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The f/number is an indication of the brightness of the lens. It is the measurement of the ratio between the focal length and the diameter of the entrance pupil (where the light enters the lens). The f/number is directly proportional to the focal length and inversely proportional to the effective diameter of the lens. It determines the amount of light reaching the camera sensor. The smaller the value, the larger the opening and the brighter the image produced by the lens.
With high-speed photography, light is king. The Navitar f/.95 lenses are the best that I have seen at light gathering, which means that you need less additional lighting in most applications when you are using one of these lenses. We offer Navitar lenses in 12.5 mm(f/1.2), 17 mm, 25mm and 50 mm and a 16 to 160mm zoom lens (f/2.0). Other focal lengths and f-ratings are available.
To determine which lens you need, check out this lens calculator at:
http://machinevision.navitar.com/opticalwizard/OpticalWizard.asp
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