It could well be argued that a good eyepiece is the most important asset to the casual and more serious Astronomer alike. After all, a good telescope is important but will inevitably be changed or upgraded many times over the full course of your Astronomical life, where as a great eyepiece only needs to be bought once and will stay with you for many years (perhaps even forever if it's looked after well).
As purchasing eyepieces can at times be somewhat of a minefield it really pays to know which eyepieces to look out for and sadly as is the case with Celestrons omni plossl which ones to avoid like the plague.
Starting off with the technical specs, this is a fairly standard plossl with 4 elements (lenses), the lenses are grouped in identical pairs (both sets of two are equally spaced and positioned). It has a 50 degree apparent field of view, some plossls have 52 or even 55 degrees so the omni comes up a little short on fov. Dissecting the eyepiece reveals internal baffling which is a good sign (internal baffling refers to the use of dull matte paint in coating the inside of the ep, this helps keep the light from bouncing around inside the plossl and therefore yielding sharper more contrasted views). The very edges of the lenses have been blackened as well, further preventing any light scatter.
The barrell has been anodised which in this day and age you would expect from all eyepieces, this not only makes the ep more durable but makes it easier for the matte paint to stick to the aluminium barrell and stops it from easily scratching off.
Every lens that contacts the air (ie the two outer ones) is multi coated to improve light transmission which again is a good sign. All in all this little plossl is looking promising so far and appears to be well worth its £30 price tag, but oh dear, what is this? Things just took a major turn for the worst when attempting to look through the eyepiece.
I should mention before tearing into this ep that when I bought it I was still very much a newcomer to the hobby, had I known a little more about what I was buying I would have stayed well clear.
You see, the eye relief in a standard plossl (how far ones eye must be to see the whole field of view) is usually around 80% of the focal length of the eyepiece, in the 4mm it's closer to 150%. This is a necessity in such a short focal length, but even this boost in eye relief is not enough to make viewing through this eyepiece anything other than unpleasant. After a few minutes you're getting eye strain and headaches from shoving your eye a hairs width from the ep lens, not to mention that on an undriven telescope objects fly out of the field of view in mere seconds.
The field stop is absolutely tiny as well, I couldn't find the information so I measured it and wasn't at all suprised to see it was a miniscule 3mm. Imagine gazing through a pin prick at the Mona Lisa a few feet away on the other side.
It has to be said that based purely on image quality this plossl isn't bad, views are sharp and clear. Clearly the quality of the glass used, the internal baffling and lens coatings have come together to produce an eyepiece that is capable of some pleasing views in the budget category of plossl, however this soon becomes redundant when looking through the ep for more than a few seconds. It's awkward and in fact even painful to look through the 4mm omni plossl, every single time I've used it I've ended up with that type of headache you get when trying to read in the car.
In conclusion, it's a shame I didn't know all this prior to buying the ep. It is possible nowadays to get a decent eyepiece of a low focal length with a more comfortable field stop and eye relief, you will have to fork out a little more but believe me you'll be very glad you did.