The Nikon D3300 is a compact digital. This camera’s image sensor measures approximately 16mm height by 24mm length. This format is called APS-C (Advanced Photo System – Classic). The focal length of lenses we fit to cameras is based on the format size. First we compute the corner to corner measure of the format rectangle. This works out to 30mm (for your camera). This format, when fitted with a 30mm lens is said to deliver a “normal” view. The term “normal” translates to an angle of view of 45° with the camera held in the horizontal position (landscape). Additionally the perspective delivered by such a lash-up is said to match the human experience -- thus the tag “normal”.
When we change lenses or zoom away from the 30mm “normal”, we can choose a shorter lens to gain a wide-angle view. Such a lens would be 70% of normal or shorter. This works out to 30 X 0.7 = 20mm or shorter. If we choose a focal length 2X “normal” we enter the realm of telephoto; that’s 60mm or longer.
What I want you to know is, once you move away from 30mm, with this format, your images will yield a view that is not the normal perspective. That’s OK because preserving a “normal” perspective is unimportant for the majority of pictures we make. In fact, violating the “normal” perspective is often praiseworthy. As an example, when doing portraiture, we benefit if the chosen focal length is a moderate telephoto. The preferred focal length range is about 2X thru 2.5X of “normal”. For your camera that’s 50mm thru 80mm.
Now, everything in photography has its reasons. The 2X thru 2.5X rule of thumb is based on how people perceive what they look like. Our view of ourselves comes from the makeup or dressing mirror. Things in the mirror appear to be behind the mirror, double the distance you are from the mirror. We use a moderate telephoto to duplicate this view.
The real point is, if you choose too short a focal length for portraiture the facial features will likely be distorted. The usual result is a slightly enlarged nose and petite ears. This distortion can be minuscule however they are a big deal when it comes to the human face. What I am saying is, use a moderate telephoto, this forces you to step back when doing portraiture and this mitigates facial distortion.
As to the buttery effect: This comes about when the depth-of-field is shallow. A short lens has a vast depth-of-field, just what you don’t want. A long lens has a shallow depth-of-field and this is what you want. This dovetails into the rule of thumb I have describe. Additionally, a lens with a large working aperture like f/1.4 or f/2 or f/2.8 delivers a shallow depth-of-field. One trick to achieve the buttery effect with a “normal” is to have the subject extend their hand a few inches forward of their face. You then focus on the hand by partially depressing the shutter button. Keep the button half depressed and then compose your subject. The focus point in front of the subject stays in place, until you complete the snap. This technique throws the background out-of-focus however the depth-of-field carries to keep the subject in focus. Try it, you’ll like it.