# Help with a question please?

• Jul 1st 2012, 07:25 AM
Libertine
Hi all,

I just joined this Forum to get an answer to a question really, it's not for Collage or Uni or anything just a curiosity really.

What I'd like to know is.... If an Object is 3 miles away, and this is what complicates things a little (for me at least), it is 3mm wide by 4mm heigh through a pair of Binoculars (15x70).
What size would the Object be?

I'm also not 100% sure of the distance so if anyone could help me out with this could you also do the same thing but with the Object at 2 miles and also 4 miles away if possible?

If anyone can help me with this I'd be really grateful.
• Jul 2nd 2012, 04:11 AM
Re: Help with a question please?
well im not sure this is completely possible, im not sure, not saying its not possible... HOWEVER! it is impossible because there are unknown variables, like, how powerful are the binoculars, 200x? more, less?
• Jul 2nd 2012, 05:22 AM
HallsofIvy
Re: Help with a question please?
Saying the binoculars are "15x70" tells us the diameter of the lens and the focal length of the objective lens but says nothing about the magnification. The magnification is the focal length of the objective lens divided by the focal length of the eyepiece.
• Jul 2nd 2012, 05:50 AM
emakarov
Re: Help with a question please?
Quote:

Originally Posted by HallsofIvy
Saying the binoculars are "15x70" tells us the diameter of the lens and the focal length of the objective lens but says nothing about the magnification.

Are you sure? This site says that the first number is the level of magnification and the second one is the diameter of the objective lens.

Quote:

Originally Posted by Libertine
What I'd like to know is.... If an Object is 3 miles away, and this is what complicates things a little (for me at least), it is 3mm wide by 4mm heigh through a pair of Binoculars (15x70).
What size would the Object be?

I am not an expert on this, but I think that the question "what size" does not make sense unless one also specified the distance to the object. For example, a coin at the arm's length can have the same size to me as the Moon. Alternatively, one can talk about the angle of view. As long as the objects are small enough, making an object appear n times closer is (approximately) equivalent to increasing the angle of view n times.
• Jul 2nd 2012, 06:20 AM
Libertine
Re: Help with a question please?
Hi,

Emakarov is right,The Binoculars are 15x magnification.
The object was to far away to see with the naked eye because it would have been 15 times smaller.

Hi Emakarov.

I have, the distance from me to the object was 3 miles.
What I'd like to know is how big would the object be if I were standing right next to it?

Thanks all for taking the time to have a go.
• Jul 2nd 2012, 06:52 AM
emakarov
Re: Help with a question please?
What I should have said, I am not sure what it means for an object to appear to be 3 x 4 mm through a pair of binoculars. I think the linear size (say, width) can be used in one of the two contexts. Either you say that an object itself has a particular width (this obviously does not depend on the distance from the observer to the object), or you say that an object O₁ at distance d₁ from the observer has the same apparent width as an object O₂ at distance d₂.