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Archive 2015 · APO logistics

  
 
nehemiahphoto
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p.1 #1 · p.1 #1 · APO logistics


How complicated, expensive and how much additional size is required for apochromatic correction?

The VC 125 isn't especially large for it's specs, and it's APO. I am wondering why lens designers aren't putting them in large aperture tele's, like the 85 1.2L or Contax N 85, or Batis 85. I realize the Otus line is APO, but they are huge.

Realistically, could we expect something like a 100 MP with APO correction at the same size? In some situations fringing is fine with some PP, in some it can't be undone.

edit:

1) Is it just an additional lens element, or does the entire optical formula change?

2) Does it significantly change the "stamp" of a lens in terms of color, bokeh, focus transition etc?



Oct 28, 2015 at 12:13 PM
telyt
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p.1 #2 · p.1 #2 · APO logistics


nehemiahphoto wrote:
How complicated, expensive and how much additional size is required for apochromatic correction?

The VC 125 isn't especially large for it's specs, and it's APO. I am wondering why lens designers aren't putting them in large aperture tele's, like the 85 1.2L or Contax N 85, or Batis 85. I realize the Otus line is APO, but they are huge.

Realistically, could we expect something like a 100 MP with APO correction at the same size? In some situations fringing is fine with some PP, in some it can't be undone.

edit:

1) Is it just an additional lens element, or
...Show more

1) It's a different optical formula
2) it depends on the particular lens



Oct 28, 2015 at 12:48 PM
JonPB
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p.1 #3 · p.1 #3 · APO logistics


I think it helps to know that, by default, lenses produce chromatic aberrations. Consider your basic prism, which takes white light and distributes it into a pretty rainbow. That's the result of dispersion, and all transparent materials that I'm aware of demonstrate some level of dispersion within their range of transparency. Color correction in a lens is a matter of using two or more lens materials that counterbalance each other. In theory, you could have one element that takes white light and makes a rainbow, then a second element that takes the rainbow and turns it back into white light; a single element will never do, but two or more might. Color correction is all about balancing lens types in order to minimize the amount of dispersion exhibited by the system as a whole.

Of course, dispersion is only theoretically independent from refraction, which is how strongly a lens element will redirect light that passes through it. Refraction is what makes a thicker or thinner element attractive, or one with more or less curvature. In practice, available glass types have only a limited range of combined dispersion and refraction characteristics. Use a new glass type to improve its dispersion characteristics and you'll likely change its refraction as well, requiring a whole new calculation for the lens.

But let's play the theory and say that you could retain the refraction characteristics of each lens element while changing their dispersion characteristics. Here, a given lens -- e.g., Zeiss 100 MP -- could retain the same style but have improved chromatic performance. And that'd be great; same rendering style, same optical signature, but with less CA. Even if that's possible, though, the glass will likely cost several orders of magnitude more. If the end product is going to cost two, five, or ten times as much, chances are that other adjustments can be made to the lens that are more cost effective, which could make for a better lens overall but with still some residual CA and with a different optical signature.

Which is to say:

1) Definitely not just adding one element.
2) Theoretically, you don't have to add any elements at all, just swap existing ones with glass types that perform better. This would allow the designer to recalculate for dispersion/chromatic effects only while retaining the same refraction/rendering "stamp."
3) Practically, doing so would be rarely cost effective (if it were, the original lens would have incorporated that design), and a cost effective redesign would change the entire calculation, including both for chroma and ultimately for rendering.

Note that I'm not an optical designer; I didn't even study physics in college, much less optics, much less exercise those theories in practice. I try to teach myself about the subject from time to time, but everything here should be taken with a slab of salt. I hope this helps, though.

Cheers,
Jon



Oct 28, 2015 at 03:06 PM
Phillip Reeve
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p.1 #4 · p.1 #4 · APO logistics


Lets hope that Brandon Dube stops by to give us an idea how much effort it takes to correct CA to such a degree that we can call an lens APO corrected.


Oct 28, 2015 at 03:13 PM
JonPB
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p.1 #5 · p.1 #5 · APO logistics


I think wikipedia does a good job on the technical definition of APO correction: basically, three parts of the spectrum are in focus on the same plane.

In practice, that definition doesn't mean much. While I have no reason to doubt that some lenses branded as APO are in fact APO (I'm thinking of Sigma and Minolta here), my impression is that comparable Leica and Zeiss lenses have better color correction even without being designated as APO. Meaning, "APO" is more of a marketing term than a useful description of a lens' performance.

Cheers,
Jon



Oct 28, 2015 at 03:17 PM
phuang3
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p.1 #6 · p.1 #6 · APO logistics


It just implies a well selection of glass types to minimize the color aberrations. It may sound easy but in reality, special glass needs special care, handling or process. They can be soft, brittle or oxidized easily, not to mention the cost they can have or the time required for annealing.


Oct 29, 2015 at 01:13 AM
nntnam
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p.1 #7 · p.1 #7 · APO logistics


APO is quite a relative "term" as far as I know. Some lenses are APO on low resolution camera, but not anymore on high resolution cameras.

And each manufacturer defines their own degree of color correction. There's Sigma's APO, there is Leica's APO and then there is Zeiss' APO. Zeiss' APO is (probably) currently the best in wide-normal/short-tele range which explains the huge size of Otus line.

More information could be found in this interview with Dr Hubert Nasse at Zeiss.





OP's #2 question is very interesting. I hope someone with knowledge in the field will explain this to us.

Edited on Oct 29, 2015 at 01:52 AM · View previous versions



Oct 29, 2015 at 01:47 AM
sirimiri
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p.1 #8 · p.1 #8 · APO logistics


Minolta called lotsa stuff "APO".


Oct 29, 2015 at 01:47 AM
paulhofseth
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p.1 #9 · p.1 #9 · APO logistics


Colour correction, like other specs is a question of tolerances; how much blueish, greenish and reddish images are allowed to differ across the plane of focus, in front of, and behind it.

Advertisements do not usually make finer distinctions very clear. Consider "waterproof" cameras. A properly maintained Nikonos goes very much deeper than a pocket phone guaranteed to withstand a few metres subrmersion for half an hour. One type of "stainless steel" bolt can perish in some environments, a proper spec. will do fine.

The classical apochromats of Kinoptik (f.2\100 etc) were also big and heavy because they were fast. Kerns apochromatic 50mm Switars were not particularly large and heavy and the Leitz 180\3,4 ApoTelyt is quite compact.

p.



Oct 29, 2015 at 02:44 AM





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