Author Topic: Totally confused as to why more aperture = more resolution  (Read 217 times)

Sam Citadelle

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Totally confused as to why more aperture = more resolution
« on: December 26, 2017, 10:59:18 PM »
As I know it as aperture goes up so does resolution, but I can not understand why.

If I have a camera lens and I use the iris to stop it down to half its aperture, I do not loose resolution within my digital photos.

Am I missing something?



Jesse Kaine

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Re: Totally confused as to why more aperture = more resolution
« Reply #1 on: December 27, 2017, 12:22:36 PM »
Yes, cameras capture light over timeand build an image. A flash
is still needed if the light intensity isn't strong enough. A large lens
or mirror collects more light than a small one. Think about it like this:
A candle will allow you to see very little detail in anotherwise dark room.
Switch on the electric lights and you can see all the details. More light
equals greater detail.

Coco Moten

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Re: Totally confused as to why more aperture = more resolution
« Reply #2 on: December 30, 2017, 05:40:35 PM »
Quote
As I understand it as aperture goes up so does resolution, but I can not understand why.

If I have a camera lens and I use the iris to stop it down to half its aperture, I do not loose resolution in my digital photos.

Am I missing something?


Your camera lens is not operating at the theoretical limit of it aperture, if it were, stopping it down would reduce its resolution.

As to why larger apertures have greater angular resolution, one has to look at diffraction, the effect of the waves of light passing through an aperture.

We like think that a telescope produces a point image of a star but that is not quite true. I actually produces a small disk surrounded by a series of faint rings. That small disk is called the Airy disk and its angular diameter is inversely proportional to the aperture, the larger the aperture, the smaller the Airy disk and therefore the greater the resolution.

A simple model is to think of the Airy disk as a pixel on your screen. The smaller the pixel, the greater the resolving power.

In general, astronomical telescopes are able to operate at nearly the full resolution

Jon

Jon Venning

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Re: Totally confused as to why more aperture = more resolution
« Reply #3 on: December 31, 2017, 04:02:37 PM »
Photographers generally refer to pixel count as resolution. Astronomical imagers refer instead to the actual resolution achieved, in terms of arcseconds per pixel. The aperture determines the maximum usable resolution in those terms, but that generally isn't achieved anyway. A limit is usually reached for other reasons (pixel size, focal length, seeing, tracking accuracy, etc) before the instrument's basic limit (determined by aperture)O is reached. Stopping a lens down has no affect on the photographic measure of pixel count, but does cut the maximum usable true resolution in half (though, again, you probably aren't achieving that anyway).

Steven Morrison

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Re: Totally confused as to why more aperture = more resolution
« Reply #4 on: December 31, 2017, 04:50:16 PM »
you don't loose pixels in photographic terms but detail in the image is reduced, not to a noticeable degree on everyday photos.

The issue with resolution in telescope terms has to do with how small of detail can be resolved in arc seconds. So lets say there is a large tarp setup on the moon, say 100x100 meters. that is much too small for any current earth/earth orbiting telescope to resolve (it would cover just 1 pixel on Hubble for example) it would take many more pixels to identify it's size or shape. by resolve I mean identify it to some identifiable object not just detect it.

A better description with formula is given here:

http://curious.astro...e-moon-beginner

exvermabo

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Re: Totally confused as to why more aperture = more resolution
« Reply #5 on: January 01, 2018, 07:24:30 AM »
Wikipedia actually gives a pretty good explanation on "why"
https://en.wikipedia...ular_resolution

TLDR: the fact that the light is restricted by an aperture creates diffraction "waves" at the edges of the aperture. The smaller the aperture, the bigger the impact of the diffraction and more blurred the "dot" would be after we focus this light (even assuming perfectly figured lens/mirror).

Todd Topcic

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Re: Totally confused as to why more aperture = more resolution
« Reply #6 on: January 07, 2018, 11:25:43 PM »
Adding another analogy to Jon's explanation: Dot Matrix Printers ( for those who remember them).
The more dots per inch the printer produced the better looking the final product. One can think of larger apertures as producingmore dots per inch ( because the dots are smaller)thanlesser sizeapertures -hence better resolution.
Hooray for physics.

John Weiland

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Re: Totally confused as to why more aperture = more resolution
« Reply #7 on: January 08, 2018, 08:48:38 AM »
The smart guys have spoken. I am the dumb guy. This is how I understand it.Walk into a room with a dimmer on the light.

Look around the room at the details.

Now, dim the light to half. You can still see everything in the room but some of the detail is not as rich.

Now dim it to 1/4.  You find things harder to see. Colors are faded.  The red cup on the table now looks kinda brown and maybe the texture is gone.

