Should I get a 35mm lens for Nikon DX, or use a focal reducer with a 50mm?

Question

I shoot with a Nikon D5200 (DX/APS-C) and want a "normal" field of view. A 50mm lens on DX gives about the same framing as a 75mm lens on full frame, so I’m considering either:

- buying a 35mm lens for a roughly 50mm-equivalent view, or
- buying a 50mm lens and using a focal reducer/speed booster.

My thought was that the 50mm might be more future-proof if I move to full frame later. Are there real drawbacks to using a speed booster for this, and is that even a practical option on a Nikon F-mount DSLR? Which route makes more sense?

user95237 · Answer

Speed Boosters

The primary design intent of speed boosters is exactly what the name suggests. They make the lens "faster" (i.e. provide more light to the image plane).

They are primarily an alternative to increasing ISO and secondarily an alternative to spending lots of money expensiver faster lenses. Being an alternative to higher ISO's sets the bar for optical quality...potential image degradation via the optical path is traded off for potential image degradation due to the noise of higher ISO.
The changes to field of view is a side effect (and often a desirable one).
The primary market for speed boosters is video/film making. There is more latitude for overall optical quality because images move. And noise reduction in post is a computationally expensive calculation which has to be done 24 or more times per second of video. Also variations in noise artifacts between frames can be problematic.
Keep in mind that slow shutter speeds are far less of an option for video due to frame rates, and even less of an option for movie making where a consistent 1/50 second is desirable.
The standard for video/film making is Super 35. This is approximately APS-C. It is not full frame. One of the motivations for speed boosters is to bring Micro 4/3's cameras closer to Super 35. This simplifies thinking about lenses. Also the smaller Micro 4/3's sensors tend to have weaker low light performance due to smaller size.
Speed boosters make more sense when the one stop of speed is the difference between a $4000 cinema lens and a $8,000 cinema lens. In part due to cost, in part because cinema lenses come in series with consistent controls to make setting up camera rigs easier.
The wider image provided by a speed booster is less important for traditional video and film making because the visual language tends to favor tighter crops. There's less time to ponder details at the edge of the frame when frames are moving 24 or more times per second. Also, tighter crops mean smaller sets...less to build, less to light, more space for everything that has to happen off camera.

A Little Gear Philosophy

When in doubt, get the simplest thing that might work. If that's a 35mm lens, then that's simpler than a 50mm lens + a speed booster.
Avoid buying gear for cameras you don't have. Between now and the time you get a full frame camera, you probably will have learned something and often learned enough that you will want to make pictures that you haven't thought of yet...in the future you might find that 50mm equivalent is not your preferred focal length or that you actually prefer zooms or even that your preferred upgrade is to medium format or Micro four-thirds.

Remarks on Normal Lenses

Traditionally the normal lens is a focal length equal to the diagonal of the image plane.
The normal lens of 35mm or full frame is ~43mm.
50mm is "normal" for 35mm film in the sense that there is a visual language built up around it because Oscar Barnak used a 50mm on the first Leica designs and everyone copied that.
The closest match normal lens for APSC with 1.5 crop is 28mm (42mm equivalent).
A 35mm lens is about as close the 35mm film normal of 43mm as 50mm is. Or to put it another way, a 35mm equivalent lens (24mm on APS-C) is about as normal as a 50mm equivalent lens. Just in the other direction.