 BetterPhoto Member |
F Stops and Lenses I would like to know the relationship with f stop and lens opening. I always thought the lower the f stop, the smaller the opening. Today I heard the reverse. Which is it? Also, a 300 mm zoom. Is that the length of the lens in mm?
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Wayne Attridge |
The smaller the f stop number, the larger the opening. f22 is a very small opening and f1.2 is a very large opening. The mm number is the focal length of the lens. On a normal lens, the focal length is approximately the distance from the center of the lens to the film plane. A telephoto lens, as your 300mm will be at a point somewhere in front of the lens. What that number really means to you is the magnification it produces. If your regular lens is 50mm the image will be about life size. Your 300mm lens has a focal length 6 times that of your regular lens and will give you 6 times magnification.
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John A. Lind |
John, The 300mm you asked about is the focal length of the lens. Technically this is the distance from the rear lens node to the film plane when the lens is focused at infinity. Wayne is correct that the rear lens node is typically located inside the lens, but not always at its physical "center," especially with zoom lenses which have a variable focal length. Since you mention this is a "zoom" lens, it should have a focal length range, such as 35-105mm. The lower number is its shortest effective focal length and the higher number its longest one. In your case, I suspect 300mm is the higher of the two numbers. If it is a "fixed" focal length lens (or "prime" lens) with only one focal length (300mm), a lens this length is not a "zoom" but a "long" or "telephoto" lens. The f-numbers used for aperture settings are the focal length of the lens divided by the effective aperture diameter. If you set the lens to 300mm and set the aperture to f/4, the effective diameter of the aperture is 75mm. The notation itself refers to the math. If "f" is the focal length of 300mm, f/4 = 75mm. Reason for this? To get the same amount of light through two different lenses with different focal lengths, the aperture diameters must be different. The f-number scheme "normalizes" aperture settings so that f/4 on a 50mm lens admits the same amount of light as f/4 on a 300mm lens. It makes setting exposures much, much easier. Without it, you would be doing quite a bit of math about aperture diameter and lens focal length. The f-numbers do this for you up front. You can see that dividing a focal length by a larger aperture diameter gives you a smaller f-number (e.g., 300mm / 150mm = f/2; 300mm / 75mm = f/4). This is why smaller f-numbers mean more light (wider aperture) and larger f-numbers mean less light (narrower aperture). -- John
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T Lee |
Hi John, Here is an easy way to think of Aperture ( or F-stops) and how they relate to the lens opening. Indeed, what you've been told is true. The smaller the FStop the bigger the openning. The reason this is true is the "weird" way that we refer to f-stops. Aperture, or your F-Stop, is the size of the opening of your shutter. A large F-stop number, such as F2.8 or F5.6, will give you a very shallow depth of field, whereas a small depth of field value, such as F11 or F22 will give you a greater portion of your photograph in focus. Now, I know what you're thinking.... What is she talking about? 2.8 is bigger than 11? It is. :) All these numbers must be thought of as the denominator of a fraction with a numerator of 1. Therefore, an f2.8 is an f-stop of 2.8 thus being 1 / 2.8th of the shutter open. (Or roughly 36% of the shutter is opened when the shutter is released.) An f11 then, means that 1/11th of the shutter opens when the shutter is released, or 9% of it. Therefore, f2.8 -IS- larger than f11. This is important because it determines the Depth of Field in your picture. Depth of Field is the amount of your image that is in focus. A shallow depth of field will blur the background. This is a technique that is used by many portrait photographers. A wide depth of field is going to give you sharpness throughout more of your photograph. This is something that is desirable for landscapes for instance. hope this helps,
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