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Apochromatic correction (1 Viewer)
- Thread starter MarkHows
- Start date
If I remember correctly, the apo- bit means that the lens will bring all three primary colours (red, blue, green) to focus at the same point. 'Normal' scopes (acrochromatic?)will only bring two of them (can't remember which ones) to focus at the same point, the third colour will be ever so slightly out, hence the apo- scope will give a sharper image.
Stands by to be shot down in flames by those who know better.
AndyC
Stands by to be shot down in flames by those who know better.
AndyC
Jay Turberville
Well-known member
AndyC said:
Well, from what I've read, there are differing critieria. But what all have in common is that an APO is very well corrected at three specific locations int the visible spectrum. Because of this, the errors between these three locations tend to be relatively small and overall color correction is quite good.
Some of the variations in the criteria have to do with how well corrected "corrected" is and what specific type of aberration are considered. That may not be quite right, but that is the jist of it.
An Achromat, OTOH, is well corrected at two locations along the visible color spectum. Consequently, the in-between points tend to wander off target more significantly.
Here is where I'll probably get shot down. Most of the scopes that tout ED, HD and Fluorite but don't claim to be APO are essentially acrhomats that are better corrected by the use of the more exotic optical glass in one (or more?) elements. So they have significantly better correction, but not to the same degree as an APO.
Read the following if you really want to understand.
http://voltaire.csun.edu/tmb/definition.html
nigelblake
don't re member
Here is a fact sheet that answers many questions re lens design and aberations including apochromatic issues.
http://www.photo.net/photo/optics/lensFAQ.html
http://www.photo.net/photo/optics/lensFAQ.html
I was under the impression that in scopes and binoculars, APO is used as marketing talk. Back focal distance for blue, red and green. I am sure that someone once told me that no one has APO correction in scopes. Fluorite is not used (CAF2), but fluoride, which I think is different.
Adey Baker
Member
For the record, this is what Leica say in their brochure produced in about 1997, I think:
'Advantages of the APO-TELEVID
High-resolution, three-element optical system with fluoride glasses with partial anomalous dispersion
High contrast rendition
Corrected apochromatically to the farthest corners
No colour fringes in the entire image area
Optimal rendition of the finest structural details in bright as well as dark areas.'
I think the term 'apochromatic' was coined, in the 19th century, by Ernst Abbe of 'Carl Zeiss' when he developed the first fluorite optics (in a microscope).
When all photography was black and white with even the negatives being 'inspected' under red safelights most films were 'orthochromatic,' i.e. not sensitive to red light so lenses that were corrected for blue and green light were called 'achromatic' from the Greek a = non, chrome = colour.
Thus the term 'achromatic' (non-coloured) which is what apo lenses really are had already been used.
'Advantages of the APO-TELEVID
High-resolution, three-element optical system with fluoride glasses with partial anomalous dispersion
High contrast rendition
Corrected apochromatically to the farthest corners
No colour fringes in the entire image area
Optimal rendition of the finest structural details in bright as well as dark areas.'
I think the term 'apochromatic' was coined, in the 19th century, by Ernst Abbe of 'Carl Zeiss' when he developed the first fluorite optics (in a microscope).
When all photography was black and white with even the negatives being 'inspected' under red safelights most films were 'orthochromatic,' i.e. not sensitive to red light so lenses that were corrected for blue and green light were called 'achromatic' from the Greek a = non, chrome = colour.
Thus the term 'achromatic' (non-coloured) which is what apo lenses really are had already been used.
In their 2001 brochure it says:
Top of its class spotting scope for professionals. With fluorite containing optical glasses for images without colour fringes and with unmatched sharpness. It then goes onto say: Leica APO Televid 77 with its high grade fluorite - containing optical glasses.
Adey why did they change the text from fluoride to fluorite
Top of its class spotting scope for professionals. With fluorite containing optical glasses for images without colour fringes and with unmatched sharpness. It then goes onto say: Leica APO Televid 77 with its high grade fluorite - containing optical glasses.
Adey why did they change the text from fluoride to fluorite
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Adey Baker
Member
The plot thickens!
Although Leica uses the term 'fluoride' three times on the same page of the brochure, in the 'Tecnical Data' section at the end under 'number of objective elements' it says: '3, fluorite, all with multiple coatings.'
I thought I'd check the instruction manual to see what it says and again under 'Technical Data' for 'Lens' it says: 'uncemented, 3 lensed, apochromatic system special fluorite glas (sic) with anomalous partial dispersion multiple layer coating; outer lenses hard coated.'
They misprinted 'glas' so I suppose they might have got their 'd's and 't's mixed up (twice)!
Also, interestingly, the 77mm UVa filter is recommended for only the Apo-Televid not the Televid, to protect the front lens.
Although Leica uses the term 'fluoride' three times on the same page of the brochure, in the 'Tecnical Data' section at the end under 'number of objective elements' it says: '3, fluorite, all with multiple coatings.'
I thought I'd check the instruction manual to see what it says and again under 'Technical Data' for 'Lens' it says: 'uncemented, 3 lensed, apochromatic system special fluorite glas (sic) with anomalous partial dispersion multiple layer coating; outer lenses hard coated.'
They misprinted 'glas' so I suppose they might have got their 'd's and 't's mixed up (twice)!
Also, interestingly, the 77mm UVa filter is recommended for only the Apo-Televid not the Televid, to protect the front lens.
Adey Baker
Member
Our posts crossed there, CDK, but that 'fluorite-containing optical glasses' phrase is probably the key.
As far as I can make out (mainly from past articles in photo magazines), 'Fluorite' is a pure crystal of Calcium Fluoride. It only occurs in small amounts, good enough for a lens, naturally and has to be grown artificially for bigger lenses.
