Distagon, Biogon and Hologon by H. H. Nasse
Well worth a read for Zeiss fans, and anyone interested in lens design, Zeiss or not.
Distagon, Biogon and Hologon by H. H. Nasse
The article discusses the new Zeiss 25mm f/2 Distagon design, starting on page 6. It also discusses why the thickness of the glass over the image sensor can degrade optical performance of both color and resolution.— very interesting stuff for those inclined to use a symmetric design steep ray angle rangefinder lens on a camera like the Sony NEX-7— don’t assume the results will be all that great.
A few interesting quotes—
To achieve compactness despite the higher maximum aperture, the design of this lens is not quite so complex, consisting of 11 elements in 10 groups. However, these also include three elements made of glass types with a high anomalous partial dispersion, achieving chromatic correction which is not quite as good as with the legendary 2.8/21, but which produces almost no visible color fringes in most cases.
I agree with this based on field shots; a trace of lateral chromatic aberration can be detected, but we will need full frame cameras offering 30-40 megapixels for it to be of any concern.
The older Distagon T* 2,8/25 focuses by means of an overall movement without variable air spaces, and therefore it cannot be seen in any way as a macro lens despite its very near short focus.
That this is true is evident at close range: poor image quality, but it is also true that it is great fun, offering an almost water appearance. Never forget that lens technical performance and desirable imagery are not always the same thing.
At f/2:
A large part of the image area displays high, uniform quality, a brilliant picture with excellent reproduction of details. Stopping down is only necessary if the outer edges are important or greater depth of focus is desired.
Stopping down is definitely required to achieve high definition in the corners, as I have found with numerous field shots. Some of this is due to mild field curvature to the edges/corners. This is not to say that other lenses are better in this regard.