Now dim to 1/8. Your iris opens up more and more as the lights dim, to try and compensate.  The lines in the wall paper seem to disappear the farther they are from the bulb.  Dust in the corner is no longer visible.  You see the book but can you make out the page edges?

Now dim it to 1/16, and so on.

the dimmer light = small aperture
the brighter light = larger aperture

How much detail you can make out is dependent on how much light can be gathered by your eyes. The less light the less detail you can see.Likewise with magnification.

As you increase magnification in order to be able to resolve details you discard light.  The more dense the light you have the more you can magnify and still have enough light left to see the details. The larger scope gives you more light per observed degree.

I don't know if this is exactly technically accurate, but it is true.

risodachest

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Re: Totally confused as to why more aperture = more resolution
« Reply #8 on: January 10, 2018, 08:05:18 AM »
It isn't about brightness, it is about maximum theoretical resolution. Google "Dawes' limit" to see how it works.

vieproltesro

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Re: Totally confused as to why more aperture = more resolution
« Reply #9 on: January 11, 2018, 02:00:11 AM »
It's instructive to set up two telescopes of very different apertures side by side and look at things....terrestrial and in the sky.

One way to look at it is that the larger aperture is capturing more information. Information in the form of reflected light (for terrestrial and planets etc.) or emitted light (for stars and galaxies) is gathered up by aperture and the more of it you have the more you can see.

Over the years I've done a fair amount of observing my neighbors' doorbell, it's very convenient for calibrating and sighting. In the 80mm you get a good view at 280x but you're using your eyepiece to blow up (like a microscope) a disc of light--the focal plane--that has been collected by the aperture and brought to a focus point where it can be used. The smaller scope doesn't have much information compared to bigger scopes. One consequence: when you go to a high magnification, like 280x, it begins to go dark (called dimming out). Nothing there because the information wasn't gathered. The lack of information shows as a dimmer image; and if there is light, at high enough powers, it will become a fuzzy image, because there's no additional information there.

Images also dim out in microscopes, one of the ways they force-feed you more reflected light information in a microscope is to have very high intensity lamps zeroed in on those poor paramecia. But we can't do that with galaxies. So we collect MORE of what light there is.

In a C14 at the same power the image will be very bright and you will see things...little weathering marks on the brass, etc., that are invisible in the 80mm. 

If you go to this site you'll see these funky giant acoustic listening devices that were used during WWII and earlier. They built huge ears which gathered more information and funneled it down the ear canals of the operators.  The bigger the ears, the greater the sensitivity, the more information (resolution) that was gathered (some of these devices were mounted on huge tracking rigs).

A telescope gathers more light information and funnels it into a small area that is usable to the eye, exactly the way these huge acoustic devices funneled distant sounds into the human ears.

What *IS* amazing is how MUCH information there is in smaller apertures. I have often said here and elsewhere, that a 5 mm eyeball pupil has the same relationship to a 4 inch telescope, as a 4 inch telescope does to an 83 inch telescope.  It has been quite surprising to me (and I've posted about it) to see Saturn's rings in a 41mm aperture--a pretty good view, too!--at 173x. In the sense of a ratio...every amateur aperture including up to 30 or 40 inches is really a marginal enhancement compared to the first three or four inches. (But we can see the difference!)

Anyhow if you think of a telescope has scooping up information it follows that the bigger the scoop the more data you get packed into that little focal plane of light that you examine with your telescope. That's the basic point, never mind the endless wars over which telescope is best.

Greg N

otisover

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Re: Totally confused as to why more aperture = more resolution
« Reply #10 on: January 12, 2018, 02:54:02 AM »
I don't know, to me it seems that some subjects do not benefit from oversimplification. Similar to how you cannot explain gravitational lensing or strong force using Newtonian physics, you cannot explain optical resolution using terms like "brightness" and "dimming".
"more information" is correct in principle, but if you think about it, "higher resolution" is by definition the same thing as "more information", which means it doesn't answer OP's question of why.

The reason resolution is tied to the aperture lies in the wave-like properties of light and diffraction in particular. You do not directly "suck in" more pixels because you have a bigger tube. I guess the simplest analogy would be to say that edges produce a certain amount of noise because they interrupt infinite flow of the wavefront (similar to how doing Fourier transform on non-infinite signal produces ripples). The farther away from the edge, the less noticeable this noise is. So with bigger aperture the ratio between the amounts of good signal (from the center) and noisy signal (from edges) is better.

This article has a nice animation showing the phenomenon and a good explanation overall.

http://www.rocketmim...lvingPower.html

Lcs King

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Re: Totally confused as to why more aperture = more resolution
« Reply #11 on: January 12, 2018, 11:41:06 AM »
Quote
I don't know, to me it seems that some subjects do not benefit from oversimplification. Similar to how you cannot explain gravitational lensing or strong force using Newtonian physics, you cannot explain optical resolution using terms like "brightness" and "dimming".
....The reason resolution is tied to the aperture lies in the wave-like properties of light and diffraction in particular.

