clipka <ano### [at] anonymousorg> wrote:
> Am 06.08.2016 um 13:25 schrieb pkrskr:
>

> No, there is no such deviation through refraction.
>
> First of all, it may be necessary to clarify a potential cause for
> misunderstanding: POV-Ray does /not/ actually simulate a lens at the
> position shown in the diagram.
>
> Instead, POV-Ray uses the "virtual aperture" as the ray origin
> /specifically/ to /avoid/ having to simulate the lens. The whole idea of
> the virtual aperture is that (to the extent required by POV-Ray) it
> gives the same results /without/ the lens as the true aperture does
> /with/ the lens.
>

Understood!


> I'm not sure I understand exactly what you are asking.
>
> In a simple camera with no diaphragm, in which the lens opening also
> constitutes the effective aperture, the aperture plane does indeed
> coincide with the lens.
>
> Consequently, in such a scenario, jittering of the ray origin does
> indeed occur at the lens plane, and across the entire lens opening.
>

This was what I was trying to say earlier, about the method being similar to
simulating a camera where the aperture stop and the lens are effectively at the
same location.

> (Shooting rays toward P on the image side is not necessary, as P' is
> computed from P using a quite simple formula, and there are no
> intervening objects between the aperture and the image plane.)

Yes this is true. I used the 'tracing both sides' phrase loosely. Apologies for
that.


>
> In a scenario with a diaphragm in a different plane than the lens, this
> simplification does not hold, as we would have to place the virtual
> aperture at different X/Y coordinates depending on the point P (compare
> the diagram, in which the red ray -- which should be at the center of
> the virtual aperture -- passes through the lens at an offset from the
> camera axis).

This is the most interesting point. In your first explanation, you mentioned
that the red ray is originally drawn through the center of the nominal camera
location (which is the virtual aperture), and I thus presumed that the ray went
through the pole of this virtual aperture (since we were tracing through a
pinhole). However, now that you mention it, I see that the pinhole ray was
traced through the center of the real aperture and thus happens to be shifted in
X/Y on the plane of the virtual aperture and the jitter is applied across this
shifted point. This is indeed different from the above scenario where the lens
and the diaphragm are at the same plane.

However, could you clarify how the locations of the real aperture and the
virtual aperture are decided? And does this mean that the nominal camera
location is the position of the real aperture?

Thanks for spending time on such detailed explanations. You have been very
helpful.

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