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Old Mar 19, 2010, 11:15 AM   #31
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Light gathering ability - i.e. diameter is the key aspect of resolving power ...
Actually, no. A large diameter does not help resolving power. A large diameter optical element can focus badly, and therefore, resolve poorly. And a large diameter lens has a larger polished surface and is therefore more likely to have optical flaws which would decrease its resolving power.

So "Light gathering ability" is not a key aspect of "resolving power". In fact, "Light gathering ability" is an impediment to "resolving power". The more photons entering an optical system, the less likely they are to all focus in the same place.
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Old Mar 19, 2010, 12:00 PM   #32
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Actually, no. A large diameter does not help resolving power. A large diameter optical element can focus badly, and therefore, resolve poorly. And a large diameter lens has a larger polished surface and is therefore more likely to have optical flaws which would decrease its resolving power.
If you're referring to a telescope that I build, you would be perfectly correct. I would use a cardboard mailer and my magnifying glass. Of course, that's not what we're discussing.

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Resolution = detail = information
More light gathering area translates into a more detailed image, that is more information. That concept is referred to as "resolution" or resolving power. All things being equal, an 8-inch telescope should have twice the resolving capability of a 4-inch.
Now, bear with me while we let out a little more air from this term. A telescope's resolution is measured by how well it can separate two distant objects that are very close to each other. It's purely a theoretical value and also depends quite a bit on the quality of the optics, but it gives you a "ballpark" feel for how well your telescope should perform optically.
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Old Mar 19, 2010, 12:33 PM   #33
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OK guys.... As Mark as already mentioned:

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Originally Posted by Mark1616 View Post
This thread was designed (very well I might add) by TCav to help give insight to the different sensors out there and how they effect field of view.
This is a sticky thread designed to help members with how different sensor sizes will result in different angles of view for a given focal length, thanks to the work of TCav, coordinating with Mark1616 to make this thread a sticky that we can refer members here to.

From what I can see, this continuing debate will only serve to confuse new (or potential) digital camera owners. ;-)

So, getting back to this thread's intended purpose....

If you have a larger sensor or film size, you will have a wider angle of view for a given focal length lens.

If you have a smaller sensor or film size, you will have a narrower angle of view for a given focal length lens.

To see how angle of view compares between different formats, see the first post in this thread.

Let's not continue to debate other differences in formats in this sticky thread, as not to confuse members trying to see how different formats impact angle of view with a given focal length lens.

Thanks.
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Old Mar 19, 2010, 12:51 PM   #34
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OK guys.... As Mark as already mentioned:



This is a sticky thread designed to help members with how different sensor sizes will result in different angles of view for a given focal length, thanks to the work of TCav, coordinating with Mark1616 to make this thread a sticky that we can refer members here to.

From what I can see, this continuing debate will only serve to confuse new (or potential) digital camera owners. ;-)

So, getting back to this thread's intended purpose....

If you have a larger sensor or film size, you will have a wider angle of view for a given focal length lens.

If you have a smaller sensor or film size, you will have a narrower angle of view for a given focal length lens.

To see how angle of view compares between different formats, see the first post in this thread.

Let's not continue to debate other differences in formats in this sticky thread, as not to confuse members trying to see how different formats impact angle of view with a given focal length lens.

Thanks.
Mark, has created a new and different Sticky. This thread is no longer a sticky...

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Old Mar 19, 2010, 1:07 PM   #35
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Thanks... I didn't notice that.
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Old Mar 19, 2010, 1:47 PM   #36
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Resolution = detail = information
More light gathering area translates into a more detailed image, that is more information. That concept is referred to as "resolution" or resolving power. All things being equal, an 8-inch telescope should have twice the resolving capability of a 4-inch.
Now, bear with me while we let out a little more air from this term. A telescope's resolution is measured by how well it can separate two distant objects that are very close to each other. It's purely a theoretical value and also depends quite a bit on the quality of the optics, but it gives you a "ballpark" feel for how well your telescope should perform optically.
Where did you find this?!?!

"... should have ...", "... a theoretical value ...", "... quite a bit ...", '... a "ballpark" feel ...', "... should perform ..." This is an authoritative reference?

First, the light gathering area of a 4 inch telescope is about 12.5 square inches, and the light gathering area of an 8 inch telescope is over 50 square inches! That's FOUR TIMES the "resolving area", not twice!


Second, the resolving power of any system is its ability to separate two phenomena. Your car's AM radio has the ability to resolve one station from another. But most important, it is certainly not a theoretical value. It is objectively, predictably and repeatable quantifiable.


Third, it's interesting that the author would gloss over the fact that resolution "... depends quite a bit on the quality of the optics." That's all it depends on.


