An Infusion of Knowledge

Though I didn’t learn a lot about birds during yesterday’s birding workshop (and we didn’t go out bird watching, thanks to the weather), I learned a lot.  So, in lieu of an attempt at writing a creative post, I’ll record here a bit of what I learned.

First, we learned about binoculars.  There are two main types of binoculars:

  • Porro prism binoculars, in which there is a jog in the path of light between the front lenses and the eyepieces.  The front lenses are generally set further apart than the eyepiece lenses, though in some compact Porro prism binoculars that is reversed.
  • Roof prism binoculars, which appear much simpler, but which have a much more complex light path and which require far greater precision in manufacturing, as illustrated in the images below (which I borrowed from Wex Photographic’s website).

From Wex Photographic’s website

Magnification and Objective Lens Size

As shown in the image above, the objective lens is the lens which receives light from an image.  And that leads me to the odd numbering that I have never been able to fully understand.  Binoculars are identified with numbers like 7 x 35 or 8 x 40 or 10 x 20.  The first number is the power of magnification. The second number is the size, in millimeters, of the objective lens, which controls the amount of light that enters the binoculars; the higher the number, the more light is allowed in and the greater the potential resolution of the image.  A set of 8 x 40 binoculars magnifies images by eight times and has a 40 mm objective lens.

Exit Pupils

The utility of binoculars in relatively low light is a function of the exit pupils, which are the small bright circles in the center of the eyepiece when you hold the binoculars away from the eyes and up toward the light.  The exit pupil is calculated by dividing the size of the objective lens by the power.  So, for example, the 8 x 40 binoculars has an exit pupil of 40/8, or 5. For people over 50, the size of the pupil in dark conditions is considerably less than younger people, which is why older people cannot see as well at night.  If a younger person’s pupils open to 7 mm in dark conditions, an older person might only open to 5 mm in the same conditions. A beam of light wider than the pupil cannot be seen, so a pair of binoculars with an exit pupil greater than 5, then, would be of no benefit to the older person.

Other Factors

Field of view is a measure of the breadth of the visible image.  The greater the magnification, the smaller the field of view. It is expressed as an angle or as the width in feet of the image at 1000 yards (420 ft.).

Eye relief is the distance back from the eyepiece one’s eye can be and still see the whole field of view. Normal eye relief for binoculars ranges from 9 to 13 mm, but people who wear eyeglasses should get binoculars that have long eye relief, which manufacturers define as 14 mm or greater.


Porro prism binoculars tend to be less expensive than roof prism binoculars, but roof prism binoculars tend to have better optics, are better waterproofed, and are more rugged.

The general advice about deciding between the two types comes down to money, the presenter told us.  He advised not to buy cheap roof prism binoculars; a similarly priced Porro prism pair would be a better way to go, he suggested.  But he says it’s possible to get a nice pair of Porro prism binoculars for $500, though the best quality instruments would begin in the $1500 range, for the premium brands, which he said are Zeiss, Leitz, and Swarovski.

Several comments were made about weight of binoculars.  The best set to buy, some said, is the lightest one with the best optics and the price suited to your budget.  The factors to consider were said to be:

  • How do they feel in use?
  • Is the image sharp & clear?
  • How does the center image compare to edge image?
  • Does the image have any color fringe, particularly in poor light (chromatic aberration)?
  • Is there adequate weather-proofing?
  • Are the binoculars in your price range?

Another factor to consider is the type of straps to use.  At the very least, a neck strap should be wide and soft.  At the best, a harness that allows you to let the binoculars hang down from low on your chest was suggested.

Yet another factor was the minimal focus distance, that is the closest an object can be to the objective lens and be in focus; shorter is better.

As to where to buy, high praise was given to Eagle Optics (www.eagleoptics. com).

My sister-in-law already bought a pair of Nikon-brand roof prism binoculars in the $500 range.  My wife and I will look for a good pair of roof prism binoculars, probably in the same price range.

Other topics of conversation included birding field guides, general birding references, smart-phone apps valuable to birders, etc.   All in all, I’d say the workshop was extremely informative.  I’m glad I went.


About John Swinburn

"Love not what you are but what you may become."― Miguel de Cervantes
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2 Responses to An Infusion of Knowledge

  1. Yep, that’s what I meant and what we’ll buy. Text corrected. above.

  2. Carol Morgan says:

    Hi John – My binoculars are Roof Prism – not Porro. Just sayin’. 🙂

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