The Field Stop: A Behind-the-Scenes Look at Binocular Field of View

The field of view (FOV) in binoculars is a crucial parameter determining the observable area. While magnification brings distant objects closer, it often comes at the expense of a narrower FOV. But have you ever wondered what dictates the exact boundaries of what you see through your binoculars? This is where the field stop comes into play.

The Hidden Gatekeeper: The Field Stop Explained

Image Credit: Amateur Telescope Optics (telescope-optics.net)

The Field Stop is the limiting aperture which defines the Field of View (FoV) of an Optical System

Consider the intricate internal workings of your binoculars or scope: Light hits the outer surface of the objective lenses, travels through all the lens elements within them, goes through the prisms, and only then does it finally reach your eyes after it travels through all the elements in the eyepieces!

The field stop acts like a gatekeeper within the eyepiece, regulating or controlling the portion of the image (formed by the objective lenses and prisms) that reaches your eye. So you can think of it like when looking through a window, the window frame edges limit how much of the scene you can see.

Here’s a breakdown of the field stop’s role:

  • Physical Location: The field stop is typically a physical diaphragm or aperture located where the objective and eyepiece focal lengths meet. This is because this is where the real image (from the objective lens) is formed. So with binoculars that have reticules, the reticule is usually located at the field stop point (not if more lenses are added). This is so that the image of the reticule lines as viewed from the eyepiece coincides with the same point as the real image coming from the objective lens.
  • Image Restriction: It restricts the light rays reaching your eye to those originating from a specific area within the image circle. This limited area defines the actual field of view you perceive.
  • Circular Shape: When you look through binoculars, the field stops appear as sharp edges that define the visible area. The field stop is usually circular, mirroring the shape of the human pupil.

Why Use a Field Stop in Binoculars?

Creating a binocular that delivers a high-quality image that also has a very wide viewing angle is very difficult to do and costly:

For example one of the most expensive and technically difficult aspects of binocular design is in designing and cementing the lens elements together, especially within the eyepieces. Making these more complicated with more elements not only costs more in terms of the amount of lens elements needed but it also multiplies the technical difficulty and thus expertise needed to pull it off.

So for lesser-quality optics, manufacturers will generally put a smaller aperture in so that more of the light that reaches your eyes comes from the centre of the lens and not the edges, thus they maintain a sharper view, but the trade-offs are a loss in light gathering and a narrower field of view.

Eye-Relief
Another aspect to remember is a narrow field of view tends to come with more eye-relief. Most wide-angle lenses have much less eye relief (shorter focal length to get that wider apparent angle of view). A long eye-relief is important, especially for those who wear glasses and thus there is a balance to be had.

This is why it is common in areas where glasses or some sort of extra eye protection is likely to be needed for binoculars not to have very wide fields of view: So with military binoculars (eye protection, goggles/gas masks etc) and even marine binoculars where uses will often use sunglasses and thus need a long eye-relief.

Connecting the Dots: Field Stop and Field of View

The size of the field stop directly impacts the field of view you experience:

  • Larger Field Stop: A larger field stop allows a wider portion of the image circle to reach your eye, resulting in more light and a broader field of view. This translates to seeing more of the scene at once, ideal for scanning landscapes or tracking moving subjects.
  • Smaller Field Stop: Conversely, a smaller field stop restricts the light rays, limiting the visible area and offering a narrower field of view. This might be preferable for situations where precise focusing on a specific detail is required or to keep manufacturing costs down.

Wide FOV and Design Trade-offs:

  • A wider FOV is often preferred because it allows you to see more of the scene without moving the binoculars.
  • However, achieving wide and sharp FOV without chromatic aberration near the edges is challenging and thus expensive. Sharpness and even image brightness can dramatically fall away towards the edges of the view on inexpensive binoculars that use lower-quality optics.
  • Therefore some optics manufacturers will purposely compromise FOV to maintain better image quality near the edge of view on some of their binoculars.

    Additional Considerations

    While the field stop plays a significant role, the relationship between it and the field of view isn’t always straightforward. Here are some additional factors to consider:

    • Eyepiece Design: The design of the eyepiece, particularly the number of elements and their arrangement can influence the effective field of view even with a specific field stop size.
    • Apparent vs. True Field of View: Binoculars often advertise the apparent field of view, which is the angular width of the view you perceive. The true field of view, however, refers to the actual width of the scene observed at a specific distance. These values can differ slightly.

    Summary:

    The observable area, or field of view (FOV), is a critical parameter in binocular selection. While magnification increases detail, it often sacrifices FOV. A lesser-known element, the field stop also plays a role in defining the viewable area.

    The field stop is an aperture placed at the focal length of the eyepiece. It restricts the field of view to the desired size and shape by regulating the light rays reaching the eye, effectively acting as a gatekeeper for the image circle.

    In summary, the field stop, eye relief, and design choices all contribute to the FOV and the overall viewing experience in binoculars. Finding the right balance between factors like FOV, cost and image quality is important.

    When choosing binoculars, consider the trade-off between magnification and field of view, keeping in mind that a larger field stop often translates to a wider viewing area, but can result in a lower-quality or less sharp image which is especially noticeable at the edges of the view on cheaper binoculars.

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    […] Quite often larger objectives at the same magnification results in a reduced¬†Field Of View (FOV). This may seem counter intuitive, but is because the larger size ripples through the entire optical design, forcing the use of bigger prisms and a bigger field stop to get a wider FOV, all of which costs more and gets technically more difficulat to do – for more see The Field Stop: A Behind-the-Scenes Look at Binocular Field of View […]

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