Glasses 'containing' fluorite would thus, by definition, not be pure fluorite and I would imagine that this is where the term 'fluoride lenses' comes in.
Hopefully, someone who really knows about these things will come along and clarify the situation for us!
As far as I can make out (mainly from past articles in photo magazines), 'Fluorite' is a pure crystal of Calcium Fluoride. It only occurs in small amounts, good enough for a lens, naturally and has to be grown artificially for bigger lenses.
Glasses 'containing' fluorite would thus, by definition, not be pure fluorite and I would imagine that this is where the term 'fluoride lenses' comes in.
Hopefully, someone who really knows about these things will come along and clarify the situation for us!
Yes the definition of the term APO is rather fluid. Basically an APO lens gives better correction for chromatic AND spherical aberrations than a normal achromat. This means that an APO lens will be sharper and show less colour fringing. It will also have better contrast and 'colour purity'. The difference is most noticeable at high powers.
I have a Leica APO 77 Televid and it DOES show some fringing under certain conditions even at the field centre, but it is insignificant. I have not used a non APO Leica, but I have used an Opticron 66 non-APO (non-ED) scope, and saw prominant colour fringing when viewing ducks on a lake.
I would say that the Leica APO 77 is a near-apochromat - or at least mine is (sample variation?) - and the Zeiss FL 85 is a true apochromat in that it has as far as I can tell perfect colour correction.
I have a Leica APO 77 Televid and it DOES show some fringing under certain conditions even at the field centre, but it is insignificant. I have not used a non APO Leica, but I have used an Opticron 66 non-APO (non-ED) scope, and saw prominant colour fringing when viewing ducks on a lake.
I would say that the Leica APO 77 is a near-apochromat - or at least mine is (sample variation?) - and the Zeiss FL 85 is a true apochromat in that it has as far as I can tell perfect colour correction.
Adey Baker said:
Again, someone may know better...
1. Fluorite = Calcium Fluoride. A crystalline mineral. Has a very low dispersion of different wavelengths of light.
2. Fluoride = a compound containing fluorine. ED/HD glass may contain some kind of fluorides ( -> almost as low dispersion as with fluorite).
3. Fluorine = an element.
IMO "fluorite containing glass" is not correct - should be fluoride or fluorine.
I am with Jay about the definition about Fl/ED/HD vs. APO - usually an APO objective requires three lenses.
Ilkka
iporali said:
Some while back I found an article on this issue by no less than Thomas M Back. To the uninitiated, Thomas M Back is a highly regarded American optical designer who designs what some people consider to be the finest astronomical refractors currently available. Here is the article:
http://voltaire.csun.edu/tmb/definition.html
Leif said:
Abbe's traditions are still very strong among us biological microscopists
When we buy objectives to a microscope, we have roughly three choices: Achromats (€), Fluorites (€€) and Apochromats (€€€€)
The difference between Fl and APO here is that APOs have a lot more glass inside. We don't understand the theories very well, we just use the optics. :hi:
I remember reading somewhere in Takahashi's web pages that the Zeiss, Leica and Kowa would use "real" fluorite.
Ilkka
henry link
Well-known member
I'm pretty certain Takahashi still uses "real" Flourite. Their approach has been to improve the techniques for working with the material. I've owned 5 Takahashi scopes over the years (3 now) and have never experienced any of the problems attributed to CaF2. The oldest scope (FC-50) is about 15 years old with an uncoated Flourite element and still produces a nearly perfect star test. The oldest scope with a multi-coated Flourite element (FS-78) is 10 years old and shows no signs at all of coating deterioration. These are both 2 element APO's, which is possible because the focal ratio is F:8.
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Jay Turberville
Well-known member
Leif said:
A cracking good analysis, Grommit!
gthang
Ford Focus Fanatic: mmmmmm... 3.1415926535.....
In chemistry, fluoride is the name of any chemical compound containing Fluorine.
an example of a fluoride compound could be NaFl, or Sodium Fluoride.
Fluorite, on the other hand, is the common name of an mineral containing either pure fluorine or a compound containing fluorine.
PS. Periodic Table facts on FLUORINE: Atomic number 10; Type: Inert Gas (Noble Gas).
an example of a fluoride compound could be NaFl, or Sodium Fluoride.
Fluorite, on the other hand, is the common name of an mineral containing either pure fluorine or a compound containing fluorine.
PS. Periodic Table facts on FLUORINE: Atomic number 10; Type: Inert Gas (Noble Gas).
gthang
Ford Focus Fanatic: mmmmmm... 3.1415926535.....
iporali said:Again, someone may know better...
1. Fluorite = Calcium Fluoride. A crystalline mineral. Has a very low dispersion of different wavelengths of light.
2. Fluoride = a compound containing fluorine. ED/HD glass may contain some kind of fluorides ( -> almost as low dispersion as with fluorite).
3. Fluorine = an element.
IMO "fluorite containing glass" is not correct - should be fluoride or fluorine.
I am with Jay about the definition about Fl/ED/HD vs. APO - usually an APO objective requires three lenses.
Ilkka
Does "Fluoride Toothpaste" fit into category 2?
AlanFrench
Well-known member
If you are willing to limit the aperture and go with a long focal length, you can get color free images even with the traditional achromat choices - BK7 and F2.
By using an ED material, be it a fluor-crown glass or true fluorite, you can make a color free doublet of larger aperture and shorter focal length. Using three elements does allow better color correction, and is a good choice if you want to keep the focal length even shorter.
Clear skies, Alan
By using an ED material, be it a fluor-crown glass or true fluorite, you can make a color free doublet of larger aperture and shorter focal length. Using three elements does allow better color correction, and is a good choice if you want to keep the focal length even shorter.
Clear skies, Alan
iporali said:
