I know that the effective focal length of the 50mm FX lens is actually 50 x 1.5 = 75mm because I shoot with an APS-C sensor.
Not exactly. Not "incorrect", but you have to understand what it means. The 75mm will not be a useful number on DX. The 50mm lens is NOT 75mm, and there is no real reality called effective focal length. It is a purely hypothetical concept. No matter what lens (at any zoom) has only one focal length, where it actually focuses the light to infinity.
The 50mm lens is still only 50mm, whether it's a DX or FX sensor. It's 50mm, end point. That's why it is marked 50mm. 50mm is the only focal length it has.
Now, you may want to compare his field of view to another lens on another sensor. And it's true that the small size of the DX sensor reduces its field of view, so that (if with a 50mm lens) its reduced field of view compares to that of a 75mm lens on a film frame 35 mm see (or an FX frame has the same size as a 35 mm film). But if your lens is marked 50mm, there is still a 50mm lens on any sensor.
The effective focal length only concerns this other lens on this other sensor (35mm film), only because this other 75mm lens seems to have the same field of view on a 35mm film (or the same FX format) as the 50mm lens of the DX sensor. The focal length on the DX sensor is 50 mm. Only this other lens measures 75mm and on this other larger sensor, it has the same field of view as the 50mm on DX. We are talking about two different lenses and two different sensors.
The fact is that many people have used 35mm film for years, even decades. They are very used to what a 50 or 75mm lens will see and do on a 35mm film. Their experience just knows.
Today's smaller digital sensors are changing things (smaller field of view of smaller sensors). This smaller sensor requires shorter lenses now, to see the "same width of view" as that of larger 35mm films. So, their experience only knows (still) about their new camera. The goal of this "effective focal length" is to compare, to tell users familiar with 35mm film what a lens will do on their new cropped sensor. If we say that this 50mm lens works on DX, just as we are used to 75mm on 35mm film (in regards to the field of view), so it makes sense to them, they know what to do with it. ;expect. However, if you are not familiar with the use of a 35mm film, the effective focal length of a 35mm film will probably not be helpful.
Effective focal length published with objectives for smaller sensors always compare to a 35mm film size (same as FX size, called Full Frame). However, we can compare the field of view of any two sensor sizes. For example, imagine 1/2 inch and 2 inch sensors (movie maybe). The largest is 4x larger than the smallest, so the framing factor is 4x, and (with the same lens), the larger one will have a field of view 4x wider than the smallest one and will need to. an effective focal 4x longer to see. the same reduced field of view as the smaller one. Different sensors can have different shapes (3: 2, 4: 3, 16: 9), so the crop factor actually compares the diagonal of the images.
For FX and DX, this ratio is 1.5.
A similar report was still true for different film sizes, but it was only up to FX and DX Digital that we were able to use the same lens on different size sensors. This is therefore becoming a topic of discussion today.