![]() ![]() So you want to be sure before you dip into your wallet that you really NEED that wide opening and that you'll leverage it often. It's possible to spend several hundred more on a lens with a wider opening. ![]() Unfortunately there's a cost issue here: the wider a lens opens, the more it costs. I should just run out and get a lens with a super-wide opening and then I can take pictures whenever and wherever I like! Since light is the foundation of every photographic image, the more that the lens lets through, the more flexibility you have to take pictures in all kinds of available light - from bright sunlight to dim twilight.Īha! you say. Maybe you will be the one to hit upon a better method.Not all lenses are created equal: some can open up much wider than others and the ones that open up wider can let a lot more light through onto the sensor. Many attempts to simplify have been tried but optical engineers carry on the f/number system. Thus a lens 4 meters in diameter with a focal length of 16 meters delivers the same light energy as a lens 4mm in diameter with a focal length of 16mm. ![]() The f/number method assures that a lens set to say f/4 delivers the same light energy to the film or chip as any other lens so set, regardless size. The f/number mergers these into a ratio (focal length ÷ working diameter). The f/number value intertwines the two major properties of a lens – focal length and working diameter. The symbol f/number is an abbreviation or acronym for focal ratio. Today the Waterhouse Stop has been replaced by a mechanical iris consisting of thin overlapping metal leaves that mimic the human eye iris. Because they passed some light and stopped some, they were called Waterhouse Stops. These inserts came in a set with different size aperture holes. ![]() John Waterhouse devised metal inserts that slipped into a slot in the lens barrel. Sidebar: Early lenses did not have an adjustable diaphragm. All you will ever need or ever be able to set will be 1/3 f/stop increments. Likely this is wishful thinking as it would likely cost thousands of dollars to deliver such precision. Īdditionally some display indicators on some digital cameras indicate a super fine f/number setting. I have seen scientific instruments that permit adjustment in 1/6 f/stop increments but they are atypical. Likely 1/3 f/stop is the best one can expect. The mechanical limitations of the gear train of the iris diaphragm system within the lens barrel prohibit making super fine adjustments. (Moderator's note: The 1/3 f/stop number set has been corrected as per Paul's post below.) The multiplier is the fourth root of 2 which is 1.189. With the initiation of electronic light metering and precision lens making, the f/number it became possible to fine-tune the diaphragm using 1/2 f/stop adjustments. We can also divide by 1.4 to get the same answer but the preferred way is multiplication by 0.707. Ĭonversely to reduce the light entering the camera by half, we multiply the current diameter by 0.707. We don’t need to be so exact so we can use a shortened value of 1.4. Stated another way, to double the light gathering power of a lens we multiply the current diameter by 1.414. Factoid: If we multiply the diameter of any circle by the square root of 2 we have calculated a revised diameter that is twice as big. Since most diaphragms are circular holes we must fall back on circle geometry. The camera iris is more communally called a diaphragm. This is the colored portion of our eye named for the Greek goddess of the rainbow. Now the entry way into the camera is a mechanical device that mimics the human eye’s Iris. Conversely smaller diameters reduce the light energy gathered. Meaning the larger the working diameter of the lens, the more light gathered. Now the lens acts much like a funnel gathering light. Because of the way films preformed the ideal sequence was thought to be a doubling or halving of the light energy allowed to play on the film. The idea is provide a method that permits photographer’s to open up or close down the camera lens with geometric precision. The f/number set of numbers (number set) seems weird. ![]()
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