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Apochromatic levels of performance

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#1 Tropobob

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Posted 01 August 2017 - 08:29 PM

With achromatic refractors, there is a general rule that if the F ratio divided by the lens aperture in Cm is 1.2 or larger, then Chromatic Aberration will be at a tolerable level.
(If there is a name for this rule or ratio, kindly let me know.)

Is there any such general rules on the F rations of ED doublets using FPL51, 53 or fluorite glass to qualify as capable of providing a satisfactory level of apochromatic performance?

Also, while I am a roll, can an achromatic with a long F ratio ever be considered to be providing an apochromatic level of performance? (For example, a 60mm F20 telescope?)

#2 photoracer18

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Posted 01 August 2017 - 08:52 PM

Not by the Definition of Zeiss and Abbe. A not ED doublet can't bring more than 2 primary colors to a common focus even if it looks like it is close when the F-ratio is very high.

The Zeiss definition goes like this for colors reaching a common focus:

 

2 colors = Achromatic

3 colors = Apochromatic

4 colors = Super Achromatic

5 colors = Super Apochromatic

 

The term semi-APO is a marketing term by people who want you to believe that the term means something, but its not scientific anyway. Obviously Zeiss was not thinking about marketing. Otherwise calling something Super Achromatic because it focuses double the number of colors compared to Achromatic is not good marketing because its also above Apochromatic (which the name actually translates to 3 color focus). So the scale if invented today would look like this with input from marketing:

 

2 colors = Achromatic

2 colors + = Semi-Apochromatic

3 colors = Apochromatic

4 colors = Super Apochromatic

5 colors = Hyper-Apochromatic

 

And what does this all mean? Absolutely nothing. Any 2 random people are not going to have the same opinion of false color.


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#3 Max Power

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Posted 01 August 2017 - 10:10 PM

Most any achromat under 100x is exactly the same as an apo.

Most f/8 to f/10 achros are exactly like an apo at under 200x.

Most any f/15 or so achro is the same as any apo under 300x.

It is when you goto 350+ powers on planets that apo's still give perfect planetary views and achromats are mostly scattered colors out.
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#4 Sketcher

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Posted 01 August 2017 - 10:19 PM

Of course, an achromat is an achromat . . .

 

Nevertheless, I have two achromats that I would be hard pressed to notice any false color with - regardless of the object being observed; but since it's been a while since I've observed Venus, I'll try to do so soon and report back.

 

One of the telescopes is a 60mm f/11.7 achromat.  The other is a 50mm f/10 achromat.  Both have razor-sharp, essentially 'perfect' images.  Yet, the first was free - having been discarded by at least two previous owners; and the second (a Galileoscope) cost me $15 plus shipping -- new.


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#5 Tropobob

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Posted 02 August 2017 - 12:33 AM

Most any achromat under 100x is exactly the same as an apo.

Most f/8 to f/10 achros are exactly like an apo at under 200x.

Most any f/15 or so achro is the same as any apo under 300x.

It is when you goto 350+ powers on planets that apo's still give perfect planetary views and achromats are mostly scattered colors out.


I would love this to be true, but I have too many refractors that demonstrate otherwise... (Although the last point may be right)

Sorry, I don't mean to be harsh and still appreciate that U have replied.
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#6 Hesiod

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Posted 02 August 2017 - 12:44 AM

As far I know, "empyrical rules" such as Conrady's and the like wete obtained by solving the equations which describes the amount of defocus for an objective designed according the given prescription (something like: to have xx mm defocus for a Fraunhofer objective of yy mm I need a zz focal, which was simplified in the well known aperture/focal ratio formula).
Since "apos" are designed according very different prescriptions, a single empyrical formula will not work.
Anyway, it is often said that a CaF doublet behaves more or less like a standard Fraunhofer with twice the CaF's focal ratio: never checked the math, but could be not far from truth.
In any case chromatic aberration is only one of many, to meet the "apo" standard the spherical aberration must be addressed too, and across the widest spectrum of wavelenghts (otherwise no halos, but soft views nonetheless)
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#7 Uwe Pilz

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Posted 02 August 2017 - 02:41 AM

With ED Apos you get about twice the color correction. That means an ED may have half the focal length in comparison to a FH.

Real Apos are able to get an color correction 5..7 times better than an FH. But that does not mean tat one can use an incredible short focal length. Other deviations cannot be cured wit the same factor. Most important is the spherical aberration and the Gauss error. 

