“Correct” perspective is not critical for most images. Portraiture is an exception. Things close to the camera reproduce large and things far from the camera reproduce small. If the camera is worked in too close, the nose reproduces too large and the ears too small. This is the dreaded facial distortion that makes portraits look weird.
Factorial: An image will display correct perspective if viewed from a distance about equal to the focal length multiplied by the magnification used to make the displayed image.
Say we make a portrait imaged with our FX camera. The frame size is 24mm X 36mm. This is a miniature image that must be enlarged to be viewed and scrutinized. We make an 8 X 12 inch image and display it. To make the 8 X 12 we were required to magnify (enlarge), the FX frame 8.5X.
We mount an 85mm. lens to do this task. The viewing distance for correct perspective becomes 85mm X 8.5 (magnification) = 723mm (28 inches). In other words, a portrait taken using an FX camera with an 85mm lens is enlarged to make an 8 X 12 inch print is suitable for viewing and the viewing distance becomes 723mm (28.4 inches). This image presents “correct” perspective.
Now we mount the 85mm on an FX. The frame size is 16mm X 24mm. To make the same 8 x 12 inch print we must enlarge this miniature frame 12.7 X. This magnification is 1.5 X greater than used to make the same image with the DX camera. With this lash-up the viewing distance becomes 85mm X 12.7 (magnification) = 1080mm (42 ½ inches). If this image is viewed from about 42 inches, the image displays correct perspective.
If we choose to view the 8 X 12 image made with the FX from 28 inches, the same as the image made using the DX, we must mount a shorter lens to achieve “correct” perspective.
The math is 85 ÷ 1.5 = 60mm (rounded). We mount a 60mm and make the portrait. The displayed print is viewed from 60mmn x 12.7 (magnification) = 762mm (30 inches).
Bottom line, when we a mount an 85mm on the FX and a 60mm on the DX, we can make a displayed images that when viewed from the same distance (about 30 inches) display similar perspective.
We choose the subject to camera distance to obtain a desired linear perspective. In portraiture correct linear perspective is the major contributor when it comes to pleasing the client. Fist we select the camera position then we select the focal length. Subject distance defines the perspective. Next we select a focal length. Focal length defines the how the principle subject is positioned within the frame. Short focal lengths reduce the size of the principle subject at the price of including more of the surroundings. Longer focal lengths enlarge the principle subject thus excluding the surroundings.
Linear perspective is a function of camera to subject distance. The liner perspective remains the same regardless of focal length or format size. The answer is: If the camera position is not changed, the perspective perceived is the same regardless of focal length or camera format.