Clearly, the intended audience for the reference material you've used is the budding amateur with a scant knowledge in geometry and a considerable tolerance for ambiguity in language, and is intended to reassure the novice that anything commercially available is good enough for what they want to do. I would be very disappointed if the original author actually believed the entirety of the text you attached.
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Last edited by TCav; Mar 19, 2010 at 1:50 PM.
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Old Mar 19, 2010, 2:02 PM   #37
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Where did you find this?!?!

"... should have ...", "... a theoretical value ...", "... quite a bit ...", '... a "ballpark" feel ...', "... should perform ..." This is an authoritative reference?

First, the light gathering area of a 4 inch telescope is about 12.5 square inches, and the light gathering area of an 8 inch telescope is over 50 square inches! That's FOUR TIMES the "resolving area", not twice!


Second, the resolving power of any system is its ability to separate two phenomena. Your car's AM radio has the ability to resolve one station from another. But most important, it is certainly not a theoretical value. It is objectively, predictably and repeatable quantifiable.


Third, it's interesting that the author would gloss over the fact that resolution "... depends quite a bit on the quality of the optics." That's all it depends on.


Clearly, the intended audience for the reference material you've used is the budding amateur with a scant knowledge in geometry and a considerable tolerance for ambiguity in language, and is intended to reassure the novice that anything commercially available is good enough for what they want to do. I would be very disappointed if the original author actually believed the entirety of the text you attached.

Oh? You did the math?

Quote:

In astronomy, the resolution of the telescope, or light gathering ability, is more important than the magnification. Although a 60mm refractor might be capable of 400x magnification, the effective magnification is only 50 x 2.4inches = 120x as the lens does not have good light gathering ability.

The highest effective magnification in a telescope is 50 x aperture in inches.

i.e. 8inch (200mm) Newtonian = 50 x 8 = 400x

To calculate the actual magnification of a telescope, use the following formula:

Magnification =
focal length of telescope
focal length of eyepiece

i.e. 200mm f/6 with 10mm eyepiece

mag = (200 x 6)/10 = 120x
So, let's see, a 4 inch telescope, 50 x 4 equals 200! Which by a strange coincidence is one half the eight.

My original link states:

Quote:
So an 8-inch (20-centimeter) telescope has four times the light grasp of 4-inch (10-centimeter) telescope -- not two times. Putting it in practical terms, an 8-inch, given a dark, clear sky and good seeing conditions, should be able to detect stars as faint as magnitude 14 or better.
But then optics come into question along with focal length...

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Old Mar 19, 2010, 3:34 PM   #38
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"... the resolution of the telescope, or light gathering ability, is more important than the magnification ..."

The resolution of a telescope is determined by its ability to distinguish between two stars in a binary system. You could increase the magnification without increasing the light gathering ability, but then the image gets dimmer because less light is entering the telescope. (Remember? as you increase the focal length, less light enters the optical system?) So in order to increase the magnification, you need to increase the light gathering ability (use a larger objective element) so you can more easily see the image you're trying to resolve.

So, increasing the light gathering ability only indirectly increases the resolution. You can't increase the resolution without increasing the light gathering ability, but you can increase the light gathering ability without increasing the resolution. That's a "Wide Field Telescope".
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Old Mar 19, 2010, 3:50 PM   #39
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"... the resolution of the telescope, or light gathering ability, is more important than the magnification ..."

The resolution of a telescope is determined by its ability to distinguish between two stars in a binary system. You could increase the magnification without increasing the light gathering ability, but then the image gets dimmer because less light is entering the telescope. (Remember? as you increase the focal length, less light enters the optical system?) So in order to increase the magnification, you need to increase the light gathering ability (use a larger objective element) so you can more easily see the image you're trying to resolve.

So, increasing the light gathering ability only indirectly increases the resolution. You can't increase the resolution without increasing the light gathering ability, but you can increase the light gathering ability without increasing the resolution. That's a "Wide Field Telescope".
You seem to be stuck in the same rut of confusing resolving power and real magnification with the word "magnification." True magnification is the ability to increase the size of what you are viewing to bring out what is resolved. When you magnify past the point of the ability to resolve, you have "empty magnification." There is no limit to empty magnification.

The actual power of a telescope is to optimally magnify what it is capable of resolving. This is done by a combination of the ability to gather light, and the focal length.

The same holds true for a lens. Once you crop an image past the point of the lens resolving power, you achieve nothing but empty magnification.

Now that I've proved all the above, leaving you once again to ignore all the science I've posted, you should be able to see that cropping a 300mm lens to a 450 equivalence does NOT mean that you can resolve as much with that 300 as the 450 can. No amount of cropping can add to the resolving power of the lens.

This thread has gone from you first denying that light gathering has anything to do with this, to claiming that on every lens, f2 is f2, to God knows what else.

In fact, it's why I haven't posted the links - You seem to have the ability to speed read them without learning a thing.

But I degress...

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Old Mar 19, 2010, 4:29 PM   #40
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But I degress...
Oh, you do more than digress.
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