Please forgive what is likely a stupid question....
But rather than a dedicated crop sensor camera, could Canon not have a "crop mode" setting for say, the R5 and have it do the same thing a dedicated crop sensor camera does?
Seems that would be the route to go....give you everything you'd want in one body?
C
The way this actually works is - a lens of a given focal length and design makes an image circle of a certain size. So when you put a FF lens on an APSc camera, only the portion of that image circle is used - the rest falls outside the smaller sensor. This 'crop factor' means you only get the field of view of a lens that is equivalently longer. 1.6x is the factor IIRC. So your 100mm lens on APSc shows only the same field of view a 160mm lens would if it were on FF. If you put that 160mm lens on APSc, it would show the same FOV on the sensor as a 250mm lens on FF. Note though that the magnification of the image at the image plane is not any larger! Its only field of view that is condensed.
If your APSc sensor has the same number of pixels as your FF sensor, then you have more pixels in a given area of the sensor (the pixels are smaller than FF pixels in this case, because the same number fit in a smaller area). Therefore you have 'more pixels per duck'. in your 100mm APSc image than you would in your 100mm FF image. Since targets like ducks are small and far away, many bird shooters like this feature of APSc cameras.
Crop mode on a FF sensor doesn't achieve the objective of more pixels per duck, as stated above. It would just mean less pixels from the image sensor are used. In the example above that means a lower megapixel image when cropped to the same FOV as APSc. The two ways to solve that are 1) increase the pixel density of the FF sensor to match that of the APSc sensor you are comparing it to. Now you can crop the same FOV and have the same number of pixels. or 2) Buy a longer lens so the duck fills more of the image, such that there are an equivalent number of pixels per duck as the APSc example above.
1) is up to Canon to produce something with equivalent density (I think the R5 is getting close). Once they match pixel density of the 7DII, the APSc advantage goes away, (except maybe on cost!) 2) is costly and heavy!
Dedicated crop cameras were not really about this pixel advantage, though it was a niche realized as the technology developed. It was more about technology and cost at first. The bodies were not necessarily smaller, as the EF mount and flange to sensor distance was the same for crop and FF sensors. Though the mirror was smaller so there was some advantage there, and the lenses needed to produce a smaller image circle so they could be smaller as well.
-Brian