>> What if it automatically gave you circular-cross-section spheres 
>> regardless of
>> the pixel ratio?  How many people *want* elliptical-looking spheres when 
>> they
>> change the pixel ratio?
>
>  What happens when you *want* to render the image for a resolution with
> non-square pixels?

You specify in some way that you want this, rather than it being the default 
behaviour.

>  (Yes, there are such situations, and they are pretty common. Search for
> "anamorphic" if you don't believe me.)

Whilst they may be pretty common, I can't believe they are as common 
compared to people who change the aspect ratio for rendering as a desktop 
wallpaper or printing out on certain sized paper.

>  And if the aspect ratio of the pixels were always assumed to be 1:1 when
> changing the resolution of the rendered image, which dimension or the 
> camera
> should change when the aspect ratio of the image changes?

The scene file author should get to decide.  How about the following syntax:

camera{
 hfov 90
}

This would fix the horizontal field of view to 90 degrees, POV would 
automatically calculate the vertical FOV to ensure square pixels based on 
the image aspect ratio.  This would work if you made some landscape scene 
and didn't want any more to be visible left/right, or any part to be cropped 
left/right, by rendering at different aspect ratios you just get to see more 
or less sky.

Of course the scene author could also use just the "vfov" keyword if they 
wanted the vertical FOV to remain fixed and the horizontal to be calculate 
automatically.

Then, of course you could specify both "hfov" *and* "vfov" if you really 
wanted both directions to be fixed, this would allow for non-square pixels. 
POV *could* even warn you that this was about to happen (maybe just once for 
each scene file, IDK), and give you three options: 1) render non-square, 2) 
adjust the horizontal image resolution to give square pixels, 3) adjust the 
vertical image resolution to give square pixels.

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