Mikehit said:
The '15-feet' explanation was discussed ad nauseam several years ago and roundly disproved.
The '15 feet' thing was more of a back-calculation from the experience that shooting frame-filling with a lens between 90mm and 135mm on a 35mm format camera gave the correct perspective on facial features - shooting closer with a wider lens gave distorted nose/cheeks, shooting further away with longer lenses gave a flattened perspective.
The human visual system does not assess the full-length body from 15 feet and putting a 30mm lens on 35mm format camera and shooting full-length from 15 feet will give you horribly distorted body shapes. The human visual system will assess full-length from further away and at those distances perspective is less important.
To a large extent, I agree with you.
In full frame terms, I usually work use this "guideline"
35mm : Environmental Portraits
50mm : Full Body Portraits
85mm : Partial Body / Bust up Portraits
100~135mm : Shoulders / Headshots.
In crop frame terms (APS-C / DX), this would translate to:
20mm : Environmental Portraits
35mm : Full Body Portraits
50mm : Partial Body / Bust up Portraits
60-85mm : Shoulders / Headshots.
However, the focal length used will introduce distortion. To achieve the similarity of the field of view factoring the crop factor, a wider angle lens have to be used. If one uses say a 85mm on a crop frame body, the resulting image is a 85mm lens distortion effect with a ~135mm perspective effect by standing far away.
This become a kind of compromise issue for crop frame shooters. A 50-100mm lens on a crop frame body will result in a 50-100mm lens distortion effect with a ~80-160mm perspective effect.
For full frame shooters, a 85mm lens will result in a 85mm distortion effect with a 85mm perspective effect. An 1:1 ratio.
In my opinion, I would recommend a 85mm lens for crop frame shooters to enjoy less distortion but due to the crop factor, one has to stand further.
