Thanks for an overdue discussion on this, Richard!
I have wondered about this very thing for years. I fellow that I know who is
likely "in the know" told me that while he couldn't give me the technical
requirements for true APO status he did know that sales and marketing had
gotten pretty casual with the term.
This might explain why we were often underwhelmed with the performance
improvements of these "APOs".
Tim
----- Original Message -----
From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Friday, January 05, 2007 6:11 PM
Subject: [rollei_list] Re: 500/5.6 Tele Tessar for SL66 Comments
>
>
>
> I wonder if the lens is truly apochromatic. A lot of
> current lenses with the name prefix APO are just well
> corrected achromats. A true apochromatic lens is corrected
> for longitudinal chromatic aberration for three colors and
> spherical aberration for two colors. In addition, most are
> corrected for lateral chromatic aberration. The published
> data for the current crop of "APO" lenses often includes a
> graph of longitudinal chromatic deviation, i.e., the change
> in focus for color. These cross the zero line twice, an
> apochromat will cross the center line three times.
> Note that the overall chromatic deviation of an
> apochromat is not necessarily better than an achromat. The
> apochromat may be correct for three colors but still have
> greater deviation outside of these colors. This is probably
> rare in a commercial true apochromat but still quite
> possible. An apochromat is not necessarily a better
> performing lens than a acromat. The overall performance
> depends on many other factors beside longitudinal color
> correction.
> I will add that many telephoto lenses, especially some
> of the early ones had severe color fringing. This may have
> been due to poor longitudinal chromatic correction but the
> extreme asymmetry of such lenses can lead to large amounts
> of lateral chromatic. This aberration is one where the
> _focus_ of the colors is not too different but the _size_ of
> the image varies. A completely symmetrical lens operating at
> equal image and object distances (completely symmetrical
> optical path) is inherently free of lateral color, however,
> symmetrical lenses, even for objects at infinity, have
> substantial cancellation of lateral color. Non-symmetrical
> lenses, like the Tessar or any telephoto or retrofocus lens,
> must be corrected for lateral color in other ways than
> relying on symmetry or partial symmetry.
>
> ---
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk@xxxxxxxxxxxxx
>
>
>
>
>
>
> ---
> Rollei List
>
> - Post to rollei_list@xxxxxxxxxxxxx
>
> - Subscribe at rollei_list-request@xxxxxxxxxxxxx with 'subscribe'
> in the subject field OR by logging into www.freelists.org
>
> - Unsubscribe at rollei_list-request@xxxxxxxxxxxxx with
> 'unsubscribe' in the subject field OR by logging into www.freelists.org
>
> - Online, searchable archives are available at
> //www.freelists.org/archives/rollei_list
>
---
Rollei List
- Post to rollei_list@xxxxxxxxxxxxx
- Subscribe at rollei_list-request@xxxxxxxxxxxxx with 'subscribe'
in the subject field OR by logging into www.freelists.org
- Unsubscribe at rollei_list-request@xxxxxxxxxxxxx with
'unsubscribe' in the subject field OR by logging into www.freelists.org
- Online, searchable archives are available at
//www.freelists.org/archives/rollei_list
