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Celestron's basic Plossl comes in the shape of the Omni. It uses 4 lenses (or elements as they're sometimes known) in a symmetrical layout, that is to say two groups of two lenses spaced closely together. This is a classic Plossl design that has pluses and weaknesses.
On the plus side the use of only 4 elements makes images crisp and clear whilst keeping the price of the eyepiece down. On the down side this configuration of lenses yields only a 52 degree field of view. The 52 degree field would matter more on a higher focal length but at 12mm objects are small enough to be held within the field longer than a few moments.
As this is a medium powered eyepiece it will get used not only on the moon/planets but also on deep sky objects such as nebula and galaxies. The small field of view and the magnification will make it useful on all but the largest objects (such as the veil and M31). As this will be used on very faint and elusive objects outside the solar system, contrast is very important. The blackened edges of the lens elements help wring every bit of contrast out of this Plossl.
As well as contrast a good eyepiece should transmit as much light into the observers eye as possible. Celestron have used their renowned lens coatings to keep images bright, especially important for detecting galaxies.
Optically the Omni performs well, there is no significant light scatter and objects stay sharp throughout the entire field. Where the Omni really excels is contrast, especially apparent when observing the Lunar surface. Craters are dark in shadow and the various ejecta found near recent (astronomically speaking) impact sites is bright and defined. Colour rendition is okay but not great providing a somewhat colder view than other eyepieces. Saturn in particular looks almost drab due to the lack of strong colour in this eyepiece.
Visually the Omni is a mixed bag, while images are crisp and contrasty accross the whole field the lack of colouring has a profound effect on viewing pleasure, making the planets and moon seem almost sterile.
Comfort wise the Omni has a rather disappointing eye relief of 8mm (eye relief being the maximum distance you can position your head away from the objective lens). This makes viewing quite uncomfortable and I doubt whether this eyepiece would be enormously useful to anyone wearing glasses.
Strong points of the Omni include crisp, sharp and well contrasted images and a reasonable objective lens size while poor points include lack of colour rendition, a short eye relief and a lack of a rubber grip ring to help prevent dropping when fingers are cold and numb. There are better eyepieces out there in the £40 price band but it isn't completely useless.