An ocular evaluation of a Barlow lens’ construction quality can be seen on its inside and external polish. The majority of high-quality Barlows feature housings made of high-quality aluminum with rubberized grip insulation wrapped around them. There is a non-reflective black surface and a brass compression ring to grip and secure the eyepieces inside the tube. Some of these lenses also include safety grooves to prevent them from falling. There are other threads on the thumbscrews that are strong enough not to pull or strip. Of course, securing an eyepiece with a quality brass ring does not require much force.