Binoculars: deciphering the terminology and how to use them: Part III

As promised, this is part III of the previous thread. Enjoy.

http://boards.cruisecritic.com/showthread.php?t=550923

Part III: More in depth discussion about binocular

• Which magnification should I choose?

It really depends on who you ask this question to because every user has his unique reason to use binoculars. Birders like to use 7x or 8x, Stargazers like 15x or 25x giant binoculars mounted on a steady tripod. But the most versatile format is between 8x and 10x. Don’t overlook the difference between 8x and 10x, they are more than just magnification power difference. Few years ago, I was a stubborn believer of 8x for bird watching. Below are the things I didn’t like about 10x format back then.

• Narrower field of view: 8x has 7.4 degree, whereas 10x has only 6 degree. It makes tracking birds in the wooded area difficult
  
• Worse field curvature: 8x has much better edge-edge sharpness, where most 10x ones have soft 1/3 outer edge. Unless you have more than $800 budget for some special aspheric lens, the problem is always present there.
  
• Dimmer image, relatively speaking. Higher power always allows less light to come in due to smaller exit pupil

Over the years, I finally overcome my “fear” of 10x mostly due to the craving for higher power. With the improvement of coating technology, today’s 10x is brighter than 8x a decade ago. Also, I realize I didn’t use my peripheral vision at all, so edge softness does not bother me. #1 is still the biggest concern. But I know I cannot get both wide angle and high power at the same time, without sacrificing other qualities, or depleting my wallet. :) So I would recommend 10x so you won’t be overwhelmed by the vastness of Alaska wilderness. However, you need to take into account some disadvantages of it comparing to 8x.

• Size of binoculars

Typically, the bigger the size, the better the light gathering capability, hence brighter image. Of course, the price goes much quickly when size of lens increases. I think anything between 36mm and 50mm should be reasonably good. Even though I don’t recommend small binocular in general, I found they do have one big advantage, smaller lens tend to have better edge to edge sharpness. My Zeiss BT 8x20 is one outstanding example. I cannot speak for those $15 small binos you got from fleabay though. :)

• Edge Distortion

If you focus your binoculars to a straight line (like horizontal roofline or vertical edge of your furniture), then move the object to the edge of the field, you will notice the center of the straight line will slightly bend inward. It is called pincushion distortion. This distortion is normal and intentionally introduced by most manufacturers to eliminate the rolling ball effect when you are scanning with your binoculars. However, if you see another type of distortion with center of line bending outward, you will suffer dizziness, even nausea when you use it. It is called barrel distortion, which sometime is present in some low quality wide angle binoculars. With barrel distortion or no distortion at all, when you scan your binocular, you will feel the object is becoming bigger from edge to center, then smaller from center to the other edge. It is very uncomfortable to use.

Below is from a Leica brochure to describe those two effects. Leica is famous for pushing pincushion distortion to the limit.

• Twilight factor

I don’t understand why some manufacturers still bother to quote this number. It is a theoretical performance measurement of binocular at twilight. It is a square root of product between magnification power and objective lens diameter. So, a 10x42 binocular has a twilight factor of sqrt(10x42)=20.5. This value is close to useless since it does not count for lens coating, resolution, color fidelity, many other important optical qualities. Many lower quality binoculars like to quote this number to compare them against Leica, Zeiss model. In my view, we should completely discard this number.

• Contrast

Just like in photography, contrast is defined as the brightness difference between light and dark areas of the image. High contrast image is perceived to be sharper or brighter. Together with shallow depth of focus, a vivid 3D image rendering can be achieved by high quality binoculars

Dave - Thanks for such great posts. I've read your older ones and printed them out. Nice to have the terminology explained!

BDL

How much do you feel a reasonably good pair should weigh? I found a pair that met many of the criteria set out in your two threads, but I was surprised that they weigh nearly 2 pounds.....and I was thinking, gee, this is a good price but do I want this on my neck all day?

Thanks for any input. Both of these threads have been very informative.

Maddle

Maddle, some good binoculars are heavy too, like Swaroski EL 10x42 goes by 28oz. When a binocular weighs over 2 lb, in most cases, it uses Porro prism, hence, bulkier and heavier. Optically, not much difference is observed between Porro and roofprism in modern binoculars, if they use the same quality material and same quality control standard. Porro is normally cheaper because of simple design. So, you have to balance between budget and weight. For me, it is no brainer since I am trying to lower my payload as much as possible together with a Canon dSLR and a 3 pounder 400mm lens.