 

For larger instruments it may be safe to say that you can use a foacal length 3..4 times shorter in comparison to a FH.



#8 Kevin Barker

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Posted 02 August 2017 - 04:40 AM

I rather like the definition based on the C-F Blur diameter for different telescopes. It is used by Rohr on astro-foren and is called RC index?? It looks at three points on the spectrum, C E F.

I recall a value <1.0 is considered apo. 1.0-2.0 is considered semi apo and greater than 2.0 achromatic.

There are a large number of test reports on the astro-foren website where coloured filters are used to determine this value. It appears to be measured??(not repeatedly)

This can also be calculated with a simple mathematical formula as well but one needs to know the fraction of the secondary spectrum. From memory RC index =920 X aperture in mm / f ratio X secondary spectrum.  

 

e.g a 100 mm f-10 achromatic doublet with 1/2000th or 0.005 secondary spectrum will have a RC index of 920 X 100/10X1/2000=4.6

 

A f-8 152 mm FPL 51 doublet would have an RC index of 920 X 152/8X1/6000 =2.9 so it is a good achromat.

 

 

Another way of looking at achromatism and apochromatism is by calculation using the refractive properties of the glass types for a doublet. This is described in "Telescope Optics" by Rutten and van Venrooij. One needs the optical glass specifications. From memory typical achromatic doublets might have about 1/2000 secondary spectrum, FPL 51 type glass about 1/6000 secondary spectrum, FPL 53 1/8000 secondary spectrum. Flourite does better at about 1/16000 secondary spectrum. This depends on the glass types and is probably the best outcome possible.

 

 

From all of this I would say a FPL 51 type doublet seems to have about 3X the correction of a typical achromatic doublet, a FPL 53 about 4 X and a Fluorite about 8 X. All dependent on matching glass type.


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#9 Cotts

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Posted 02 August 2017 - 07:06 AM

A point which I noticed:   My old eye lenses, cataracts and all, were quite yellow.  I have owned a few achros in the past and found the purple halo only noticeable on the brightest objects.  

 

After I had lens replacement surgery I have had a chance to look through a couple of achros and - WOW - so much purple!!  Way more than i remembered...

 

And the kicker was a 6-inch Explore Scientific Triplet APO on Jupiter.  A very noticeable purple fringe all around (not atmospheric dispersion, BTW)....  CA in an APO???  Yes.

 

My theory is that old, yellow lenses act as a minus violet filter - the condition of a person's eyes will have a role in determining the level of false colour...Two people looking through the same Achro can differ in the amount of colour they report....

 

Dave


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#10 CounterWeight

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Posted 02 August 2017 - 08:18 AM

Interesting.  I know if imaging (at least satisfied by my own experience), all flaws are bare when it comes to color crossings in focus.  Because of this I think the distinctions are more critical for imagers than for visual. Here I mean apo to super apo.  I do trust Takahashi specs and data about this.

 

For visual I believe the variation in eyes can affect the distinctions, at least I cannot think of a reason to doubt that.  I've been comparing my TSA-120 and my FS-128 and the distinctions are subtle, and I'm always dancing with the seeing and transparency when I try to make a critical distinction.  I have some decent ep's and I feel they contribute slightly to the rendition so there are subtle and some less subtle distinctions there as well.  These present differently depending on the object under scrutiny, and it's because of the subtle difference that I find it difficult to make any pronouncements... to point of being unsure of which word to use.. 'crispness?', 'fidelity?', cold or warm?... and will it stack up the same the following night.  Here taking notes or recording audio and making notes has benefits.

 

This is the second time that I've compared the FS-128, (and I'm using the 'new to me' 128 I recently purchased, interestingly seems zero sample variation between the two 128's optics to my eye)  to an 's' apo.  The 's' apo(s) being the TOA-130(s) and the now TSA-120.  I have not had the opportunity to compare a TOA-130 to a TSA-120.

 

Complexity.  The TOA-130 I remember being flawless up to any mag the sky would hold and into empty... you could not see any sort of breakdown, the atmosphere is the final arbiter.  And it was only in pushing at and near the edge that I noticed what I am calling subtle differences (to my FS-128), and really - how much of that is my atmospherics contributing.  I am finding the same with my TSA-120. 

