Carl Zeiss 21mm f/2.8 Biogon T* 2.8/21 ZM

SLRgear Review
December 14, 2010
by Andrew Alexander

Announced at Photokina 2004 with a series of other prime lenses, the Carl Zeiss 21mm ƒ/2.8 Biogon T* uses the Zeiss Ikon lens mount, making it suitable for any Leica body going back over forty years. Or, in the case of this review, the lens is also compatible with the Sony NEX-5 digital camera, by using a third-party lens adapter.

The lens was developed to fit a 35mm rangefinder camera, with a fast ƒ/2.8 aperture. Fitted to the Sony NEX-5, the lens provides an effective field of view of approximately 32mm.

A lens hood is available for but does not ship with this lens, which is available now for approximately $1,300.

Sharpness
We were a bit surprised at the results for sharpness noted with the Zeiss 21mm ƒ/2.8 Biogon. Mainly - it offers quite soft corners when used at its widest aperture of ƒ/2.8. Given that it was designed to cover a full 35mm frame, we were surprised to see as much softness in the corners of the APS-C size frame of our test body when shooting wide open. Sharpness results in the center at ƒ/2.8 are a pretty decent 1.5 blur units, which quickly degrades in the corners, approaching 5-6 blur units on the left side and 8-10 on the right. Stopping down helps greatly to improve sharpness: at ƒ/4, we note around 1 blur unit in the center, softening to 4-5 blur units in the worst corners. Stopping down further doesn't produce any more center sharpness, but it does help to tame the corners, until about ƒ/8-11.

Diffraction limiting sets in by ƒ/11 but is more noticeable at ƒ/16, where the lens shows results of 1.5-2.5 blur units across the frame. Fully stopped-down at ƒ/22, the lens is pretty uniformly soft at about 3 blur units across the frame.

Chromatic Aberration
Chromatic aberration is well-controlled by the lens, though it's possible to note magenta fringing in the corners, in areas of high contrast.

Shading (''Vignetting'')
There is a constant amount of light falloff for this lens; when used at ƒ/4 or smaller, the corners will be 2/3 EV darker than the center. At ƒ/2.8, this differential is just over 3/4 EV.

Distortion
Using the full-frame lens on an APS-C sized sensor helps reduce the distortion produced by the lens, which is gratifyingly little. In the extreme corners, the lens produces just +0.25% barrel distortion.

Autofocus Operation
The Zeiss 21mm ƒ/2.8 Biogon is a manual focus lens. Particular to use on the Sony NEX-5, focusing the lens manually will not engage the NEX-5's automatic 7x / 14x enlarged view mode - you have to push a button to activate that.

Macro
Look elsewhere for macro - the lens offers just 0.047x magnification. Minimum close-focusing distance is 50cm, or just under 20 inches.

Build Quality and Handling
There's no mistaking that the Carl Zeiss 21mm ƒ/2.8 Biogon T* is an all-metal lens. Weighing in at 280 grams (just under 10 oz) it is about the same length as the Sony 18-55mm kit lens, but slightly slimmer. The lens is made in Japan with Quality Assurance supervision by Zeiss.

The lens was attached to the NEX-5 with a Leica bayonet mount to NEX E mount adapter purchased on Ebay. There are several brands to chose from - this particular adapter is made by Metabones and cost about $80. A Voigtlander adapter is also available for about $180. Normally, when the lens is attached with this adapter, it will focus slightly past infinity, but a shim of paper has solved that problem. When mounted on the NEX-5, we found the combination quite front-heavy.

There are only two control features for the lens - an aperture ring, located near the front element of the lens, and a focusing ring, located near the body mount. The focus ring is silky smooth and easy to turn and the aperture ring is marked in 1/3 stop settings. The lens uses ten diaphragm blades so bokeh results should be very good.

The focusing ring is metal with raised ribs, about 3/8'' wide, with hard stops at closest focus and infinity. The focusing ring features an infrared index marker, and has about 90 degrees of rotation. The lens extends very slightly as its focuses.

The optional lens hood for this lens is very wide and very flat with a rangefinder window cutout. It is a bayonet-mounted hood which looks like it could reverse onto the lens for easy storage.

Alternatives

Given the current number of lenses available for the NEX system is three, the use of adapters to mount non-E-mount lenses is currently quite popular.

Sony E 16mm ƒ/2.8 ~$250
While the Sony 16mm ƒ/2.8 wasn't amazing, it was quite good for its price tag, as well as being very small, light and portable. Optically it's even slightly better than the Zeiss - sharper at ƒ/2.8 and ƒ/4, though at ƒ/5.6 and up they're about even. The Zeiss handles chromatic aberration slightly better, while the Sony has less corner shading. Distortion is handled somewhat better in the Zeiss than the Sony.

Sony E 18-55mm ƒ/3.5-5.6 ~$300
The only other out-of-the-box option is the NEX kit lens, which we haven't yet tested - offering the 21mm focal length, though not the constant ƒ/2.8 aperture.

Sony 20mm ƒ/2.8 ~$680
By using the Sony E-mount to alpha-mount adapter, the NEX series of cameras can use standard alpha lenses. The Sony 20mm ƒ/2.8 offers a similar focal length and constant ƒ/2.8 aperture.

Conclusion
While the combination of Zeiss lenses and the NEX camera makes an appealingly small camera platform, it didn't amaze us with its test results: wide-open sharpness was decent in the center, but quite soft in the corners. You're limited to manual focusing, made much easier by the NEX 7x / 14x magnification, but most importantly - you're not getting dramatically better performance than the wider Sony-branded 16mm ƒ/2.8, which offers autofocus and a lower price tag. The Zeiss 21mm does have a wider sweet spot at the center at f/4 and above, and a good bit less geometric distortion, but individual users will have to decide if those factors are enough to justify the price tag.

Zeiss glass does offer a particular look for the images it produces, which is outside the realm of our testing: For that reason alone users with large Zeiss collections can be tempted to find an adapter for their NEX camera. There's also the undeniable appeal of the silky-smooth manual focus operation offered by a high-quality manual focus lens like this. If you're inclined towards the more thoughtful approach to photography that comes with manual-focus lenses and appreciate the "Zeiss look," you'll find the Zeiss 21mm f2.8 Biogon on a Sony NEX body a delightful combination that produces very nice-looking images. Casual users would find it hard to justify the cost vs an inexpensive autofocus wide angle, but for Zeiss fans, it may be less about finding the lens to use with their camera, than one of finding a camera to use with their lens - And the NEX makes a great platform in this regard.

Sample Photos

The VFA target should give you a good idea of sharpness in the center and corners, as well as some idea of the extent of barrel or pincushion distortion and chromatic aberration, while the Still Life subject may help in judging contrast and color. We shoot both images using the default JPEG settings and manual white balance of our test bodies, so the images should be quite consistent from lens to lens.

Still Life shot

VFA target

As appropriate, we shoot these with both full-frame and sub-frame bodies, at a range of focal lengths, and at both maximum aperture and ƒ/8. For the ''VFA'' target (the viewfinder accuracy target from Imaging Resource), we also provide sample crops from the center and upper-left corner of each shot, so you can quickly get a sense of relative sharpness, without having to download and inspect the full-res images. To avoid space limitations with the layout of our review pages, indexes to the test shots launch in separate windows.