"Kenneth" <kdw### [at] earthlinknet> wrote:
> "Edouard Poor" <pov### [at] edouardinfo> wrote:
>
> > I agree with Ricky - while the POV focal blur system has it's flaws, I think
> > it gets the actual blur amount in front of and behind the focal plane correct.
>
> The concept of 'hyperfocal distance' has been with me since I was a teenager,
> when I used to shoot lots of B&W 35mm stills.  But I decided to look up the
> actual definition(!) To my surprise, there are *two* definitions, both equally
> valid (if slightly different as to the 'acceptable focus' range.) The first
> assumes setting the camera focus at a point somewhere off in the distance
> (though not at infinity) to get the maximum focus-range from 1/2 that distance
> *to* infinity. The other assumes setting the camera focus *at* infinity, which
> produces a nearly equal 'acceptable focus' range. (My own concept has always
> been the former one.)
>
> I looked at the actual equations for both 'hyperfocal distance' and the
> 'thin-lens approximation', and I'm still wondering if POV-Ray takes it all into
> account. (I'm no optician, so I'm still trying to get the hang of how the two
> equations are coupled together.) I guess I should just set up a POV test scene
> looking at objects that proceed off into the distance, and just *see* if POV's
> blur does mimic HD.
>
> One thing I did come across was that hyperfocal distance is intimately tied to
> the size of the film frame itself (or the size of the CCD sensor in a digital
> camera), in relation to the lens diameter or focal length--or something like
> that. For hyperfocal distance to have any meaning at all in the POV world, one
> would have to assume that POV's rendered image size (say, 640 X 480 pixels)
> *is* the film or sensor 'size'. I guess that's obvious, I don't know.

I understood the hyperfocal distance to be the same as you - where the most
distant area that could be considered to be in focus happened to be infinity.

As for calculating the hyperfocal distance in POV, you're right, the resolution
of the image can be taken into account - but that equates to the circle of
confusion (the size at which you cannot tell if a pixel is in focus or out of
focus). For 35mm film, that is often taken to be around 0.03mm. For a render,
it would usually be one pixel, but if you rendered a very large image and
displayed on a high res device (or printed it at very high res), it could be
more than a pixel.

The "film size" or "sensor size" also needs to be taken into account, along with
the lens size etc, for the actual calculations, but that's different from the
resolution of that virtual sensor or film.

Like I said, that's one of the improvements I'd like to make. I'll have a play
with the maths this weekend. I'm not actually very good at maths, but the web
has literally thousands of pages on this stuff, so it shouldn't be too hard.

Cheers,
Edouard.

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