This page describes tests carried out on tthe Olympust 12 mm f/2 lens for Micro 4/3 cameras. A general discussion of this lens is available here.
This page is part of a set describing tests of small-format lenses in UV photography. The main page describing these tests is available here.
Olympus 12 mm f/2
This is an excellent native lens for VIS photography on Micro 4/3 cameras, capable of high resolution and relatively low chromatic aberration even without in-camera correction. This lens allows manual focusing (albeit with focus-by-wire, not true mechanical focus). A mechanical clutch allows instant switching between manual focus and autofocus. The dedicated lens shade available from Olympus and third-party sellers cannot be used with UV filters larger than 46 mm, and a conical lens shade must be extremely broad and short to avoid vignetting. At the same time, filters smaller than 46 mm do vignette. For this test, I used a 2" Baader U remounted in a 52 mm filter ring and no lens shade.
Because of its complex optical formula with 11 elements in 8 groups, this lens attenuates NUV by over 5 stops with the Baader U filter, with respect to the reference lens. This is an excessive absorption for virtually all types of NUV photography. Use with the Asahi Spectra XRR0340 is unrealistic because NUV transmission with this filter is barely visible.
Image resolution is one of the poorest among the lenses tested in this occasion, even when the flash power was increased by 1-2 stops, compared to the other lenses. I repeated the test a few times with this lens, but results did not improve. One possible cause is trouble for AF to lock onto the subject (albeit the camera did confirm a correct AF with the subject illuminated by a 365 nm torch). Poor optical resolution, possibly caused by a massive amount of axial chromatic aberration within the band transmitted by the Baader U filter and this lens, remains a possible cause of the lack of detail. The strongly violet false-color with Baader U filter indicates that wavelengths below 370-380 nm are not transmitted.