I own a Canon Rebel T7i with only a kit lens and the APS-C sensor size is bothering me for some reason.
Is it a good idea to purchase a speed booster in order to take my APS-C sensor into a full-frame sensor, or should I just purchase a 35mm prime at f/1.4?
Another option is purchasing Canon's "Nifty Fifty", but then I am worried the field of view will be too "zoomed" or "cropped."
Can anyone give any advice?
A speed booster does not change your sensor. Not in any shape, way, or form. It changes the size of the image circle projected by your lens, making it smaller.
The usefulness of a speed booster is to combine lenses that cast large image circles, such as lenses made to be used on full frame cameras, with camera bodies that have smaller sensors. By reducing the size of the image circle, the entire field of view of the lens can be projected onto the smaller sensor.
For example, if I have a 35mm full frame lens and I put it on an APS-C camera the smaller sensor only captures the center portion of the image circle and the angle of view visible in the image will be about the same as a 55mm lens on a FF camera would give.
The following chart shows what happens when you use different combinations of lenses and sensors.
If your lens only projects an image just large enough to cover your sensor, then using a speedbooster will have the same effect on your photos as using a lens with a too small image circle: The image circle will not cover the full sensor and the edges and corners of your image will be dark.
I own a Canon Rebel T7i ... Is it a good idea to purchase a speed booster ...?
There is no focal reducer available for your camera.
... should I just purchase a 35mm prime at f/1.4?
Not all 35/1.4 lenses are the same. What do you need it to do? Perhaps a 35/2 or 35/2.8 would satisfy your needs.
If you were to switch to a crop-sensor mirrorless system, focal reducers could expand the usefulness of manual-focus lenses. For instance, a 50/1.4 could double as a ~36/1.2. However, they will not cure non-specific full-frame envy because the sensor remains cropped.
They trade off some aspects of image quality for others. For instance, edge and corner sharpness may be sacrificed for improved center sharpness. This could work well for portraits, but not for landscapes.
They don't work well with all lenses. They may introduce vignetting, unusual flares ("blue dot"), "glow", and other aberrations. You pretty much won't know whether a lens is a good fit without trying it. Some afflicted with lens-buying addiction may enjoy trying out different lenses. Others may find it frustrating.
The crop factor is not entirely canceled out, which requires 0.62-0.67x. Most focal reducers are 0.72-0.74x.
At wide apertures (~F1.4 and faster), the gain in light is less than a stop, limited by the reducer optics.
Michael C gave you a really nice answer about the speed booster. But I wan to address the other question.
Can anyone give any advice?
Well, stop reading things about the "disadvantages" of a cropped sensor because they are bothering you for "some reason", but it does not look for any particular reason.
Take your camera and shot. Enjoy taking photos, enjoy experimenting with light from a window, from the opposite window, lay on the floor, step up o a ladder.
KNOW your existing lens this is the most important thing.
I am worried the field of view will be too "zoomed" or "cropped.
You already have a 50 mm lens. Is it too zoomed or cropped for your intended usage? you should not just "worry", you should test it.
If you need a lens for a particular reason, which I am feeling is for indoor photography for group shots on low light you could get a 35 mm 1.8 lens, or use a flash.
Is a 35 mm lens for you? You can also find out with your current lens.
I doubt it is about the sensor size.
A "speed booster" concentrates a full-frame image circle to an APS-C size image circle, sort of the antithesis of a teleextender. If you have only an APS-C size image circle to start with (which a kit lens would certainly have since it would otherwise be overkill), only a central crop of your image would show a useful image and such an image size reduction in order to reduce noise is much simpler to do digitally.
So a speed booster only makes sense when buying at the same time one or more full-frame capable lenses. Even then, an actual full-frame body without speed booster will likely produce better images. And it's not like a fast lens that is APS-C from the start will likely deliver worse results than the combination of a full-frame lens and a speed booster.
So the main use of a speed booster is when for past or future reasons a number of your lenses happen to be full-frame and you want to get best value from them in spite of their mismatch with your APS-C body.
I recommend considering the Canon 24mm f/2.8 lens in addition to the 50mm nifty fifty. Ok, it's only f/2.8 but it's very small, cheap and also is sharp wide open. Compare that to a nifty fifty f/1.8 that becomes sharp only at around f/2.2 and the difference isn't very large.
A 35mm f/1.4 lens is quite expensive. For the money, you will probably get at least 24mm f/2.8, nifty fifty and perhaps some other lens.
I agree that lens options may be somewhat limited with an APS-C camera. Most lenses are designed for full frame cameras.
