Kit and Optics
Birdwatching Fundamentals
Course 1 · Chapter 2 · Kit and Optics: Seeing Further
Birdwatching Fundamentals 1 listed the basic kit. This chapter looks at the three items that cost the most thought: binoculars, a spotting scope and a field guide. The most useful skill is reading the two numbers printed on a pair of binoculars, such as 8×42, and knowing what they trade off against each other.
Reading the Numbers
According to Wikipedia's Binoculars article, in a description such as 8×42 the first number is the magnification and the second is the diameter of the objective lens in millimetres. The article says the objective diameter "determines the resolution (sharpness) and how much light can be gathered", so a larger objective gives a brighter, sharper image at the same magnification and quality. The cost of a larger objective is weight: the article says a 40–45 mm objective works better in low light but makes a heavier binocular than a 30–35 mm one.
Magnification
The first number. The article says 7× to 10× is typical for hand-held use, because higher powers need steadier support.
Objective diameter
The second number, in mm. Bigger gathers more light but weighs more.
Field of view
Per the article, in general inversely proportional to magnification: more zoom, narrower view, harder to find a moving bird.
Exit Pupil
The article defines the exit pupil as the objective diameter divided by the magnifying power, and says an exit pupil matching the eye's pupil (about 7 mm at night, about 3 mm in daylight) makes the best use of the light. Applying the formula by hand:
| Binocular | Calculation | Exit pupil |
|---|---|---|
| 8×42 | 42 ÷ 8 | 5.25 mm |
| 10×42 | 42 ÷ 10 | 4.2 mm |
| 10×50 | 50 ÷ 10 | 5 mm |
Prisms, Coatings and Eyeglasses
| Feature | What the article says |
|---|---|
| Porro prisms | Z-shaped configuration; wider bodies with offset objectives and eyepieces |
| Roof prisms | Objectives almost or totally in line with the eyepieces; narrower and more compact. Schmidt–Pechan designs are less bright than Porro or Abbe–Koenig at the same specs, because of mirror-coated surfaces |
| Coatings | Anti-reflective coatings reduce light lost at each surface; "fully multi-coated" means all air-to-glass surfaces have multi-layer anti-reflective coatings. Roof prisms also need phase-correction coatings |
| Eye relief | Over 16 mm "should be adequate for any eyeglass wearer"; over 17 mm may be needed for thicker frames |
| Image stabilisation | Allows binoculars up to 20× to be hand-held, but stabilised models tend to be more expensive and heavier |
Spotting Scopes and Tripods
Wikipedia's Spotting scope article describes it as a compact, lightweight, portable telescope, usually mounted on a tripod, used for birdwatching among other things. It says magnifications usually fall between 20× and 60×, since higher powers reduce brightness, narrow the field of view and increase shake. Objectives are "typically between 50 and 80 mm", larger being heavier and dearer. Most eyepieces are removable or zoom, and either straight-through or angled (usually at 45 degrees). Because the image is sensitive to vibration, scopes are normally mounted on a tripod, or occasionally a monopod.
Field Guides
Wikipedia's Field guide article says a field guide helps readers identify wildlife and other natural objects and is meant to be "brought into the field". Popular guides are usually a browsable picture guide organised by family, colour, shape, location or other descriptors; scientific references follow taxonomic order. It says Peterson's 1934 bird guide placed paintings of similar species side by side with arrows marking the differences, called the Peterson Identification System, and quotes his wife Virginia Marie Peterson that artists can emphasise key features better than photographs because drawings can omit clutter.
Not Yet Verified, and Sources
- Sources. Binoculars: first 100,000 of about 169,000 characters read, so later sections were not. Spotting scope and Field guide: the fetch tool returned summaries of the pages; I did not see the full text.
- Not in the articles. Why 8×42 is popular, the advice to test in a shop, and the ordering of scope after binoculars (check).
- Not covered. Prices, weights and brands; the articles give none and none are stated here. Sound-recording kit and apps are left to a later chapter.
Exercises
Work out the exit pupil of an 8×32, a 10×32, an 8×56 and a 12×50, using the formula in this chapter. Which comes closest to the daylight eye pupil of about 3 mm, and which suits the night pupil of about 7 mm?
Explain in a short paragraph the difference between Porro and roof prism binoculars in shape and in brightness, and say which you would choose for a day of walking, giving a reason.
You are watching ducks on a lake about 300 m from the car park. Describe how you would use binoculars, a spotting scope, a field guide and a notepad, and in what order.
Quick Reference
- 8×42: 8× magnification, 42 mm objective
- Exit pupil: objective ÷ magnification (8×42 = 5.25 mm)
- Hand-held: typically 7× to 10×
- Scopes: about 20× to 60×, tripod needed
- Eye relief for glasses: over 16 mm
- Check: 8×42 popularity; why one source says 1890 for the 1889 guide