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Why are some lenses soft?

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#1 Binofrac

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Posted 10 April 2024 - 10:24 AM

In one of those brain meandering sessions I was wondering why cheaper lenses and so cheaper binoculars don't give a sharp focus albeit in a small sweet spot. As far as I know any lens should theoretically be able to provide a sharp focus somewhere even if it's not where you want it. I understand why CA would affect this by not focusing all wavelengths to the same plane, but it is possible for CA prone optics to still provide a sharp image. Is there generally microscopic flaws in the glass that make the image slightly fuzzy or other factors? 



#2 JoeFaz

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Posted 10 April 2024 - 11:02 AM

If I understand your question correctly (I'm not 100% sure if I do), I think you're just describing field curvature (not to be mistaken for magnification distortion, or other off-axis aberrations), in which the image focuses on a curved plane and, practically speaking, when the center of the FOV is in focus the periphery is increasingly out of focus. Field curvature is always present in a spherical lens and I think the simple answer to your question is that substantially correcting for it with a field flattening element, costs dollars in design/production.



#3 PEterW

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Posted 10 April 2024 - 04:04 PM

There are many other optical aberrations that need to be corrected for to provide a wide and fully sharp field, any design will balance these and other factors. Using aspheric and exotic glasses can help achieve a better result.

Peter

#4 mati93

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Posted 10 April 2024 - 04:04 PM

I am not sure I understand your question. In optics there are simple formulas like the "thin lens formula", among others, that are just approximations to the full theory. These approximations and geometric optical diagrams of a telescope and eye system, suggest that the light of every source would be perfectly focused in a point that lies in the focus plane, but this is far from reality. Like the other user explained, there are several aberrations that prevent the light of a source to be perfectly focused, even at the optical axis, and thus they make the image fuzzy. That is even with perfect spherical lenses.
So the manufacturers use several lenses in order to correct the image, without compromising the light transmission, contrast, weight, etc. (the more lenses the more reflections, so less contrast and transmission). Expensive binoculars could have complex optical designs capable of correcting the image in a big sweet spot, without compromising the light transmission too much thanks to high quality coatings, and also have higher quality standards for the lens construction and alignmets.



#5 Binofrac

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Posted 11 April 2024 - 02:35 AM

Thanks everyone. I was just trying to understand the difference between theory and practice.



#6 Jon Isaacs

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Posted 11 April 2024 - 04:37 AM

Thanks everyone. I was just trying to understand the difference between theory and practice.

 

 

In one of those brain meandering sessions I was wondering why cheaper lenses and so cheaper binoculars don't give a sharp focus albeit in a small sweet spot. As far as I know any lens should theoretically be able to provide a sharp focus somewhere even if it's not where you want it. I understand why CA would affect this by not focusing all wavelengths to the same plane, but it is possible for CA prone optics to still provide a sharp image. Is there generally microscopic flaws in the glass that make the image slightly fuzzy or other factors? 

 

The simplest objective lens is a doublet.  A doublet has 4 curved surfaces that must be precisely made so that they are essentially perfect, that means to a small fraction of a wave length of light.  And the curves must match the design specifications.  And the surfaces must be very smooth.  This is a very difficult task.  

 

Cheaper lenses are not as accurately made, they do not focus the light as precisely.  

 

A wave length of green light is 550 nanometers,  0.000022 inches....

 

Jon



#7 Binofrac

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Posted 11 April 2024 - 04:49 AM

The simplest objective lens is a doublet.  A doublet has 4 curved surfaces that must be precisely made so that they are essentially perfect, that means to a small fraction of a wave length of light.  And the curves must match the design specifications.  And the surfaces must be very smooth.  This is a very difficult task.  

 

Cheaper lenses are not as accurately made, they do not focus the light as precisely.  

 

A wave length of green light is 550 nanometers,  0.000022 inches....

