Carl Zeiss Biogon T*2,8/25 ZM ($1,201 on Amazon)(Opens a new window). This wide-angle lens is designed for M-mount rangefinder cameras. The lens is compatible with mirrorless cameras such as the Sony Alpha 7, even if you do not have a Leica digital 35mm rangefinder. The lens's wide field of vision is attractive to photographers who think a 21mm lens might be too broad or a 28mm lens not enough.
This Biogon, like the rest of Zeiss ZM's ZM series, has a steel lens barrel and a manual focus. It features a smooth, long-throw, tight tolerance and smooth operation. To make it easier to focus, the focus ring features a smaller grip and can be adjusted from f/2.8 to f/22 at third-stop increments. It measures 2.5 by 2.1 inches (HD), is 9.2 ounces in weight, and can support 46mm front filters.
The lens does not come with a hood, however you may purchase one. Two hoods are available from Zeiss, one for square and one for circular lenses. Each hood costs $84. The barrel has a printed depth-of-field scale with marks to the nearest f/16. This makes it easy for you to focus on scale.
If you are using the Leica rangefinder with Live View, it is not possible to focus to less than 0.5 meters. Instead you will need to estimate distances. You will need to either use the 28mm frames and lines loosely, or buy an external finder. Leica M8 uses a smaller APSH image sensor. It has a set of lines that roughly approximates the 24-mm field-of-view, making it possible to use a 25mm lens with an external finder.
Imatest was used to examine the distortion characteristics when the lens is paired with the Leica M full-frame Leica M (Typ240) ($4,999.99 Amazon). The barrel distortion is only barely visible in the field. It measures about 1.1 percent. The center-weighted sharpness at f/2.8 is impressively 2,213 lines per pic height.
This is much higher than the 1,800 lines required to call a photograph sharp. There's some drop-off at outer edges of the frame—they show just 1,070 lines. This is a small enough amount to indicate some visible fuzzyness.
This problem can be solved by reducing the aperture to 4x. This score is now 2,806 lines. The edges are sharpened to 1,580 lines. The lens reaches f/5.6 at 3,000 lines and the edges exceed 2,200 lines. The best performance is at f/8 where you get an incredible 3,122 lines and edges hovering around 2,600 lines.
The Carl Zeiss Biogon T*2,8/25 ZM wide-angle rangefinder lens is an excellent choice if you are looking for one. The price is about half that of the Leica Elmar M 24mm f/3.8 ASPH. It also captures twice as much light at the maximum aperture. Although the Biogon's optics are very good, you will need to slow down to get sharp images from corners to corners.
There is also a small amount of distortion. It's still a great deal at $1,200 compared to Leica lenses. The Biogon's compact design and manual focus can't be beat by cheaper SLR lenses with similar focal lengths.