Fake magnification and other information

[sauroman1]

Hello, I bought 20x50 (150m/1000) binoculars that were not expensive from untrusted market but they were relatively cheap. But when I decided to sell them to another person he told that they zoom no more than 12 or even less.
I don't understand how magnification is calculated. I didn't thought that zoom number is calculated by how much zoomed image becomes bigger because image looked smaller on other binoculars also.

 

[wabbit]

It s not so clear from your post whether these are zoom (variable magnification) or regular (fixed magnification) binoculars.
In any case, magnfiication is angular. You look at an object (say the moon, which is 1/2° appparent angular size), it will appear bigger. If the magnification is 10x the moon through the binos looks about 5° wide (about the size of your fist at armlength), etc.
Lastly, assuming these are handheld, I don't know what 20x would be useful for - even if the optical quality is decent (unlikely for cheap zoom binoculars) the image will move around too much since your own hands' jitter is also magnified 20x

 

[sauroman1]

Fixed magnification. So magnification isn't just calculated by multiplying how much view got bigger? Yes, they jitter but not a lot.

 

[wabbit]

Well yes it is "how much view gets bigger" in a sense although that wording is rather vague. More precisely it is the ratio between the size of the image of an object as seen through the binoculars, and the size of the object as seen without binoculars.
You can very roughly estimate the magnification using the test I described, but that's certainly not the only way. And of course if you have another pair then the one showing the bigger image of the same oject viewed from the same distance, has the higher magnification.

 

[Chronos]

A 20x bino would jitter like a june bug on a hot skillet. Even a 10x bino is a challenge to use hand held. The formula for magnification is focal length of the objective lens divided by focal length of the eyepiece. Assuming the eyepiece is removable, it is pretty simple to deduce the actual magnification.

 

[Doug Huffman]

7 x 50 (7 power, 50 mm objective) are standard navigation binoculars, hand held on a moving, perhaps vibrating ship.

 

[wabbit]

7 x 50 (7 power, 50 mm objective) are standard navigation binoculars, hand held on a moving, perhaps vibrating ship.

That's probably one reason they're popular. 7x or maybe 10x is about the highest magnification for comfortable handheld use. Some people with steady hands use 15x but for me at least those are a strain without a tripod, not at all what I'd call comfortable.