Optics 101
OVERVIEW
In this guide we hope to give you the knowledge base you need to select the perfect optic for your application as well as giving you an understanding of how each item in our specifications list affects the optic you are looking to purchase. We will be adding to this guide continually, so check back often for updates and additional information.
The diagram to the right (or below on mobile) shows the core parts you will find on most binoculars - use this as a reference when we are describing different parts of the binocular. At its most basic level the objective lens gathers light while inverting the image, that light goes next into the prisms which correct the inverted image (rotate and flip it back to the correct orientation), which then goes into the ocular lens which magnifies the original image. The focus wheel moves a lens in the housing to bring the image into focus while the diopter adjusts for differences between the left and right eye. Most of our optics come with strap attachments, and a method to attach to an adapter.
MAGNIFICATION
Magnification will be one of your first considerations when it comes to selecting an optic. Are you (relatively) near or far away from the object you want to view? Is the subject you are viewing moving?
In the case of wildlife viewing magnification is very important. If you are cataloging the birds in your backyard for instance you will be relatively close to your subject and (being a bird) it is likely to move about quickly. For this application you would want a low magnification - this will increase your field of view and make it easier to find your subject - and see identifying characteristics. If you look at the diagram below you may be looking at a pronghorn, because both sexes can have horns one needs to find a dark cheek patch to signal the animal is male. For this application you may want a mid to high magnification for making that determination.
FIELD OF VIEW
Field of view (FOV) shows up a lot in our specs as well as in articles about what optics to buy, but what does it mean? Simply it is just the amount of space you can see through the optics measured in degrees.
You can see in the diagram to the left the higher the magnification the smaller the field of view. It seems important to note that the largest drop in field of view is between the 9x and 11x (or 8x and 10x in other models). Optics with a wide field of view equate to being better able to find your subject, following moving targets, and are less likely to magnify shaking from the user.
According to Wikipedia a typical human visual field is over 200° with both eyes (though much of it is peripheral)[1]. These B.2's are rated at 7.4° (7x). All are measured at 1000yds. At 1000yds every degree of view is equal to 52.5 feet. So If you take the amount of degrees and multiply by 52.5 you end up with the horizontal distance you can see in your optic. The last spec that you’ll see FOV in is Apparent FOV; this is the FOV in degrees multiplied by the power. In our example to the left the 7x comes out to 51.8°. This number describes your field of view through the optic.
[1]Wikipedia contributors, "Visual field," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Visual_field&oldid=993364595 (accessed December 14, 2020).
OPTICAL PERFORMANCE
Another consideration when choosing your optic is the category of optical performance - these measurements give you digestible numbers to show how much light is making it to your pupil. This is generally represented by a percentage of light and three common calculations to show how much light is moving through your binocular. The light transmission percentage is determined by a machine that measures the entire optical system. This can be a bit misleading as usually, you get the maximum measurement and not the full visual spectrum.
The first formula, Exit Pupil, is determined by dividing the size of the objective lens by the power. In an 8x42 binocular 42/8 = 5.25. This represents the diameter of the beam of light coming through the binocular eyepiece.
In the diagram above you can see that the size of the objective (along with magnification) dictates the diameter of the beam of light hitting your pupil. Ideally, the exit pupil should be about the size of the pupil of your eye. In general, larger is better – an exit pupil of greater than 5 is considered best.
The second formula is Brightness; this is the exit pupil squared. So in an 8x42, this number is 5.252² = 27.56. Greater than 25 is considered ideal.
The last formula is Twilight Factor, this is another formula for brightness/resolution in low light for a binocular – because it is a more complex measurement, some feel it’s a better representation. This is calculated as the square root of (objective diameter x magnification). So in an 8x42 bino, 8x42=336 – √ 336 = 18.33. Again, the higher the twilight factor, the better the resolution in low light.
PRISM TYPES
The prism assembly serves to correct the image so that it doesn’t appear flipped. It also shortens the optic to make it more compact, streamlined, and lighter.
PORRO-PRISM
This is the prism system used in traditional, old-school binoculars. The objective lenses are offset from the ocular lenses and are set farther apart than the ocular lenses. These binoculars are simpler to build and usually less expensive to make. Porro-prism binoculars produce a high quality image and have great light-gathering capabilities. They are, however, heavier and significantly less durable than newer optical systems, so high-end binoculars made with Porro-prisms are less common.
SCHMIDT-PECHAN
These prisms are oriented in a straight line and are located within the optical tube. This creates a smaller, more compact, and durable binocular. Because this prism splits the light wavelengths (which are then recombined with the lenses), phase shift can occur. Because this needs to be corrected light gathering qualities and image contrast can suffer. For this reason, high quality roof prism binoculars are typically more expensive due to improved coating technology.
ABBE-KOENIG
This reflective prism system is used primarily in telescopes and more recently in some high-end binoculars. The Abbe-Koenig prism bends the light 3 times as opposed to the more common Schmidt-Pechan prism which bends the light 5 times. This creates a mid-sized binocular that produces the same high quality image and light gathering capabilities as a Porro-Prism, but is more compact and durable. Paired with high quality coatings and lenses, this optical system produces a very bright image.
