2. A crop sensor body does not turn a 200mm lens into a 300mm lens, and crop sensor bodies do not "stretch" faces out less. The reason you might think this is true is that you're generally standing farther away with a crop body than a FF body (see point 1).
So repeat after me: focal length does not affect perspective. At all. Ever.
Get your camera and any zoom lens. Frame a head shot with the lens fully extended. This may involve a walk as Aaron did. Take a photo. Zoom to the lens's shortest length and take another.
On your computer later, examine the head shot and compare that with the same area in the other.
You don't need a person, I've done this using a kit lens and random stuff on the café table as I enjoyed a coffee.
You would use a wider lens for a group portrait or an environmental portrait.
If I were a wedding photographer I'd happily photographer the happy couple on the doorstep of their church with a TS-E17. I'd position myself to capture the church, with the lens shifted up, with the camera held level.
Of course, this needs to be tempered according to the size of the church, but the couple will look completely fine.
Perspective has nothing to do with focal length. The reason wide angle lenses show more perspective distortion than longer lenses is that you're standing closer to the subject. That's it. Crop a 20mm shot 10x and compare wit the same image shot FROM THE SAME PLACE with a 200mm. Perspective is identical.
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