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Relation: aperture-mag when view extended objects?

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#1 Guest_**DONOTDELETE**_*

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Posted 22 December 2003 - 04:11 AM

I had to trim down the title in order to fit it all. Sorry if it looked strange.

If any of you remember my previous post asking for advice on what binocular to get, this is related. I still haven't purchased a new one, and am happy to say that I've had no problems with my 15x70's. Because of their weight and size I can hold them as steady as a 10x50 pair. But I've been thinking about what would be a good complement to them, and need to find out for sure how things work.

In my 15x70mm Binoculars I know that I can see more stars than a smaller pair, but what about things other than stars?

Suppose I'm looking at M42 with my 15x70's. I assume that magnification doesn't matter too much as it's already pretty big, aperture big enough to match the magnification is more important. I will rate the 'brightness' of M42 through this pair of binoculars as 1.

Now suppose I use a pair of 7x50's. I had thought that these couldn't compare to the 15x70mm's, but now I'm not so sure. If I was to look at M42, it would look smaller, but also stand out more, right? I'm only getting half as much light as I was with the 70mm lenses, but I'm also only magnifying it to have an area about 1/4 the area of the 15x70's. So I would give these a brightness rating of 2. Which is twice as good as the 15x70's.

I know that the bigger image scale does make up for the dimmer view somewhat, but how much does this matter?

It seems to me that a good complement to a high power binocular is a large aperture one. So all I would need to do now, is decide whether I want a pair around 10x70mm for the better view, or something closer to 7x50 for the big field of view.

Sorry if this is basic stuff, I thought this would be the best place to get a good answer. Thanks in advance.

#2 EdZ


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Posted 22 December 2003 - 07:59 AM

There's a lot of factors that determine whether an object will stand out more in a lower power larger aperture binocular.

The larger aperture has greater light gathering ability.

Image scale has a lot to do with whether you can see detail in an object or not.

Extended onjects, especially faint diffuse objects benefit from greater aperture.

In binoculars, even globular clusters and dense open clusters can be considered extended objects. These will benefit much more from larger image scale provided by magnification.

If your eye pupil does not get as large as the exit pupil, the binocular equivalent aperture is determined by your eye pupil. For a 10x70 binocular with a 7mm exit pupil, if eye pupil max is 6mm then light gathering will be determined as if deleivered by a 10x6mm = 60mm effective aperture, then 60x60=3600. Resolution will be determined as if delivered by 70mmx6/7=60mm lens, same as light gathering.

I've seen the North America nebula with my 15x70s. I've never seen it with my 10x50s. If you consider the amount of light gathered spread over the area of the magnified image, (assume a 1 arcminute square) then the 15x70s gather 1.96x the light (70x70=4900 vs 50x50=2500, so 4900/2500=1.96) and they spread it over an area 2.25x as large (15x15=225 vs 10x10=100, so 225sqarcmin/100sqarcmin=2.25x). If 1.96x the light is spread over 2.25x the area you would think the 15x70s would make it more difficult to see, but that is not the case. larger image scale can make it easier to see faint light.

The faintest magnitude that can be seen in a telescope will increase by approximately 1 magnitude as magnification increases from about 30x to about 100x. The same holds true for binoculars. As magnification increases, fainter objects can be seen.

I do not have a formula to determine a break point.


#3 Guest_**DONOTDELETE**_*

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Posted 22 December 2003 - 09:31 AM

Thanks very much for posting that EdZ. So magnification is more helpful than I had thought. It seems that any improvement gained by a large exit pupil is smaller than one would expect. I guess if I want a big improvement over my 15x70's I'll be better off getting a big telescope eyepiece for a large magnification and exit pupil. When I do get another pair of binoculars it will mainly be for a bigger field of view.

Thanks again, these forums are a great place to learn.

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