BINOMANIA

Reviews and field tests of optical instruments

Internal or external focus?

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In recent years, one of the most frequently asked questions from Binomania readers concerns the focusing system of binoculars. Is internal or external focusing better? Before writing my opinion, I think it's useful to briefly summarize what focusing is and how it can be achieved. 

I'll begin by saying that focusing is the ability to obtain sharp images thanks to the mechanical movement of (some) elements of the optical train. There are two types of focusing: external and internal.

EXTERNAL FOCUS

External focusing is used on Porro prism binoculars (not all). It's achieved by a carriage that moves the eyepieces up or down. The advantage is that it's inexpensive. The downsides are numerous:

  • Cheap trolleys flex and this problem can cause misalignment
  •  During the movement phase, not only external air is sucked in and extracted, but also small particles of dust and humidity. Owners of some telephoto zoom lenses (pump-type) understand this problem well. 
  • Even the most valuable products – which benefit from excellent mechanics (Nikon EII, Nikon SE, Swarovski Habicht) they will never be completely waterproof.
  • Mechanical parts tend to wear out more, as they are more exposed to atmospheric phenomena
Image: A close-up of the central carriage that moves the eyepieces of the Nikon E II

This problem can be solved by using eyepieces with a focusing system on each eyepiece, but this results in a loss of the ease of tracking subjects at short to medium distances. Binoculars that use this technology are mainly nautical, military, and astronomical binoculars. 

Image: Astronomical binoculars Oberwerk BT-100XL-SD It offers single focus on each eyepiece, as is standard practice in this sector.

 

Some companies offer Porro prism binoculars with individual focus on each eyepiece, which they call "always in focus," when, in reality, they exploit the owner's visual accommodation, and the depth of field of binoculars, as I recall, is equal to the square of the magnification. In practice, a hypothetical 8x30 fixed-focus model will have a total depth of field starting at 64 m. However, thanks to the visual accommodation, you can still perceive detail reasonably well even at 20-30 meters, but in the long run, these binoculars will become tiring to see.

 

INTERNAL FOCUS

Image: Zeiss Victory HT and Zeiss Victory SF obviously have internal focusing
Image: Opticron was one of the few companies to offer a Porro prism with internal focusing

Internal focusing binoculars have two main advantages. They usually allow for a shorter focusing distance, and they are also completely waterproof. Thanks to the presence of O-rings, manufacturers prevent air exchange with the outside, as well as humidity and dust. The only downside is the higher production cost, given the presence of micromechanisms that move the internal elements. 

Another positive aspect of internal focusing is the longer life of the components, since they are not subject to the elements, impacts, and wearing materials such as sand or salt. Internal focusing is used on all binoculars with roof prisms, although sometimes the manufacturer may decide to include them in binoculars with Porro prisms. I recall, for example, the late Minox BD 8×44 BP, or the’Opticron HR 8×42 WP.
 

With this focusing system, it's helpful (but not necessary) to fill the inside of the optical housing with nitrogen, argon, or other gases. This will prevent internal condensation from forming, which could be caused by humid air seeping into the binoculars. Some binoculars designed during World War II circumvented this problem by providing small pockets for the silica gel.

Image: a cross-section of the Zeiss Victory FL, where the internal mechanism that moves an optical element that allows focusing is also visible



DIOPTER COMPENSATION

It's common in both systems but can be designed differently. The cheapest designs usually feature a simple dial on the right eyepiece, with an adjustment range of +/- 4D, but there are cheaper products that don't exceed three diopters.
Two more valuable solutions include the presence of an integrated ring near the central knob (such as the Swarovski EL or it Zeiss Victory SF) or on the opposite side of the focusing rotor (such as the Zeiss Victory Pocket)

WHICH ONE TO PREFER?

In most cases, it's a choice dictated by budget. Those who prefer economical solutions will often purchase binoculars with Porro prisms and a central carriage. In other cases, however, depending on the preferred field, it may be useful to prefer a single focus on each eyepiece to ensure perfect stabilization (nautical observation) or a more precise focus (astronomical observation).
Nowadays, the most demanding birdwatching and hunting enthusiasts purchase high-performance binoculars with roof prisms which, among other advantages, offer internal focusing, which is increasingly precise and high-performance.