The name of Self focusing binoculars, or as they are also sometimes called Auto Focus Binoculars is somewhat misleading, as they should really go by the names of individual, fixed focus, focus free binoculars, or at least always in focus binoculars. This is because they don't auto or self focus at all, they are basically permanent focus binoculars that have no focusing mechanism and have been designed with a wide, but fixed depth of field from a reasonably close distance (usually around 40ft) to infinity. (They have a large hyperfocal distance.)
What this means is that everything from usually about 40 feet to infinity will remain in focus once you have set them to your eyesight and so you do not have to make any additional adjustments at all.
So how do auto focus binoculars work?
Well to achieve this the binoculars have been set in the factory to focus on a distant object and their optics use your eyes natural ability to focus. To set a fixed focus binocular for your eyesight, you adjust the diopter of each eyepiece to your vision (which is why they are also sometimes refered to as individual focus bins) and then from then on they require no further adjustments unless someone with a different strength of eyesight wants to use them.
Why all the confusion with the names?
The confusion in the name and the tag of self focusing binoculars has probably been brought about by clever marketing strategies from the major manufacturers of this type of binocular trying to make them sound more sophisticated than they actually are. What ever you wish to call them, Fixed focus, or always in focus binoculars have been around for over a few decades now they have their uses and there are some excellent quality binoculars of this type out there.
Advantages: Self Focusing Binoculars
The fewer amount of moving parts means they are much easier to make and therefore tend to be cheaper, more robust and therefore often much longer lasting. This also makes them much easier to make fully dust and waterproof. So as you can see a fixed focus pair of binoculars would be ideal for uses in harsh environments and so make popular military as well as marine binoculars.
Because of the fact that once you have set the binoculars to your vision and no further focusing is necessary, it does make using them quicker than binoculars where you have to focus on every object you look at. This is perfect for fast moving erratic objects, so in theory they could make ideal birding binoculars, so long as a good close focus distance is not important to you (Fixed focus optics usually have minimum close focusing distance of around 35 - 40ft).
The lack of a focussing wheel make the binoculars "less fiddly" and therefore are much easier to use if you are wearing thick gloves for example. So once again ideally suited for cold or harsh environments and make ideal Skiing Binoculars and again ideal for marine use in colder climates.
Focus free binoculars tend to have a good deep field of view.
Disadvantages of Auto Focus Binoculars
Always in Focus Binoculars sound great, but are considered to be a compromise in design, suited for convenience, but not well suited for work that falls outside their designed range or uses.
Auto focus binoculars (fixed focus) do not usually have a very close range and many will only focus from about 35 to 40 feet and that is why they tend to be more popular for use in wide open areas like the ocean so are ideal for Marine Binoculars and would not be suitable where a close minimum focusing distance is important - Butterfly Binoculars for example.
Fixed focus binoculars rely on the flexibility of your eyes to keep the image clear and in focus. This is not a problem for children and young adults, but as you get older the eye slowly loses its ability to focus and so for many people over the age of about 40, these binoculars could produce a lot of eye strain.
Self focussing binoculars do have their place and uses, but because you set them to you individual vision, they are less suitable for sharing with others (unless you have the same vision) So don't take them to share with someone else at a sporting event or safari holiday for example.
Automatic Focusing Binoculars were initially made popular by the brand "Jason" which was established in 1947 in Kansas City. The Jason brand made a group of self-focusing binoculars that became very popular in the 1970's and early 80's. The Jason brand of binoculars was then bought by Bushnell who have now re-branded them as their PermaFocus line of auto focus binoculars. As far as I know, Jason Binoculars are no longer in production.
Video Demonstrating the Bushnell 8x25 PermaFocus Focus Free Compact Binoculars
Featured Self Focusing Binoculars
Below are a few selected Auto Focus Binoculars (Fixed Focus)
The 7x50 configuration, tough body designed to withstand the elements as well as it's bright image, wide field of view and of course the rangefinding reticle, digital GPS and compass mark these Celestron binoculars out as the ideal companion for boaters, security and military personnel as well as hunters and especially those involved in search and rescue operations.
The fixed focus compact roof prism binoculars, come with a very comfortable open bridge design and are fully waterproof and filled with nitrogen gas to prevent internal fogging. They have a very wide field of view and produce a high quality image that makes these ideal take anywhere binoculars.
The New Bushnell 7x50 Marine Binoculars (13-7507) are designed for boating, but can be used in any demanding environment or where rock-solid durability is required. Not only are they completely waterproof, but they actually float and come with a host of features that includes a digital compass for true heading and range finding scale that allows you to estimate the range to objects of a known size.
The Steiner 7x30 Navigator Pro C Marine Binoculars are waterproof, have anti glare and haze optics, come with an integrated range finder and stabilized compass, are lightweight and compact, making them ideal for any small marine vessel. These focus free binoculars have Steiner's Sports Auto Focus system that keep all objects in focus from about 20 yards to infinity that allow faster location of navigational marks.
A tough, solid compact binocular that offers plenty of magnification, rainproof construction and maximum versatility. Refined design after years of use in a variety of settings, from mountaintops to deep canyons, aboard and ashore. For spotting game or exploring natureís majesty, Steinerís increased depth-of-view puts all the action in focus at once. Long eye-relief for eyeglass wearers.