page_title: Eyepiece
section_title: Field of view
hierarchical_section_title: Eyepiece / Eyepiece properties / Field of view
caption_attribution_description: English: 25 mm Clavé Plössl eyepiece. Français&#160;: Oculaire Plössl Clavé de 25 mm.
context_page_description: An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed near the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece.
  An eyepiece consists of several "lens elements" in a housing, with a "barrel" on one end. The barrel is shaped to fit in a special opening of the instrument to which it is attached. The image can be focused by moving the eyepiece nearer and further from the objective. Most instruments have a focusing mechanism to allow movement of the shaft in which the eyepiece is mounted, without needing to manipulate the eyepiece directly.
  The eyepieces of binoculars are usually permanently mounted in the binoculars, causing them to have a pre-determined magnification and field of view. With telescopes and microscopes, however, eyepieces are usually interchangeable.
context_section_description: The field of view, often abbreviated FOV, describes the area of a target (measured as an angle from the location of viewing) that can be seen when looking through an eyepiece. The field of view seen through an eyepiece varies, depending on the magnification achieved when connected to a particular telescope or microscope, and also on properties of the eyepiece itself. Eyepieces are differentiated by their field stop, which is the narrowest aperture that light entering the eyepiece must pass through to reach the field lens of the eyepiece.
  Due to the effects of these variables, the term "field of view" nearly always refers to one of two meanings:
  Actual field of view
  The angular size of the amount of sky that can be seen through an eyepiece when used with a particular telescope, producing a specific magnification. It ranges typically between 0.1 and 2 degrees.
  Apparent field of view
  This is a measure of the angular size of the image viewed through the eyepiece. In other words, it is how large the image appears (as distinct from the magnification). This is constant for any given eyepiece of fixed focal length, and may be used to calculate what the actual field of view will be when the eyepiece is used with a given telescope. The measurement ranges from 30 to 110 degrees.
  It is common for users of an eyepiece to want to calculate the actual field of view, because it indicates how much of the sky will be visible when the eyepiece is used with their telescope. The most convenient method of calculating the actual field of view depends on whether the apparent field of view is known.
  If the apparent field of view is known, the actual field of view can be calculated from the following approximate formula:
  or
  where:
   is the actual field of view, calculated in the unit of angular measurement in which  is provided.
   is the apparent field of view.
   is the magnification.
   is the focal length of the telescope.
   is the focal length of the eyepiece, expressed in the same units of measurement as .
  The focal length of the telescope objective is the diameter of the objective times the focal ratio. It represents the distance at which the mirror or objective lens will cause light to converge on a single point.
  The formula is accurate to 4% or better up to 40° apparent field of view, and has a 10% error for 60°.
  If the apparent field of view is unknown, the actual field of view can be approximately found using:
  where:
   is the actual field of view, calculated in degrees.
   is the diameter of the eyepiece field stop in mm.
   is the focal length of the telescope, in mm.
  The second formula is actually more accurate, but field stop size is not usually specified by most manufacturers. The first formula will not be accurate if the field is not flat, or is higher than 60° which is common for most ultra-wide eyepiece design.
  The above formulae are approximations.
  The ISO 14132-1:2002 standard determines how the exact apparent angle of view (AAOV) is calculated from the real angle of view (AOV).
  If a diagonal or Barlow lens is used before the eyepiece, the eyepiece's field of view may be slightly restricted. This occurs when the preceding lens has a narrower field stop than the eyepiece's, causing the obstruction in the front to act as a smaller field stop in front of the eyepiece. The precise relationship is given by
  This formula also indicates that, for an eyepiece design with a given apparent field of view, the barrel diameter will determine the maximum focal length possible for that eyepiece, as no field stop can be larger than the barrel itself. For example, a Plössl with 45° apparent field of view in a 1.25 inch barrel would yield a maximum focal length of 35mm. Anything longer requires larger barrel or the view is restricted by the edge, effectively making the field of view less than 45°.
caption: The Plössl, an eyepiece with a large apparent field of view
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