[ptx] new optimizer: things that might appear to go wrong

[Rik Littlefield]

A couple of people have reported cases where the new optimizer makes big 
changes to fov and a/b/c, even taking fov to large values > 180 degrees.

The basic issue seems to be that the new optimizer is sometimes able to 
find "better" values for parameters than the old one was, even when 
those parameters lie far from the initial estimates.  "Better" means 
that the error is reduced for the specified control points.  You might 
think that this is always good, but sometimes it results in 
"over-fitting" the data so that error is greatly increased in areas 
where control points were _not_ placed.

I want to reinforce advice that has been posted in other descriptions of 
how to use the optimizer:

1. Do not optimize a/b/c independently for each image, unless you really 
have used a different lens for each image.
2. It is safer to optimize pitch/roll/yaw first, then optimize lens 
parameters.

I hope this is helpful.  Please continue to let me know if the new 
optimizer does not behave as you expect.

Thanks,
--Rik