Posted by
Joachim Wesner on
Oct 02, 2006; 7:29pm
URL: http://imagej.273.s1.nabble.com/More-on-pixel-shifting-tp3701425p3701429.html
Hi there,
>Sadly, if you use "zero pixel shift filters" does not mean that you will
>have zero pixel shift. This is because the magnification of the
>objective+tube-lens varies slightly over the spectrum (I think this has
>also been posted, recently). To make things worse, the shift produced by
>this effect varies over the field of view. E.g., if the magnification of
>two channels differs by only 0.1%, and you take a 1000 pixels wide
>image, the shift between the two channels on the left side and on the
>right side of the image differs by 1 pixel.
>For most objectives this effect is particularly prominent when you use
>blue-violet dye (e.g.DAPI) together with a green/red/IR dye.
Very true, in a traditional "finite corrected" system, even APO objectives
had a very
large amount of "lateral color", which would only be corrected in the
eyelens ("ocular")
resp. there were special "photo oculars" to correct for this in case you
used any kind
of camera.
Because making infinity corrected objectives that are "perfect" on their
own in any regard
would require a lens design of prohibitive complexity, usually any large
manufacturer
has it´s own design philosophy regarding what is already corrected in the
objective
(i.e. astigmatism, lateral and logitudinal color, curvature of field, coma,
spherical
aberration and so on) and what still needs to be compensated by the tube
lense (and probably
even by the ocular, but the latter compensation clearly is not used so much
any more today
because of problems with attached cameras - sometimes the approach is that
the central area of
field that is seen by todays small sensor is nearly perfect without the
ocular, while at the
outer edges of the field up to a diameter of 20-25 mm some aberration are
still handled by
the eyelens)
Therefore, you should clearly not mix infinity objectives from DIFFERENT
manufacturers, YET,
it also forces ONE manufacturer to try to induce the very same amount (in a
simplified way)
of aberrations in all objectives of it´s line for a perfect cancellation of
those after the tube
lens. This is not easy, as the "natural" aberrations of a certain design
will clearly depend
on magnification, aperture, free distance from from lens etc.
Sincerely
Joachim Wesner
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