RiceHigh's Pentax Blog: Comparing the Optical Formulae of DFA*70-200 F2.8 Vs FA*80-200 F2.8

Friday, March 18, 2016

Comparing the Optical Formulae of DFA70-200 F2.8 Vs FA80-200 F2.8

19 Elements in 16 Groups; Source: Ricoh Imaging

16 Elements in 13 Groups; Source: Bdimitrov's K-mount Page

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ToetSFX · 470 weeks ago

THX, Suberp lenses the FA* 's ..... Unfortunately I have never photographed with these lenses.

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adrianpglover · 469 weeks ago

Definitely not the same formula. I'm glad to see that they're using a lot more "special" types of glass. I know some of these are for cost sake, but all of them are more current technology than the older one. Only time will tell which one is "better", if there's a noticeable difference.

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super special glass · 469 weeks ago

marketing?

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Chris · 469 weeks ago

can anybody point me to a tutorial or textbook that helps in interpreting diwgrams such as the above?
that would be greatly appreciated.

6 replies · active 468 weeks ago

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Chris · 469 weeks ago

just to be clear: I need more than a "this is an element, that is a group, here is the diaphragm, and this is the optical axis". I'd like to learn whether or not a design is "impressive". Because, mind you, it is absolutely easy to put a lot of glass in a lens and charge a lot of money for it.

Naively, it seems to me that a superior lens design would be characterized in the lens diagram by having FEW lenses (=good flare control, lighter) but several different sorts of glass (=supressing chromatic errors) and a lot of aspherical stuff (=higher sharpness, avoiding the spherical errors). Of course, this comes on top of the total lens length being short (=easy to carry) and the front element being wide (=fast lens).

Right or wrong?

By these measures, the new lens above is "better" in all aspects but the number of elements, which has increased.

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adrianpglover · 469 weeks ago

I think you understand these diagrams as well as I do.

There is another problem not being discussed when these diagrams are posted: What is the construction quality of each of the elements and how repeatable is it? They might be able to create one that is really good, yet not be able to keep the quality consistent. At the same time a superior design with inferior manufacturing means that a seemingly "better" lens on paper could be a much worse lens in actuality. I'm not saying anything about the actual quality of the manufacturing lines used to produce either of these lenses. My only point is that since the diagrams are different all one can say is that the two lenses are in fact different designs.

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Chris · 469 weeks ago

Well then the purpose of publishing and looking at the diagrams is very basic indeed...

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Chris · 469 weeks ago

BTW, the new lens has a little closer minimum focusing distance (1.2m instead of 1.4m) but is heavier (1755g instead of 1510g).

And off-topic: I received my DFA 24-70 these days. It is lighter than the Nikon/Canon competitors but pretty heavy by my measures, I am a fan of the limiteds. I made a couple test shots with my K-3 (which of course uses the sweet spot of this FF lens). Boy are these pictures sharp and contrasty. Wow.

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Kunzite · 468 weeks ago

An element means an individual lens element. A group is simply two or more lens elements cemented together.
For example, in the second diagram, the 3rd group from the lens mount is formed by cementing an ED (blue) element with a "normal" optical glass one.

I would not say that a superior lens design has fewer elements. I'm afraid the subject is beyond by abilities, in-depth knowledge of optics would be needed... however, I do recognize the complexity and that making naive assumptions will get us incorrect answers ;-)
For example, incorporating aspherical lenses can indeed reduce the number of elements required to meet certain performance targets. But not on longer lenses, where there's no benefit; you can see the D FA* 70-200 doesn't incorporate aspherical elements, while much cheaper (and wider) lenses do (at least inexpensive hybrid glass/acrylic elements). You can see the same pattern on other lenses, and on other manufacturer's lenses.

On longer lenses, there's instead an increasing need to use special e.g. low dispersion glass; and Pentax/Ricoh Imaging put no less than 8 such elements in the D FA* 70-200.
In comparison, Canon 's 70-200 f/2.8 II has 1 Fluorite and 5 ultra-low dispersion elements, and the Nikon's 70-200 f/2.8 II has 7 ED elements - numerically fewer than Pentax.

So while I cannot provide an in-depth optical explanation, we can be certain that the D FA* 70-200 has a no-compromises optical design.