Re: The main (theoretical) differences between APS-C and Full-Frame?
"While the aps-c sensor as a whole gets less light, isn't it the case that the amount of light per square centimeter of each sensor is the same in the two formats?"
The amount of light per unit surface correlates with the ISO, not the f value. One can reasonably expect that a cropped-sensor camera at a high ISO, e.g. 12800, should produce a noisier image compared to an equivalently framed image acquired at the same ISO with a full-frame camera.
"This means that the smaller sensor must be more enlarged than the full-frame sensor to reach the same image size. This is why the aps-c image shows more noise and less resolution. But it is not the case, as is sometimes implied, that a scene shot at f2.8 on full-frame requires an exposure of f2.0 on aps-c."
No, it is the case. This is what actually happens when a TC is added to a lens on a full-frame camera. The TC optically crops the image coming from the lens and projects the crop onto the entire sensor. When I add a 1.5x TC to a f2.8 lens, the camera tells me that my aperture is now effectively f4. With a 2x TC, the camera knows that the aperture is effectively reduced to f5.6 even if the aperture ring on the lens remains at f2.8.
Ignoring the optical artifacts added by the TC, one can expect that the above image obtained with the lens and a 1.5x TC should look like the uncropped f2.8 image obtained without the TC and subsequently cropped by the factor of 1.5 in post. That is, it does not matter how an image is cropped; the corresponding effective f value increases with the crop factor (1.4 = one stop, 2 = two stops, etc.).
All of the above applies to the same sensor or equivalent sensors. Understandably, if the sensor technology is different in two cameras, then it becomes difficult to know the actual differences in the signal-to-noise ratio between the two. What remains unchanged is the difference in DOF. For example, if the crop factor is 2 then equivalently framed images obtained using the cropped-sensor camera and a full-frame camera should have the same DOF when the f value of the former is 1/2 of the f value of the latter.
There are very different reasons for choosing or not choosing a cropped-sensor camera. My own reason for sticking to FF is the the MP count. I like the ability to crop 50 MP images from my A1 when this is needed. Buying a "pre-cropped" camera is not very attractive, from this particular point of view. For others, the obsession with ever smaller, lighter, and more compact camera systems may be a compelling reason to own and use a cropped-sensor camera. I personally don't care much about the size of my lenses or the weight of the entire system in my hands.
Jun 28, 2023 at 10:16 PM
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