> I need a bit of help with the focal blur module. I am trying to render a simple
> scene with 3 vertical bars in the front and 3 horizontal ones at the back. I'm
> having trouble with setting the focal length in the scene (focal_point does not
> seem to work for this). All the scene parameters (for eg. distances of objs from
> camera, their thickness etc.) are in terms of focal length. How do I set it as a
> param and use it to define all other distances? Also generally aperture in
> cameras is defined as a fraction of 'f'. How does the aperture param in POVRay
> correspond to real apertures defined as such?
>
> Another doubt: As I am trying to increase the aperture and putting my
> focal_point on the vertical bars in the front (ref: images in the ZIP), the bars
> at the back start "splitting" into parts. Any thoughts on how this could be
> happening?
>
> Thanks in advance!
>

The breaking up of the bars is caused by the first few samples all 
falling outside of the objects. In this case, POV-Ray "think" that that 
area already got enough samples.
The cure is to use the two parameters form for the number of samples to 
take.

When you use only a single parameter, you only set the MAXIMUM number of 
samples to take, and, in some cases, it leads to what you see in your 
examples.
When you provide two parameters, you explicitly set the minimum number 
of samples to use.
The syntax is :
blur_samples Minimum_Samples, Maximum_Samples

The default minimum number of samples to use is set to 7 if you use more 
than 7 samples, and 4 otherwise. In your case, when using larger 
apertures, it's not enough. Try something larger in the 9 to 20 range. 
Use the smallest value that make the problem go away, as larger values 
will only result in longer rendering times.

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