 

Jon

Cheers John. At school many years ago one lesson held me fascinated as we measured the wavelengths of different colours. It was a way of learning the practical applications of the vernier scale which I've long forgotten in this age of digital measuring tools, although I still use a dial caliper.



#8 GlennLeDrew

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Posted 11 April 2024 - 06:18 AM

Even perfectly made lenses can and do exhibit aberrations. Take the typical 35mm f/4 acromatic objective. It introduces a small amount of spherical aberration that can be seen at the low 7X when carefully examining a star or other point source while racking through the best focus. The eyepiece is a contributor also, but the objective's contribution dominates because most or all of its aperture fills the pupil at the eye.

 

Higher end optics either use one or more aspheric elements or balance undercorrected and overcorrected spherical aberration among different elements.

 

Years ago I removed for a project the really fast 50mm f/3.3 objectives from a wide angle 10X50 bino which suffered HORRIBLE spherical aberration. That aberration inherent to such a fast lens is bad enough, but these inexpensive lenses had turned-down edges due to the aggressive polishing action to make for fast work and which notably worsened the undercorrected SA. I fixed them up by refiguring the front surfaces on a special polishing pitch lap (losing the coating in the process.) Through an iterative process of polishing for a couple or few minutes and testing with a Ronchi screen in double-pass mode, an aspheric figure resulted which GREATLY improved the performance. For example, formerly the compact star cluster M29 (at the middle of the Northern Cross) was an unresolved blur. Afterward it was a collection of sharp points. With the expected small amount of chromatic aberration, which is FAR less injurious than is spherical aberration.


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#9 JoeFaz

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Posted 11 April 2024 - 09:15 AM

Not to hijack the thread for my own personal curiosity, but I think this is still generally on topic:

 

Glenn, could you help me (and surely others) understand the difference between field curvature and spherical aberration? I think I understand the theory behind the two and why they occur, but I'm less sure I fully understand their "real world" impact at the eyepiece(s). My understanding is that they are both innate characteristics of spherical lenses, but FC is based on angle of incidence and SA on height of incidence. So, FC impacts the image as a whole (i.e., its impact is noticed off-axis), whereas the impact of SA is realized at all point sources throughout the image creating a blurry halo around each point source (like the "halo" from CA, but not from a specific color) and creating a "softening" of the image throughout.

 

Does spherical aberration create an additional blurring off-axis, or is it is just because it is generally superimposed with field curvature? If an optic is very sharp on axis, but out-of-focus off-axis, does this suggest that SA has been corrected but not FC?



#10 Binofrac

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Posted 11 April 2024 - 10:14 AM

Years ago I removed for a project the really fast 50mm f/3.3 objectives from a wide angle 10X50 bino which suffered HORRIBLE spherical aberration. That aberration inherent to such a fast lens is bad enough, but these inexpensive lenses had turned-down edges due to the aggressive polishing action to make for fast work and which notably worsened the undercorrected SA. I fixed them up by refiguring the front surfaces on a special polishing pitch lap (losing the coating in the process.) Through an iterative process of polishing for a couple or few minutes and testing with a Ronchi screen in double-pass mode, an aspheric figure resulted which GREATLY improved the performance. For example, formerly the compact star cluster M29 (at the middle of the Northern Cross) was an unresolved blur. Afterward it was a collection of sharp points. With the expected small amount of chromatic aberration, which is FAR less injurious than is spherical aberration.

That must've been a very satisfying project Glen. I wonder how far a binocular can be tuned? It's a shame that the the work involved and the cost of recoating the lenses would be far too expensive for a company to offer a tuning service like you can get for cars, rifles, etc. It would be great to have a favourite binocular improved in such a way. 



#11 GlennLeDrew

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Posted 11 April 2024 - 04:28 PM

Not to hijack the thread for my own personal curiosity, but I think this is still generally on topic:

 

Glenn, could you help me (and surely others) understand the difference between field curvature and spherical aberration? I think I understand the theory behind the two and why they occur, but I'm less sure I fully understand their "real world" impact at the eyepiece(s). My understanding is that they are both innate characteristics of spherical lenses, but FC is based on angle of incidence and SA on height of incidence. So, FC impacts the image as a whole (i.e., its impact is noticed off-axis), whereas the impact of SA is realized at all point sources throughout the image creating a blurry halo around each point source (like the "halo" from CA, but not from a specific color) and creating a "softening" of the image throughout.

