Olympus G.Zuiko Auto-W PEN F 20mm f/3.5 OM on NEX 3
This is a nice compact wide angle, much smaller than any adapted SLR lens of similar coverage, how does it perform?
The Olympus PEN F series lenses are compact lenses designed for a half-frame (135 format 35mm) film SLR, which conveniently enough is basically the same size as an APS-C digital sensor. The PEN F shot a vertical picture on a 35mm frame. The longer dimension was up and down on the film, this meant you had to shoot with the camera held vertically to get a landscape orientation shot.
Being a half-frame lens, it was designed with a much shorter flange distance than a traditional SLR lens, so the adapter on the NEX is only about 10mm thick, and an overall length is only 47 mm with the adapter and lens. The adapter also tapers down and the lens is only about 46mm in diameter. This appears to be in the same barrel as the 40 mm f/1.4 lens, which is about 5 mm longer than the 38 mm f/1.8 and 25 mm f/4 lenses, but still nice and compact. Build quality is superb, with all metal construction, nice smooth focus and aperture ring. I find the front aperture ring easier to use than some of those next to the body.
Same as with other PEN F lenses, you can pull out (towards the objective element) on the aperture ring and spin it around so you can control in specific aperture numbers (f/3.5, 5.6, 8, 11, and 16) or you can use f/stops relative from wide open (0, 1, 2, 3, and 4)
The lens has 7 elements in 6 groups, with a 5 mostly straight (slightly curved) bladed diaphragm. The minimum focus distance is short 0.2 m (7.9 inches), and the weight is light at 145 g (5.2 oz). It uses rangefinder standard filters of 43 mm.
Comments on the Results
The lens exhibits a lot of field curvature so you want to focus where you want it sharp. The corners are pretty mushy wide open, and even stopped down to f/5.6 or 8, but if you focus for the corner resolution is okay (it still lacks contrast though).
The lens has noticeable barrel distortion, but is less than the new Sigma at 1.2% barrel.
Lateral CA is noticeable, but not bad for a wide angle lens. Longitudinal CA is hardly noticeable. Unfortunately corner shading is an issue. Vignetting is high, and there is a color shift.
I don’t notice much purple fringing. Lightroom 4.1 allows for correction of longitudinal CA and it does a good job, as long as you don’t overdo it and there aren’t strong colors behind the fringes that will desaturate.
Coma and astigmatism are visible, lights in the corners are comet shaped, and line sharpness depends on if it runs radially or tangent.
Resistance to iris reflections is low, and veiling glare is high. Not resistant to flare.
Measured focal length is 21mm at 1:53 magnification.
Pros and Cons
- Similar performance to similar vintage SLR 20 mm
- Compact and well made
- Coma and Astigmatism