Are we overcomplicating discussions around focal length in portraiture re distortion?

[Tom Pinchenzo]

Discussions around which focal lengths you can use for portraits without distortion seem centre around focal length and framing e.g. you can use a 50mm for head and body, 85mm for head and shoulders and 105 and up for tight headshot. Surely a simpler way would just be to say e.g. distortion kicks in at 3m. Forget about framing and focal length - if you get within 3m, you'll start getting distortion. I feel like for beginners this would be a much easier way of explaining it.

Just a thought rattling around in my lock-down brain.

 

[woof woof]

I don't think 3m is a good starting point as many of my pictures will be taken at less than that distance and to me don't show any significant distortion. A geek may nit pick and may even be wrong or it may be a question of taste whilst things may need to be a bit more extreme before normal people notice any distortion.

There's also the possibility that distortion may actually be something we're aiming for for pictorial reasons.

I think rather than having strict rules about distance or even the amount of distortion that may be present and if it's noticeable or not or desirable or not it's maybe more important to get people to really see what's in the VF or on the screen and understand what's going on and why as once the person pressing the button can see what's happening and why they're in a much better position to make decisions.

 

[Tom Pinchenzo]

I don't think 3m is a good starting point as many of my pictures will be taken at less than that distance and to me don't show any significant distortion. A geek may nit pick and may even be wrong or it may be a question of taste whilst things may need to be a bit more extreme before normal people notice any distortion.

There's also the possibility that distortion may actually be something we're aiming for for pictorial reasons.

I think rather than having strict rules about distance or even the amount of distortion that may be present and if it's noticeable or not or desirable or not it's maybe more important to get people to really see what's in the VF or on the screen and understand what's going on and why as once the person pressing the button can see what's happening and why they're in a much better position to make decisions.

Yeah, you're right - there isn't a distance where distortion suddenly happens. 3m was a bit of a number plucked from the air. And I guess it depends on the tolerance of the photographer as to when distortion becomes unacceptable. But in that case, adding the element of 'acceptable distortion' makes the discussion even more complicated! And yes, sometimes distortion is intentional (I have a 2 year-old and close-up wide-angle shots of her are cuuuute). I just feel like it's easier to say 'forget about focal length, distortion is proportional to distance.

 

[sk66]

I think the distance correlation is more relevant than FL is, but most do not understand it...

Most of human vision is based upon monocular/2D cues. It is only at short distances that binocular vision has any significant impact (i.e. depth perception). And the issue arises at distances where the camera's monocular/2D view/recording of the scene conflicts with our binocular/3D view/experience (the brain's interpretation/conversion of it). Binocular vision is most significant w/in arms reach and rapidly decreases over distance; it is almost entirely negated by ~ 5m.

** for a typical human stereopsis reaches about 1* of offset/accuracy at 3.7m, and 1* is also the angle of maximal retinal sharpness/resolution... if I had to describe a cutoff point for perceptible "distortion" it would be near this.