I ordered two of the Celestron Omni 15mm Plossls along with two Celestron Omni 12mm Plossls for use in a Binoviewer.
The field stop in an eyepiece is the component that gives the edge of the field a crisp black border. I have had 30 or more different Plossls over the last 15 years from a variety of manufacturers, and ALL of them had field stops.
The Celestron Omni 15mm Plossls oddly did NOT have field stops and this gave a very poorly defined edge of field. The Celestron Omni 12mm Plossls that I ordered at the same time DID have field stops.
I honestly do not know if this was a defective batch or if they are supposed to be this way. I returned them as defective though because as I mentioned, having owned at lest 30 Plossls over the last 15 years, this was the only pair that I have ever owned that did NOT have a field stop, and I think that it was a manufacturing defect in this entire lot.