Effective aperture.
The diameter of the bundle of light rays striking the first lens element that actually pass through the lens at any given diaphragm setting.
EIS ...
These are the effective apertures set on the camera, not marked apertures on an aperture ring. As you focus more closely you lose light up to two stops at life size. Nikon AF cameras calculate this automatically.
" In this example f/8 is the effective aperture. If the lens were focused at other than infinity, f/8 would then be the relative aperture.
Also can be explained as numerical expression of the relative aperture of a lens at its different stops; equal to the focal length divided by the effective aperture of the lens opening and written in various forms, such as f/8, f8, 1: 8, etc.
Determining the effective aperture of a more or less pentagonal shaped opening isn't straightforward. Someone probably knows how to do it mathematically, I don't.
The relative aperture is specified as an f-number, the ratio the lens focal length to its effective aperture diameter. A small f-number like f/2.
Other consequences of the effective aperture include autofocus ability and viewfinder brightness. For example, most SLR cameras lose the ability to autofocus when the minimum f-stop becomes greater than f/5.6.
A teleconverter is a small lightweight intermediate optic that will increase the magnification of a lens, while reducing its effective aperture. So a 2X teleconverter turns a 300/2.8 into a 600/5.6.
The quick technical explanation is that as lenses zoom out, their effective apertures get smaller, and this means less light hits the sensor. The camera compensates by either boosting ISO or turning on the flash.
A scale used to express the relative area of the aperture of a lens, simply the result of dividing the focal length of a lens by the effective aperture of the lens opening (the apparent size of the diaphragm seen from the front of the lens).
It's not really the doubler which is causing the drop in resolution, it's the effective aperture being so small. You would drop the resolution by just about the same amount if you simply stopped down two stops rather than adding a doubler.
This is actually a ratio of the focal length of the lens divided by the effective aperture diameter (the actual opening diameter). This means that an ƒ-number is relative to the focal length of the lens.
-Zoom lens is a lens which is constructed to allow continuously variable focal length within a specific range. The effective aperture and focus settings remain unchanged throughout such adjustments.
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As a result, the background blur is messy and unattractive. (It also follows that achieving a correct exposure here was essentially down to luck - the effective aperture just happened to let in the correct amount of light.) ...
Besides changing the effective focal length, the effective aperture of the attached lens is increased by one or more stops as well. Autofocusing usually does not operate if the effective maximum aperture is greater than f/5.
Lighter tripods just could not dampen vibrations or hold the assembly steady at the long shutter speeds necessary at effective apertures of f32 - 48. At 3X and higher magnifications the slightest vibration will soften your image noticeably.
Calculated by dividing the focal length by the effective aperture diameter. A one stop change in f-number effectively alters the amount reaching the film by a factor of two.
 See also: Aperture, Focal, Focal length, Image, Lens
  
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