The 25mm Biogon is a wide-angle lens of a simple type that is currently specific to the M system. This imparts the lens its small size, simple distortions and excellent performance. In more ways than one this Biogon sits halfway between the 25mm Distagon ZF and 24mm Elmarit-M.
The 25mm Biogon is an example of the design relaxation philosophy of Carl Zeiss applied to the same concepts behind the 24mm Elmarit ASPH. The design of the Biogon leans on a larger number of simpler, spherical elements to deliver a performance that is roughly equivalent to that of the Elmarit at all but the widest aperture. Anecdotally this design employs a very rare triplet group, a trait it shares with the 50mm C-Sonnar and virtually no other lenses.
Immediately after I sold my 24mm Elmarit-M ASPH, I became interested in this lower-cost replacement and the usability improvements I saw in it. I contacted Roger at LensRentals.Com and he kindly agreed to send me a copy to write this evaluation. The lens I received was in great condition as usual, my own lenses don’t stay in this good condition for long.
The 2.8/25mm Biogon is unexpectedly larger and longer than both the 2/35mm Biogon and 2.8/24mm Elmarit-M. In the absolute it remains a small lens however, and it obstructs only a small portion of the viewfinder on the M9 camera. The hood for this lens (not tested) is quite a bit slimmer than the one for the Elmarit-M so the final comparison might come close to even.
The aperture ring is one of the strongest points for ZM lenses, and this lens is no different. The implementation on the 25mm Biogon is thicker and feels even more positive than the ZM lenses previously tested here.
Manual focus with this Biogon is extremely easy over the entire range that is coupled to the rangefinder. For me, the ribbed focus ring was a notable usability improvement over the focusing tab of the Elmarit-M. The key fact is that the same grip on the lens works equally well at infinity and the closest focus point, extreme wrist flexibility is not required with this lens.
The 25mm Biogon can focus closer as well, outside the Leica rangefinder’s capabilities. Despite the Leica lenses’ hard stops at 0.7m, and to my great surprise, the M9 camera is coupled for focus down to around 0.6m. This lens goes 10cm further, down to 0.5m, only using your eyes for rangefinder.
This lens shifts forward as a whole when focused closer than infinity; it does not implement floating elements.
The drawings of the 25mm Biogon ZM are reminiscent of those of the middling 25mm Distagon ZF – it’s true, and don’t panic. The Biogon performs substantially better than its bulkier sister, but astigmatism and related halation artifacts impact its usefulness at full aperture and one stop down. The net result is that this Biogon is a fantastic performer, but reservedly so until smaller apertures.
At the full aperture of f/2.8 the 25mm ZM renders very fine detail at high contrast along the axis. The outer regions render fine detail, but not very fine detail, at medium contrast with a decent amount of light fall-off. The overall performance at this aperture would be very good already were it not marred by halation at high contrast edges situated near (but not in) the focal plane. These often-colored artifacts take on a look similar to that of other astigmatic wide-angle lenses we have seen recently (35mm ZM, 25mm ZF, 28mm ZF).
One stop down to f/4 brings small improvements in terms of resolving power, the magnitude of aberrations is slightly less and peripheral illumination and contrast improve. With the second stop down to f/5.6 the lens now really stretches its legs and delivers its peak performance where micro-contrast is concerned, over the whole frame. Halation is still not completely gone at this aperture but practical photographers will agree this aperture is very good in this respect.
It is with the third stop down to f/8 that the Biogon’s drawing becomes essentially flawless on the M9’s sensor, but now with a tad less micro-contrast than seen at f/5.6. At this aperture the Biogon is an extremely powerful lens for the snapshot style of photography, saturating the high-resolution sensor of the M9 with minimal fuss and zero artifacts.
The 25mm Biogon is very resistant to veiling glare, but is a bit more prone to flare spots than other M-system wide-angle lenses tested to date. When shot against the Sun, the Biogon adds a short string of red artifacts of odd shapes (again, probably due to astigmatism).
The 25mm Biogon easily eclipses its DSLR siblings but falls short of matching the optical prowess of its closest Leica competitor. Nevertheless the Biogon delivers improvements on that lens’ few flaws, including its price. This lens is a good match for the M9 camera, and for the upcoming crop of professional mirrorless cameras.
Would I recommend this lens to other photographers? Yes. However I am most keen to see the 2.8/21mm Biogon ZM for myself.