Tube Size Matters – But Not Much

August 27 2013

#Hunting #Rifle scopes

Tube Size Matters – But Not Much

Scopes built on 30mm tubes are inherently brighter, sharper and just plain better than those constructed on 1-inch tubes.

Not really.

Brightness is a product of exit pupil (EP) diameter and anti-reflection coatings. The EP is the little circle of light seen in an eyepiece held about 18 inches from your eye and aimed at a bright wall or sky. It's diameter is determined by the objective lens diameter divided by the scope's magnification. Thus, a 50mm objective at 10X would yield a 5mm EP. At 6X the EP would enlarge to 8.3mm and at 4X it would be 12.4mm. At 50mm objective at 25 power would produce a tiny 2mm EP. These numbers are the same whether the main tube is 30mm or 26mm (1-inch.)

A scope's EP corresponds to your own pupil, which dilates from about 2mm in bright sunlight to perhaps 7mm in the dark. If your scope's exit pupil is smaller than your pupil, it can't transmit all the light you can use. If it's larger, the extra rim of light bounces off your iris and never enters your pupil to stimulate your retina. Wasted light. But an excess diameter of exit pupil does give your eye more room to wander around in without showing edge blackout, so that's something.

The reason a 30mm main tube scope isn't inherently brighter than a 1-inch scope is because both carry internal lenses much larger than 7mm, so there is no loss of light through either. The reason some 30mm scopes appear to project brighter views is probably because they were built with the absolute finest materials and  effective light transmission (how much light the scope passes through) determines brightness, is a product of the number of air-to-glass surfaces in the scope (the fewer the better) and the anti-reflection coatings on those lenses. (The more the better.)

So what, then, are the advantages of a 30mm scope tube? The walls can be made thicker for added strength and durability or the internal lenses can be made slightly larger, which increases optical performance simply because larger lenses always perform better than smaller ones, all else being equal. Alternatively, manufacturers can keep internal lenses the same size as those in their 1-inch scopes and use the extra internal room for long range reticle adjustments.


About the author

Ron is from a small town, farming family background in rural South Dakota. He began hunting small game and exploring the natural world as child, poking, prying, splashing and groping through the mud and weeds. Self-taught do-it-yourself hunter, naturalist, birder, wildlife photographer.

Ron has spent last 40 years as licensed hunter. He has hunted in South America, Africa, Asia, Europe, Australia, New Zealand.

Ron has photographed the natural world and its Wildlife around the globe; images published in hundreds of magazines, books, catalogues, ad campaigns. Self-taught birder with over 500 NA species on life list. Strong curiosity about and interest in native plants, habitats, geology, ecology and their impact on wildlife populations and behaviour. He is a student of ballistics and optics. Hand loader of centre fire cartridges and shot shells for 35 years.

Back to Top