I've hesitated posting this for some time, but since I find it useful I thought I'd put it out there. If you hate math you might not want to read the entire thing. Like many people, I like to take photos in low light, and rarely have a tripod with me. To be useful under these conditions a lens must: (a) have a large aperture, obviously; and (b) have a short focal length. The short focal length is important because hand shake is magnified with longer focal lengths, which is the origin of the oft-quoted "1/FL" shutter speed limit for hand-held shots. The focal-length (FL) in this equation is the full-frame equivalent and has to be doubled on an m43 camera; i.e. you'd want a shutter speed no slower than 1/40 for a 20mm lens. It's important to note that this relationship is more than a "rule-of-thumb"; it can be justified theoretically (Shutter speed rule of thumb). Although some people have steadier hands than others, the minimum 'steady' shutter speed is always proportional to 1/FL. In order to compare different lenses with different focal lengths and different maximum apertures, I've created a little algorithm. Here's the math: The light-gathering capability of a lens is proportional to 1/A^2 (A = aperture). In other words, a lens at f/2 will gather four times as much light as a lens at f/4 [1/4 vs. 1/16]. The slowest shutter speed you can safely use with a lens is proportional to 1/FL. Combining these effects, the 'low-light-ability' of a lens is proportional to 1/(A^2*FL). Since the numbers produced are small, I've normalized them against the common Panasonic 20mm f/1.7 lens. The low-light-ability of this lens is: 1/(1.7^2*2) = 1/60. So, for example, the Olympus 12mm f/2 lens has a low-light-ability of 1/(2^2*12) = 1/48, which is slightly better than the 20mm Panasonic. You can compute just how much better by the formula: LLA = 60/(A^2*FL); for the Olympus 12mm, it works out to be about 30% better. So here's a table of lenses, sorted by their 'low-light-ability' . A couple of clarifications might be needed. First, note that I've assumed "f/1.7" is actually f/sqrt(3), "f/1.4" is actually f/(sqrt(2), etc. I've also tried to account for the fact that many C-mount lenses vignette, sometimes badly, by redefining an 'adjusted' focal length that has the same angle-of-view as the bright central part of the image. So, for example, the 25mm f/0.95 Navitar (which I own) vignettes pretty badly, forcing you to crop out the central part of the image, which essentially increases the effective focal length to 30mm. It's the adjusted focal length I use in the calculation of LLA. Well, what's the purpose of all this? (Some might argue: NONE!, lol). For me it helps me identify lenses that might be useful for shooting at night, or indoors, or under other difficult conditions. It explains the popularity of the C-mounts, which although they often vignette, also often have relatively large apertures. And it explains why the new 12mm f/1.6 Noktor might be a really cool lens.