"Why do you care? It has the field of view it has. There is no reason to make 35mm some standard of reference, especially if you have never used a FF camera."
Gold star answer.
Hi and welcome to the forum:
As you can see this is a subject fraught with challenges! I would offer the following article I wrote on the subject that is based on physics, material from a professor in photographic optics, and has been peer reviewed.
Focal Length, Sensor Size, Equivalence and Field of View
I will simply say, for the discussion, that a lens' focal length is a physical characteristic of a lens. It does not change, whatever type sensor is behind it, from what is written on the lens. However, what the sensor captures does impact the final result. This result is the called Field of View (or Field of Capture) and should always be identified as such. The image area recorded by a smaller sensor will be smaller: however, when magnified in a screen or print to the same area or to a common size (e.g. both formats printed in an 8"x10" format), will seem to render a result similar to using a longer focal length lens. The magnification is not done in the lens.
In an effort to encompass this phenomena, and explain it in terms of the lens alone, writers and reviewers have coined the term 'crop factor', and applied it to the focal length, but actually it is applied to the Field of View, and this is where the confusion begins. The lens is not 'cropping' the image, the sensor is. Arguably, Ernie is quite correct in saying that all images are cropped, because the image projected by the lens is actually circular - as they were in the early Kodak box cameras:
However, the vernacular term 'Full Frame' came about as the 35mm format became a de facto standard in the mid- to- latter part of the last century - basically for its combination of decent imagery, combined with the smaller, lighter cameras and high-quality lenses that could use it. Certainly that dominance was driven home by the proliferation of the 35mm SLR camera. Smaller size films (and later sensors) thus became known as 'cropped'.
To put it another way, in the context of your question: "Would I still add this multiplier or is the number the same as on the box for RF-S lenses?" If using an APS-C camera, you multiply the focal length of ANY lens by the crop factor (RF or RF-S) to compare it to the focal length, if using a FF camera, to achieve the same Field of View . So, a 50mm lens (be it an RF or RF-S lens type) will create a FoV equivalent to an 80mm lens on FF camera. Conversely, if you want to put a lens on your crop-sensor camera that will yield the same "normal" FoV as a 50mm lens on a FF camera, you divide the required value by the crop factor. Thus, to get the same FoV, 50mm/1.6 = 32mm to get a 'normal' FoV.
The name APS-C (Advanced Photo System type-C) actually comes from a move by a consortium of film makers in 1996. For a full explanation of the the format see Advanced Photo System - Wikipedia. The format was essentially overtaken by the rise of the digital camera, and so died out pretty quickly, but was reborn as a digital sensor format.