 

All hold up surprisingly well looking at our moon at 'ridiculous x' magnification.  Trying to be objective while picking up jaw from the ground.  Limb, terminus, subtle gradations, shadows, looking for anything that should not be there and trying to differentiate if any meaningful difference in what is.  Not worrying about the previous nights views and thoughts.  Jupiter and Saturn also make for great test objects for different reasons.  Colored doubles... the greys in nebulas... and star colors... I think if the scope you are using is 'apo' the transition to s-apo might be difficult to truly discern visually, it will not be an obvious or a slap in the face.


Edited by CounterWeight, 02 August 2017 - 08:20 AM.


#11 John Huntley

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Posted 02 August 2017 - 08:45 AM

I guess my TMB / LZOS 130 F/9.2 triplet is in the "super apo" category ?. It and my Tak FC-100DL are certainly the best refractors I've ever looked through. The differences that I've noticed between them and my Skywatcher ED120 and Vixen ED102SS (both ED doublets) are that the Tak fluorite doublet and TMB / LZOS triplet show no CA at all, on any object even inside and outside of focus wheras the Skywatcher and Vixen show a very small splash of pale purple around the brightest stars at focus and slight rims of CA on other bright objects either side of focus. The Tak and TMB / LZOS seem to absorb high magnifications with ease making 200x - 300x well worthwhile with the Tak and even 400x not shabby with the 130mm triplet, when the conditions and target are suitable.

 

Mind you, the Skywatcher ED120 does extremely well given it's "budget" price levels, particularly on the pre-owned market. It's only really since I've owned the Tak and the TMB / LZOS that I've realised just how good the objective of the ED120 actually is smile.gif

 

I don't image though so I'm stuck trying to judge what my eye can perceive crazy.gif


Edited by John Huntley, 02 August 2017 - 08:48 AM.

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#12 SandyHouTex

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Posted 02 August 2017 - 10:20 AM

It's important to remember that ALL lenses are prisms.  All of the elements after the front one, are just trying to put "humpty dumpty's" colors back together again.  lol.gif


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#13 Cotts

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Posted 02 August 2017 - 01:10 PM

I look forward to the production of a 152mm hyper-uber-super-duper APO in the near future.  Oh wait, that's a Newtonian reflector.....

 

Dave


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#14 Element79

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Posted 02 August 2017 - 02:04 PM

It's not just color correction that make an Apo superior.  It's putting the colors where they should be!

 

When all of the colors don't focus at the same spot, even imperceptibly, then there is a 'blur' where what should be a sharp edge and you get a gradient.  The net result of this is loss of contrast and in something like planetary detail, contrast is everything.  In order to see small festoons or a moon's shadow you need extremely high contrast, something an Achro can't provide.

 

I get this feeling that these people that say there isn't much difference between an Achro and an Apo have never looked through similar size aperture examples of the two, and when they do they say something like "Oh", I see the difference now!


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#15 photoracer18

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Posted 02 August 2017 - 02:15 PM

A point which I noticed:   My old eye lenses, cataracts and all, were quite yellow.  I have owned a few achros in the past and found the purple halo only noticeable on the brightest objects.  

 

After I had lens replacement surgery I have had a chance to look through a couple of achros and - WOW - so much purple!!  Way more than i remembered...

 

And the kicker was a 6-inch Explore Scientific Triplet APO on Jupiter.  A very noticeable purple fringe all around (not atmospheric dispersion, BTW)....  CA in an APO???  Yes.

 

My theory is that old, yellow lenses act as a minus violet filter - the condition of a person's eyes will have a role in determining the level of false colour...Two people looking through the same Achro can differ in the amount of colour they report....

 

Dave

This is also the basis of the doublets with Lanthanum flints because Lanthanum doped glass has a yellowish tint and some newer optics with Lanthanum coatings. Using Lanthanum in the optical system can make a larger FPL-51/53/OK-4/FCD1/FCD100 ED doublet almost free of blue/purple fringing especially with the glasses having Abbe numbers above 90.

As for older lenses, when they get a yellowish tint it usually means the glass is thorium doped glass (radioactive) and the color change is indicative of electron (Beta radiation) loss. That glass was about the only higher Abbe number glass lens maker had to design with (except for CaF2) until the advent of LD glass types. This is common knowledge to those of us who have collected old camera lenses although I am not sure I have seen any achromats myself that exhibit this. I would have to inspect then or put a Geiger counter near them. It was used in camera lenses up till at least 1970 by some makers.  It can be reversed somewhat by exposing the glass to intense UV or polychromatic light (sunlight will work but slowly) which initiates free electron capture. At least that is the assumption of what is happening when people have seen the amount of yellow get reduced. 