All of that is absolutely correct in theory -- and if you're asking about resolution for things like splitting tight double stars, it's really the essence of the matter.

To put this in the very simplest possible terms, the image of a point source (a star) in a telescope is not a point but a disk -- called the Airy disk -- surrounded by a series of faint rings. That's due to the wave nature of light. When you try to split a very tight double star, you actually see two overlapping Airy disks. If the overlap is significant, your eye can't tell them apart, and you can't split the double.

The size of the Airy disk is inversely proportional to the aperture. Therefore, the bigger the aperture, the tighter a double star your scope can split.

But when you're observing the planets, brightness also comes into play. Jupiter gets really dim at magnifications high enough to see the Airy disk, so your views of Jupiter at magnifications too high for your aperture are limited both by the blurring due to diffraction and also by the dimming due to spreading a given amount of light over too large an area.

Stanley Edwards

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Re: Totally confused as to why more aperture = more resolution
« Reply #12 on: January 13, 2018, 11:51:43 PM »
Here's how to see most directly why aperture matters.

Look through a pinhole smaller than about 1mm. When scrutinizing small, bright light sources particularly, you will see diffraction in the form of the central Airy disk and one or more rings. Less contrasty scenery will be made 'fuzzy'. Try to go to as small a pinhole as sufficient light can get through, and note how diffraction expands in scale as the aperture shrinks.

When we push our telescopes to these ~1mm and smaller exit pupil diameters, we are resolving the same diffraction as seen with unaided eye and same-aperture pinhole. The bigger scope aperture merely supports the concomitantly higher magnification permissible for its aperture.

If we say a 1mm pupil at the eye's iris is the threshold lower than which an image 'breaks didn't due to diffraction, we see then that a 10mm telescope aperture can support 10X, a 100mm aperture 100X, a 1m aperture 1,000X..... in all cases, at the 1mm exit pupil the eye sees an *identical* sized diffraction pattern. But the higher magnification represents the same ratio in resolution. The 10mm, 100mm and 1m apertures can resolve 10X, 100X and 1,000X better than can the unaided eye, respectively.

The foregoing expresses the matter in terms we can understand via direct observation. More fundamentally, resolving power scales as the ratio of aperture to wavelength of the EM radiation being imaged. This is why optical telescopes (or the eye) can resolve well at quite small aperture, but at the very much longer radio wavelengths even a football-sized aperture resolves quite poorly.

Manuel Ghumare

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Re: Totally confused as to why more aperture = more resolution
« Reply #13 on: January 14, 2018, 04:54:44 AM »
Quote
As I understand it as aperture goes up so does resolution, but I can not understand why.

If I have a camera lens and I use the iris to stop it down to half its aperture, I do not loose resolution in my digital photos.

Am I missing something?


I can assure you that if pixel peep the 100% image from a good dSLR you will definitely see a difference in sharpness stopping f8 down to f16 and even more stopping down to f32. All good camera lens work best stopped down ~ 1-2 stops from wide open, so that's usually in the range of f4- f5.6, and then the image really begins to degrade from there.
Research the MTF tables of any lens out there or do the tests yourself using a resolution chart.
To give you another camera analogy, would you prefer a 2MP camera or a 36Mp camera ?? Aperture works the same way !

A 2" telescope will only "resolve" 2 Mp of details, a 36" scope will resolve 36Mp etc.
Carrying that a step further, that's why you can't increase your magnification beyond certain limits. It's like trying to scale up a 2Mp 640 X 480 px jpeg , to a 7360 x 4912px image from a 36Mp camera. You can do it, but it's not going to look pretty.

buckfeedssapfai

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Re: Totally confused as to why more aperture = more resolution
« Reply #14 on: January 14, 2018, 06:06:47 AM »
Quote
Quote

As I understand it as aperture goes up so does resolution, but I can not understand why.

If I have a camera lens and I use the iris to stop it down to half its aperture, I do not loose resolution in my digital photos.

Am I missing something?

To give you another camera analogy, would you prefer a 2MP camera or a 36Mp camera ?? Aperture works the same way !

A 2" telescope will only "resolve" 2 Mp of details, a 36" scope will resolve 36Mp etc.
"Bingo", a good analogy. However "seeing", has a major part to play, in the "theory matching the reality". A 2" aperture might realize its potential 90-95% under a clear night sky, whilest a 36" mostly less than 1-2% (or never, depends on ones "locale").

Light gathering, is a different proposition, with a strong correlation between "aperture squared", and photons captured.

Stephen.(45deg.S.)