Of course, it’s true that a full frame camera provides noise-free (cleaner) images in low light when shooting at the same ISO on both the APS-C and full frame cameras. If the pixel count is constant, a full frame camera typically has larger pixels (photoreceptors), and these gather more light. This will result in a stronger image signal that needs less gain. Therefore, with a full frame camera, you can increase the ISO settings confidently.
However, by using the APS-C system, we get the benefit of shooting at a lower ISO. And if you are using full frame in a low light situation, you may be required to shoot at the widest aperture (assuming you don’t want to increase ISO), which means you might run the risk of getting images that are not entirely in focus.
Let’s assume we are doing an outdoor portrait session with no artificial lights with low available light. Our goal is to achieve a nice shallow depth of field. Using the Fuji APS-C system with the 56 1.2, we would set our aperture at about 1.2 to get a similar depth of field that the full frame lens produces at 1.8. Assuming the exposure setting is 1/125, 1.2, and ISO 200 to achieve the desired bokeh and correct exposure on APS-C, we’d need to be at 1/125 shutter speed, 1.8 aperture, and ISO 320 to achieve a comparable exposure setting on the full frame. The ISO needs to be set at a higher level on the full frame (200 times 1.5 = 300, so the closest ISO is 320). So even though the APS-C is a smaller sensor, we get the benefit of shooting at lower ISO than when we’d be shooting on the full frame.
In short, the larger sensor will produce less noise when shooting at the same ISO as on APS-C, but you might get by with a lower ISO on APS-C, thereby reducing your noise. Unless you are shooting astrological images, you might not really need the noise free benefits of a full frame sensor. For most photography use cases, you’ll achieve noise free images with either format.
As far as resolution is concerned, APS-C vs full frame cameras is almost identical thanks to the latest technological advancements. But the sensor is what truly sets the full frame apart. A full frame camera is equipped with a sensor that’s more than 2.5 times the surface area of a normal APS-C sensor. This attribute gives it the capability of having larger pixels.
Larger pixels and more pixels translate to the sensor’s ability to take in more light. Hence, the sensor can read the scene more accurately, reducing the chances of digital noise. If the amount of light reaching the sensor isn’t sufficient, digital noise can occur, and the ability to reproduce an image is weak.
But if you are using a full frame camera with a maximum aperture on your lens of only 4.0, and shooting in low light conditions without an artificial light source, then you are going to be forced to crank up your ISO. Unfortunately, going to a higher ISO setting will increase noise.
Fast lenses for APS-C (apertures of 1.2 – 2.8) are less expensive than the equivalent speed lenses for full frame. Shooting with a faster lens means you don’t have to crank up the ISO, and you won’t get noise on APS-C. So, to some extent, this dispels the need to to use full frame sensors to achieve less noisy image