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#16 photoracer18

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Posted 02 August 2017 - 02:42 PM

I look forward to the production of a 152mm hyper-uber-super-duper APO in the near future.  Oh wait, that's a Newtonian reflector.....

 

Dave

If only that reflector didn't have that oversized black blob right in the center of the image destroying contrast.

Oh wait, that's why we like refractors.smile.gif


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#17 photoracer18

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Posted 02 August 2017 - 03:03 PM

I have owned SCTs up to 11", Newtonians up to 16" and refractors of all types up to 6". I prefer to own at least one of each type at the same time. While each can see the same things, each type has somethings it does better depending on what you want to see/image (except the SCT which is just more versatile than the other 2 main types).
As for refractors, just in the ED or APO category I have owned:
TEC140EDT
Borg 150ED
Meade 152ED
TMB 115T (SV)*
TMB 105T (WO)
TMB 80T SS*
Vixen ED114SS
Vixen ED102SS
Vixen FL-80S*
Vixen ED100sf
SV-110ED
SV-102ABV
AT80ED
(*= currently owned)
The list of achromats is even longer topped by a Jaegers 6" F15 (built by Barry Griner of D&G).
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#18 Cotts

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Posted 02 August 2017 - 03:20 PM

 

I look forward to the production of a 152mm hyper-uber-super-duper APO in the near future.  Oh wait, that's a Newtonian reflector.....

 

Dave

If only that reflector didn't have that oversized black blob right in the center of the image destroying contrast.

Oh wait, that's why we like refractors.smile.gif

 

Please come see either my 5.7-inch  Ceravolo Mak Newt (15% central obstr.) or my 12.5" f/6.5 Lockwood/Teeter (17% C.O.) for fully APOchromatic and contrast transfer indistinguishable at the eyepiece from equal-sized refractors.  Suiter's 'bible' on star-testing says, along with many other authorities, that a C.O. of 20% or less cannot be distinguished visually from no obstruction at all...

 

The C.O. myth lives!!!

 

Dave


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#19 Jon Isaacs

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Posted 02 August 2017 - 03:48 PM

Not by the Definition of Zeiss and Abbe. A not ED doublet can't bring more than 2 primary colors to a common focus even if it looks like it is close when the F-ratio is very high.

 

 

I recommend reading these essays and posts by Roland Chrhisten:

 

Musings on APOs

 

Color Correction in Refractors

 

Achro, Semiapo, ED, Fluorite, etc.

 

Finally, here's an annotated Abbe Normal chart from Roland.  

 

 

Abbe Normal Chart

 

My take:

 

Color correction in ED/apo refractors depends on the same factors as it does in achromats, it's just that there are more variables, more choices.  The glasses chosen for an achromat essentially fix the longitudinal color correction at one part in 1900.  With that fixed, the only other variables are focal ratio and aperture.  Increasing the aperture results in poorer color correction because while the defocused blur is the same size, the Airy disk is smaller so in relation to the Airy disk, the color blur is larger.  The focal ratio affects the diameter of the blur since it's for a given amount of defocus. the blur increases in diameter.

 

In an ED/apo, these same factors are at play but there are more choices, the glasses, the number of elements are added to the mix.  Take a particular design, fix the glasses, double the aperture, to maintain the same level of color correction, the focal ratio will need to be doubled.  

 

I have decided to use the term ED/apo.  In truth, apo is a rather vague term and as Roland points out, modern apos are generally only corrected for spherical aberration at one wave length rather than the two required by the original Abbe definition.  The number of crossings isn't whats important, it's the deviation that is important.

 

The optic with perfect color correction has no color crossing and if it corrected for spherical aberration at one wave length, it's corrected at all wave lengths.

 

They have a name for such an optic.. reflector.. Newtonian.  

 

One note, I find it interesting that Roland, who might have made one doublet in his life, refers to FPL-51 doublets as "apo doublets."

 

Jon Isaacs


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#20 Tropobob

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Posted 03 August 2017 - 02:39 AM

Thanks all for the responses. I feel that I have a much better handle on this in my mind, although, it seems the more I learn, the more complicated things become!
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#21 SandyHouTex

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Posted 03 August 2017 - 08:51 AM

It's not just color correction that make an Apo superior.  It's putting the colors where they should be!

