Full Frame vs APS-C for Photo & Video

Here we are - the big debate - do you actually need full frame as a hybrid shooter or can you use APS-C professionally and obtain the same results? This wasn't designed to be an exhaustive test by any means, more of a practical comparison "in the field" using the Canon R5 C and the Fujifilm X-S20.

Criteria: I wanted to find out several things. First, how much noise do you get on a full frame, high megapixel sensor, vs a smaller APS-C sensor? How many stops of light do you lose when using APS-C instead of full frame? Does stopping down a cheaper full frame lens (shooting at f/8 for example) ruin your images, and would you be better off using an APS-C sensor but with a faster lens? I took sample footage and images at several ISOs - 400, 800, 3200, 6400 and 10000. The other settings were the same, with a SS of 1/125 and fairly neutral WB & colour profile. The only difference was that I wanted to see how the Canon R5 C performed at f/4, against the Fujifilm X-S20 at f/2.8.

ISO noise: this was the first thing that surprised me. At identical ISOs, both cameras seemed to produce the same amount of noise. Whether at 3200, 6400 or even 10000 - neither camera was noisier and both cleaned up very well in Adobe Lightroom. This was a shock, as I thought the APS-C sensor would be much worse. They were so similar in noise profiles that I couldn't tell which was which by the end.
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High ISOs: to match the Fujifilm's faster lens, I ended up stepping up the ISO of the Canon R5 C and this is where things got interesting. The R5 C overall encouraged me to shoot at a slightly higher ISO, and the camera fared excellently - of course there was light noise, but given the easy cleanup in post production, I found that I preferred the sharper, retouched images of the R5 C against the more bokeh-licious, large aperture images of the Fujfilm. The Canon image was extremely flexible, even at an ISO of 16000. The Fujfilm didn't fare badly, but handled breakup in the shadows slightly worse. On the whole, though, I tended to overexpose rather than underexpose - as recovering shadow detail on both cameras was worse than highlight recovery.

Shadow detail: this is where both cameras showed their strengths. The Fujifilm produced a great "out of camera" image that you could easily send out as is. The Canon, on the other hand, lost a lot of detail when being underexposed. When using a high ISO, however, it really showed its strength as a full frame camera and was extremely useable. Perhaps my reluctance to shoot at high ISO on the Fujifilm is due to historical experience (their X-T2 for example was quite bad for shadow detail and noise), but once I relaxed and did this on the Canon R5 C I found it a much better experience - instead of trying to compete with the look of the Fujfilm.

Shallow depth of field: at the end of the day, it really comes down to depth of field. When noise and flexibility in post production are similar, the reason for purchasing fast lenses comes down to a specific look - i.e., do you need shallow depth of field or are you fine with sharpness all around? There are different ways of obtaining shallow depth of field, of course, whether through using a tighter lens, even a telephoto, or standing your subject closer to you and away from the background. But essentially, achieving a shallow DOF on a full frame camera will end up being more expensive, as you will need a lens that has a fast aperture and will give you that look. On APS-C, it tends to be easier to get this look as those lenses will be cheaper and more accessible.
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Filters: one thing I did notice especially on the APS-C sensor, but also on the full frame one, is that mist filters are extremely exaggerated when shooting wide open. This might give your image a "cakey" / "muddy" look across the whole frame. I ended up stopping down both cameras when using filters as the images were too hazy.