I am wondering whether to get an ultrawide rectilinear lens for my E-M1. I am in any event going to be getting the Samyang 7.5mm f/3.5 fisheye lens very soon (and I have had that lens before and liked it), but I have never used a rectilinear lens shorter than 12mm (or 24mm with 35mm film) before. I have read Ken Rockwell's guide to using ultrawide rectilinear lenses, and think that one might be quite useful for interiors, landscapes and cityscapes, all photographic subjects that I particularly like (although owing to living in London, I do not get the chance to take rural landscapes very often). I suspect that I should find better value in a secondhand lens than a new one by some considerable margin, and it is at secondhand lenses of this type that I have been looking. Setting aside for a moment the future Olympus pro ultra-wide zoom which will not be with us for over a year (and which I suspect will probably be a 7-12mm f/2.8), there are four options: (1) the Olympus 9-18mm f/4-5.6 Micro Four Thirds; (2) the Olympus 9-18mm f/4-5.6 Four Thirds; (3) the Panasonic 7-14mm f/4 Micro Four Thirds; or (4) the Olympus 7-14mm f/4 Four Thirds. No. 1 is the most convenient and compact option, the second cheapest, and will focus quickly; but it is known to be soft in the corners, which is far from ideal for landscapes or interiors. No. 2 is a feasible option with the E-M1; it is, I think, about the same size as the 12-40mm but lighter and sharper in the corners than the Micro Four Thirds version. It is also the cheapest of the four. Both 9-18mm lenses, however, overlap rather more with my existing 12-40mm lens than the 7-14mm lenses do. The overlap being on the shorter end, however, does mean that the optical quality and maximum aperture of the lenses will be at its best in the lens's most useful range. No. 3 is considerably more expensive (nearly twice as much) as the 9-18mm lenses, and has the additional disadvantages of the serious flare problems, especially with the Sony sensors on the most recent Olympus cameras (and apparently, to a lesser extent even with more recent Panasonic sensors). It is, however, a wonderfully compact lens otherwise capable of high optical quality and a very wide angle of view indeed. No. 4 is probably optically the most superior of the lot, and, although notionally the most expensive, there is actually a poor fellow who has been trying to sell his secondhand on eBay in the UK for about a month without luck and keeps reducing the starting bid by £10 each time, with the result that the thing costs less than a secondhand Panasonic 7-14mm. I was somewhat tempted by this (which is also the only weather sealed option) until I checked the Four Thirds website and saw that it weighed over 700g, which is a bit much for a lens that I cannot see myself using terribly often. I am currently leaning strongly towards the cheapest option, the Four Thirds 9-18mm, but I should appreciate input from people who have used any of these lenses on the subject before making any final decisions, not least because I have only been able to find one review of the Four Thirds 9-18mm which was quite positive, but its metrics, especially for sharpness, were not easily comparable those of other lenses on other review sites, and there are also few sample photographs to inspect. Beyond the question of the lenses themselves, I am also interested in people's views on how useful that they find ultra-wide rectilinear lenses generally, for what subjects that people tend to use them, and the extent to which (for each sort of subject) the difference between 7mm and 9mm makes a real difference. My provisional thinking on the topic, incidentally, was that there is much to be said for using the fish-eye when an extreme wide-angle view is desired (on the basis that the fish-eye, although it distorts shape, at least does not distort perspective, which is arguably a more useful thing to keep undistorted in extreme wide-angle situations) and that the less extreme 9mm is actually likely to be a more usable focal length for general cityscapes, landscapes and interiors than the more extreme 7mm, which, in its rectilinear form, can do very weird things to perspectives indeed. I am very interested in people's views on the various issues discussed here.