 

When all of the colors don't focus at the same spot, even imperceptibly, then there is a 'blur' where what should be a sharp edge and you get a gradient.  The net result of this is loss of contrast and in something like planetary detail, contrast is everything.  In order to see small festoons or a moon's shadow you need extremely high contrast, something an Achro can't provide.

 

I get this feeling that these people that say there isn't much difference between an Achro and an Apo have never looked through similar size aperture examples of the two, and when they do they say something like "Oh", I see the difference now!

Even APOs or Super APOs don't put all of the colors in the same spot, or Airy disk.  For example, for a f/8 fluorite doublet, which many consider an APO, the red blur disk is 3 times the Airy disk and the same for the blue blur disk at focus in the center of the field.  Reference "Telescope Optics, by Rutten and Van verooj.  Here's a good example of many currently available APOs today:

 

http://www.telescope...po_examples.htm

 

As you can see, all of the APOs, except for the Ortho-APO (Figure 149, bottom row, third from the left) fails to put all of the colors in the Airy disk.  Most of the designs try to bring all of the colors together at the 70% zone, but away from their, the colors are not in the Airy disk and the blur will affect the contrast.



#22 Jon Isaacs

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Posted 03 August 2017 - 08:57 AM

As you can see, all of the APOs, except for the Ortho-APO (Figure 149, bottom row, third from the left) fails to put all of the colors in the Airy disk.  Most of the designs try to bring all of the colors together at the 70% zone, but away from their, the colors are not in the Airy disk and the blur will affect the contrast.

 

 

Each color has it's own associated Airy disk.  Any color that is not perfectly focused, will have a blur that is larger than it's associated Airy disk.. 

 

Enter the reflector... 

 

Jon


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#23 MrJones

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Posted 03 August 2017 - 09:17 AM

Coatings can help. Notice what color the AR coatings on my 152mm f/5.9 is reflecting:

 

https://www.cloudyni...-1474908492.jpg



#24 KevH

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Posted 03 August 2017 - 09:44 AM

 

As you can see, all of the APOs, except for the Ortho-APO (Figure 149, bottom row, third from the left) fails to put all of the colors in the Airy disk.  Most of the designs try to bring all of the colors together at the 70% zone, but away from their, the colors are not in the Airy disk and the blur will affect the contrast.

 

 

Each color has it's own associated Airy disk.  Any color that is not perfectly focused, will have a blur that is larger than it's associated Airy disk.. 

 

Enter the reflector... 

 

Jon

 

Enter coma, diffraction spikes, tube thermals, the need for collimation, poor field illumination. smile.gif


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#25 sg6

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Posted 03 August 2017 - 10:23 AM

I think you will find there is more then just the focal length and the wavelength involved. When talked of it is often simplified. An achro will have a depth of focus and what I find is that within this band the focus appears to remain the same, lets say that on a 1000mm achro the depth of focus is 1mm either side of the optinum plane. On an APO this depth of focus is likely down to 0.2mm either side.

 

The result is that the achro when "in focus" is never quite pin sharp, whereas on the APO it becomes pin sharp but you have to use a dual speed focuser to get on it and remain there.

 

So there is more then the often mentioned factors. I guess much of this is kind of lost these days when people have a DSLR and even the previous auto film cameras where the camera does it for you. Get an old Zenit camera and you learn about depth of field and depth of focus and how to use these to get the photographic effect you want.

 

Have noticed that this "question" often comes up basically amounts to can I get apo performance from an achro, and the answer is No. If you could I for one and I guess many others would go buy the apo performing achro at the achro prices. Like many I do not go spending say £1000 on an apo if I could get the same from a £250 achro.

 

With achromatic refractors, there is a general rule that if the F ratio divided by the lens aperture in Cm is 1.2 or larger, then Chromatic Aberration will be at a tolerable level.
(If there is a name for this rule or ratio, kindly let me know.)

Since the f number is the focal length divided by the diameter the "rule" above is the same as focal length divided by aperture squared.

 

One aspect is that not all achro's use the same glass types, a cheap one may use the most convenient and least costly lumps of flint and crown at the time, may not even be the same as the previous manufacturing run. . A better one may select flint and crown such that the negative element is a bit further along the characteristics and has a lower dispersion, so a bit closer to ED glass without being so. So there are good and bad achro's.




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