thomasbarker81
Well-known member
Can someone give a layman's explanation of why the larger an objective lens is the greater the resolution is?
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Why does a bigger objective lens give better resolution (1 Viewer)
- Thread starter thomasbarker81
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Vespobuteo
Well-known member
Hi,
diameter and angular resolution are indeed connected by Rayleigh's criterion:
angular resolution = 1.220 * lambda/D
with lambda being the wavelength of the light and D the objective diameter.
The reason for this lies in the wave nature of light - a point like light source generates an interference pattern when passing the objective aperture called airy disk (called so after George Biddell Airy, an english astronomer) and thus its image is much larger than point-like. These airy disks get smaller with larger objective aperture and thus it is easier to keep them apart.
Joachim
diameter and angular resolution are indeed connected by Rayleigh's criterion:
angular resolution = 1.220 * lambda/D
with lambda being the wavelength of the light and D the objective diameter.
The reason for this lies in the wave nature of light - a point like light source generates an interference pattern when passing the objective aperture called airy disk (called so after George Biddell Airy, an english astronomer) and thus its image is much larger than point-like. These airy disks get smaller with larger objective aperture and thus it is easier to keep them apart.
Joachim
The OP's question was about resolution not "clarity" (whatever you mean by that). As jring has shown, under ideal conditions a large objective has the capability of discriminating (or "resolving") more fine detail than a smaller one of the same quality. Under less than ideal conditions when neither instrument is performing up to its potential--handheld binoculars, say, or tripod-mounted scopes peering into the hazy distance--the user might not notice much if any difference.
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The diffraction limit is rarely approached in a spotter, so that doesn't figure.
Rayleigh would figure in if you pushed your 60mm to over 150 power, but the atmosphere and dimness would ruin
your view long before that.
Aberrations force the length up rapidly, though, unless you mitigate that with ED glass,
which makes more of a difference at 80mm than at 60mm or 50mm, where it's easier to use length to
reduce aberration....so...that only makes using a bigger objective expense or trouble...
The most important reasons for aperture at high power are brightness, which goes down as the square
of power and up as the square of aperture, and the exit pupil, which can make eye placement
and contrast hard if it's too small. Aperture benefit is more a matter of power than resolution
directly. The aperture has to keep up with your intended power to keep the field bright, and your eyes
saturated and comfortable.
Rayleigh would figure in if you pushed your 60mm to over 150 power, but the atmosphere and dimness would ruin
your view long before that.
Aberrations force the length up rapidly, though, unless you mitigate that with ED glass,
which makes more of a difference at 80mm than at 60mm or 50mm, where it's easier to use length to
reduce aberration....so...that only makes using a bigger objective expense or trouble...
The most important reasons for aperture at high power are brightness, which goes down as the square
of power and up as the square of aperture, and the exit pupil, which can make eye placement
and contrast hard if it's too small. Aperture benefit is more a matter of power than resolution
directly. The aperture has to keep up with your intended power to keep the field bright, and your eyes
saturated and comfortable.
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Vespobuteo
Well-known member
Gijs van Ginkel
Well-known member
thomasbarker81, I will try to answer your question in post 1.
Resolution is defined as the capabilty of distinguishing between to separate details in an image. The resolving power of a binocular or telescope depends on the size of the diffraction pattern of the transmitted light and the distribution of the intensity in it, the so-called Airy disk. From the size of the Airy disk we might expect that a binocular could easily distinguish two Airy disks separated by their own angular diameter. Lord Rayleigh showed that stars with intensity maxima separated by roughly half that distance can be distinguished visually.
The diffraction patterns are than separated by 140/D arcseconds, where D is the objective diameter (or 13,8/D arcseconds at 550 nm, the wavelenght of optimal sensitivity of the eye at daylight (D = the objective diameter in cm here).
William Dawes sharpened the resolution criterion even further and came with the emperically determination of the resolving power of a telescope (or binocular) as 11,58/D arcseconds (D is the objective diameter in cm again).
Now real life is always more complicated, since in reality the resolving power of our eyes varies with:
-1- pupil diameter
-2- light intensity
-3- shaking of the hands
-4- decreases with age
Visual resolution is also reduced if the observed object has inherently low contrast as is the case for most natural objects. Reductions in contrast als occur because of lightning irregularities, atmospheric effects, diffraction, aberrations in the eye, focus errors and effects of misalignments and aberrations in any optical system employed.
I hope that this sheds some light on the question you asked.
Gijs
Resolution is defined as the capabilty of distinguishing between to separate details in an image. The resolving power of a binocular or telescope depends on the size of the diffraction pattern of the transmitted light and the distribution of the intensity in it, the so-called Airy disk. From the size of the Airy disk we might expect that a binocular could easily distinguish two Airy disks separated by their own angular diameter. Lord Rayleigh showed that stars with intensity maxima separated by roughly half that distance can be distinguished visually.
The diffraction patterns are than separated by 140/D arcseconds, where D is the objective diameter (or 13,8/D arcseconds at 550 nm, the wavelenght of optimal sensitivity of the eye at daylight (D = the objective diameter in cm here).
William Dawes sharpened the resolution criterion even further and came with the emperically determination of the resolving power of a telescope (or binocular) as 11,58/D arcseconds (D is the objective diameter in cm again).
Now real life is always more complicated, since in reality the resolving power of our eyes varies with:
-1- pupil diameter
-2- light intensity
-3- shaking of the hands
-4- decreases with age
Visual resolution is also reduced if the observed object has inherently low contrast as is the case for most natural objects. Reductions in contrast als occur because of lightning irregularities, atmospheric effects, diffraction, aberrations in the eye, focus errors and effects of misalignments and aberrations in any optical system employed.
I hope that this sheds some light on the question you asked.
Gijs
