them all like the 24-70mm 2.8, or on a crop, the 17-50/55 from tamron/canon (both good), is because first and foremost, I routinely shoot in lighting situations where I need the extra light. All 3 of my primes are sub f/2 and useable at max aperture. The 35 on my 5D is f/1.4, the 50 and 85 both 1.8. My X100 is 35mm f/2. I find 2.8 too slow for many situations. Secondly, I like having the space between focal lengths. It makes the decision making easier. Expanded, normal, or compressed perspective? With a zoom it's more of a spectrum with lots of choices. Many people like that, but I don't. Creatively I only ever want one of those 3 settings, and having them in set steps helps me focus. In addition, having an out of focus background becomes more difficult the wider your lens goes. So while an 85mm F/2.8 can still give you a headshot with a blurry background, a 35mm f/2.8 at normal viewing distances doesn't really. 50mm 2.8 is kind of mixed bag depending on distance from you to the subject whether you will get background blur or not, but at any distance, F/1.8, 1.4 or even 1.2 will give you much more.
Now for some examples:
35mm really allows you to get a sense of this space. 50 or 85 would have compressed it too much. You would have only gotten a small edge of the front flower box with 50, less of the branches at the top, and the white area in the back would have been projected physically larger in relation to him, making it more of a middle or high key shot instead of this smoky dark look. Backing up to make 50 work would have made him much smaller and it would no longer really be a portrait. 85 would have made all these issues worse. 35mm gives you a sense of setting.
This one needed a lot of size ratios going on, and it was already unrealistic and surreal to begin with, so I didn't want to muck with perspective at all, hence the 50mm. 35mm to keep him this size and the front bench would curve away, and the back benches would be very small, and the path wouldn't look very deep or three-dimensional. 85mm would not have shown the side-most benches at all, and the background would look very large and tunnel like. In order to get the whole bench in, I'd need to back up a lot from where I was with the 50, and that would require him being much smaller in the frame.
This one, the background was very important to the shot. That far building over his right (our left) shoulder is the Dom Cathedral in Cologne, Germany. With a 50 or 35mm, it would have been projected incredibly tiny, and the darker areas around him of the less populated residential area of Cologne would have shown more, making it a less bright and less city-ish feel. He was also on a cramped balcony and I was inside the apartment shooting out. A 50mm might have allowed me to still get all of him without showing the door frame or other parts of the balcony, but the 35 definitely would have unless I got so close as to make him look very distorted.
I chose these three shots specifically because I had decided on the lens focal length before even getting to the location to shoot (or from the original location scout) but they were specifically picked for their affect on perspective. Also because all three relied on fast, sub-f/2 apertures, as all 3 were shot at the widest aperture, either to let in enough light on the 2nd two, or the provide a bit of background blur on the first. Could I have shot these same images with a zoom and had them look very similar? Possibly, but my creative process doesn't work that way, and I like the freedom that this limitation actually brings creatively.