Please Note: Subtension Accuracy with BTX Eyepiece & Tremor 4 Reticle The Tremor 4 reticle is calibrated for use with the 95mm objective lens, ensuring precise MRAD subtension measurements Using other objective lenses introduces measurement error: 115mm: ~1.6% error (Some may be ok with this small error for quick calls in the field) 85mm: ~18.7% error 65mm: ~19.4% error When paired with the 1.7x magnification extender, these errors increase proportionally: Example: 18.7% error becomes 31.8%For optimal accuracy, use the BTX eyepiece with the 95mm objective. Swarovski's new revolutionary BTX binocular spotting scope eyepiece brings unheard-of comfort to long range optical viewing. Compatible with all ATX/STX objective lenses, the BTX eyepiece module changes the way you'll think about spotting scopes. BTX FeaturesPerfect fit to SOC Objective modules - 65mm, 85mm, and 95mm objective modules are all supportedObserver now can use both eyes all the time when scoping, enhancing user comfort and ergonomics and providing amazing image qualityDoes not have a zoom eyepieceEyepieces are adjustable for interpupillary distanceRight eyepiece adjusts for diopterForehead rest provides enhanced user comfort and stability, and is adjustable for optimum viewing distance from ocular lensesIntegrated peep sight for quickly finding remote targetsSWAROVISION Field Flattener and High Definition Lenses give perfect clarity, eliminate distortion, offer increased eye relief, and bring out ulimate color contrastsSWAROTOP, SWARODUR, and SWAROCLEAN lens coatingsFunctional Temperature: -13° to 131° FNote: Reticle appearance may vary slightly based on objective lens size. Swarovski BTX Eyepiece w/Tremor 4 Reticle 49904