 

Does spherical aberration create an additional blurring off-axis, or is it is just because it is generally superimposed with field curvature? If an optic is very sharp on axis, but out-of-focus off-axis, does this suggest that SA has been corrected but not FC?

In the main you are correct.

 

However, FC is not inherent to optics having spherical surfaces. Field curvature is the common term for the surface of best focus, which can be non flat for aspheric optics, too. This aberration (and it is very much an optical aberration) generally requires some complexity via additional elements to control and shape as desired.

 

We should distinguish between the impact of FC for imaging systems versus afocal systems. For the latter, which include instruments delivering an image requiring an external system such as the eye to form the final image, FC is simply the result of a disparity between the surfaces of best focus for the sub-systems comprising the device.

 

An objective is permitted to produce a strongly curved focal surface to no ill effect if the eyepiece has its own similarly curved surface of best focus; they would complement each other for the result of good focus field wide. Assuming other aberrations are brought to heel also, such as astigmatism.

 

Indeed, first order astigmatism by itself results in what can be considered two different surfaces of 'best focus.' An astigmatic lens or optical system having a point source some distance from the optical axis typically will form an image as a radially elongated line on one side of best focus and a tangentially elongated line on the other side of best focus. At best focus between these two, the image will have a complex shape, often + shaped. Of course, going farther out of focus will produce an increasingly large and symmetrical blur circle (or whatever is the shape of the effective entrance pupil.)

 

About that last parenthetical insertion. Binos of low to moderate magnification almost all suffer partial clipping of the entrant beam for off-axis light. The large field angle is at the root of it, combined with the fast f/ratio objective. If you put a bright star in the outer field and throw the focus far enough off so as to get a sizeable disc of light, the shape may be obviously not a circle, but instead a bit 'football shaped." This is caused by the entrance aperture of the prism system clipping part of the converging light cone which is arriving at that prism face laterally offset. Looking at the exit pupil from an angle well offset from the emerging optical axis reveals the same entrance pupil clipping, although this can pitentially be worsened somewhat by other clipping occurring farther down in the optical train.



#12 revans

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Posted 11 April 2024 - 05:40 PM

In one of those brain meandering sessions I was wondering why cheaper lenses and so cheaper binoculars don't give a sharp focus albeit in a small sweet spot. As far as I know any lens should theoretically be able to provide a sharp focus somewhere even if it's not where you want it. I understand why CA would affect this by not focusing all wavelengths to the same plane, but it is possible for CA prone optics to still provide a sharp image. Is there generally microscopic flaws in the glass that make the image slightly fuzzy or other factors? 

The way I look at it is that with a telescope, a cheap achromat objective can give good views if it has a long native focal ratio.  My old Sears 60mm achromat refractor that I used a lifetime ago in high school was an f/15 and gave very good views on most astronomy objects.  But binoculars typically have focal ratios around 5 and so aberrations in a cheap objective lens pair become more significant and views aren't as good. Plus, cheap binoculars have more frequent collimation issues.

 

Rick



#13 GlennLeDrew

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Posted 11 April 2024 - 08:37 PM

Bino objectives are faster than one might think; most are f/3.8-4. Even the long-barrelled 'classic' 20X60 are about f/4.5. And the ultra-wide angle 7X50 Tasco Model 124 is f/3.3, which is effectively reduced further to f/3 via a weak reducing lens just ahead of the eyepiece.

 

The driving force behind this is the consumer demand for compactness and light weight, resulting in pushing things to the max. And then some, when this results in aperture reduction due to the steep light cone such fast objectives cannot fully squeeze through the first prism aperture.




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