Автор: desmes
Дата: 29-03-15 02:54
Това е съвсем нормално за макро обектив.
http://s8.postimg.org/reyuzo81h/Macro_Aperture.jpg
LENS EXTENSION & EFFECTIVE F-STOP
In order for a camera lens to focus progressively closer, the lens apparatus has to move further from the camera's sensor (called "extension"). For low magnifications, the extension is tiny, so the lens is always at the expected distance of roughly one focal length away from the sensor. However, once one approaches 0.25-0.5X or greater magnifications, the lens becomes so far from the sensor that it actually behaves as if it had a longer focal length. At 1:1 magnification, the lens moves all the way out to twice the focal length from the camera's sensor:
diagram of focusing distance versus lens extension
Choose a Magnification: 1:2 (0.5X) 1:1 (1.0X)
Note: Diagram assumes that the lens is symmetric (pupil magnification = 1).
The most important consequence is that the lens's effective f-stop increases*. This has all the usual characteristics, including an increase in the depth of field, a longer exposure time and a greater susceptibility to diffraction. In fact, the only reason "effective" is even used is because many cameras still show the uncompensated f-stop setting (as it would appear at low magnification). In all other respects though, the f-stop really has changed.
*Technical Notes: The reason that the f-stop changes is because this actually depends on the lens's focal length. An f-stop is defined as the ratio of the focal length to aperture diameter. A 100 mm lens with an aperture diameter of 25 mm will have an f-stop value of f/4, for example. In the case of a macro lens, the f-stop increases because the effective focal length increases — not because of any change in the aperture itself (which remains at the same diameter regardless of magnification).
A rule of thumb is that at 1:1 the effective f-stop becomes about 2 stops greater than the value set using your camera. An aperture of f/2.8 therefore becomes more like f/5.6, and f/8 more like f/16, etc. However, this rarely requires additional action by the photographer, since the camera's metering system automatically compensates for the drop in light when it calculates the exposure settings:
Публикацията е редактирана (29-03-15 02:54)
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![[smilie5]](https://photo-forum.net/forum/smileys/smilie5.gif)
другите правят същото, но производителите са избрали да не го изписват.
![[smilie21]](https://photo-forum.net/forum/smileys/smiley21.gif)
![[smile]](https://photo-forum.net/forum/smileys/smilie1.gif)
![[smilie11]](https://photo-forum.net/forum/smileys/smilie11.gif)
Но общо взето ми стана ясно. ![[smitten]](https://photo-forum.net/forum/smileys/smitten.gif)
![[smilie18]](https://photo-forum.net/forum/smileys/smiley18.gif)
![[smilie24]](https://photo-forum.net/forum/smileys/smiley24.gif)
![[beer]](https://photo-forum.net/forum/smileys/smiley20.gif)
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