我今天新買的騰龍180mm微距頭，非常喜歡這個頭的成像。
可是我發現一個問題，就是這個頭在對焦的時候光圈也會隨之變（機器Nikon D300）。我查了一下國外的貼，很多人說在其他90微上面也會有這種情況。

直接轉載一下

> >I just bought an used Tamron SP 90mm F2.8 from ebay. The funny thing I
> >found with the lens is that when setting at maximum aperture 2.8,
> >focusing from infinity to closest distance, the aperture goes down
> >from 2.8 to ~5.6.

回答1：
> of the lens entrance pupil remain the same, but as you focus from
> infinity down to 1:1, the distance from the lens to the film plane
> doubles. The effective aperture is the entrance pupil size divided by
> the distance to the film, so the effective f-number doubles, which is 2
> stops of light loss.
>
> Normally, with manual cameras, you have to calculate the light loss
> yourself. It looks like the Nikon is calculating the effective
> aperture change for you, so you can use a light meter reading directly
> without doing those calculations.
>
> Dave

回答2：
This is just the "bellows factor" that the view camera people have to
deal with all the time. The Tamron on your camera is letting you know
the effective aperture. The formula is f(eff) = f * ( 1 +
Magnification) so for a magnification of 1 (1:1) and f/2.8 the
effective f stop is 2.8*(1+1)or 5.6. At close focusing distances, the
lens is internally farther from the film and the light exiting the
lens follows the same inverse square law as any other source. Another
way to look at it is the same light patch at the film plane is twice
as big at 1:1 and the film only receives 1/4 of that light or two
stops. The lens aperture did not physically or optically get any
smaller, you just moved it away from the film.

For magnifications of less than 0.15 or so, typical for standard
lenses, the light loss is less than half a stop so it doesn't show up
while focusing.

To make a 105mm lens f/2.8 constant aperture at 1:1, it would need an
internal light path 75mm in diameter. Then it could automatically
adjust the aperture to a constant f/2.8 at all distances but it would
still be a 105mm f/1.4 lens with an artificially small aperture at
infinity and would weigh as much and cost even more than a close
focusing 105mm f/1.4, ie. somewhere well north of $1000.

The Nikon 105mm Micro-Nikkor works the same way as the Tamron. Nikon
has to obey the same laws of physics as the rest of us.

Bob Kirkpatrick

請問哪位大俠解釋一下？

不只有Tamron,任何一家的macro鏡都是如此

这是正常的。

景深按照标称光圈算，测光按缩小后光圈算

按照缩小后的光圈算

如果是Nikon的话，就孔径而言确实是错误的。但是就通光量而言，是正确的。

微距镜头特有的情况。据说是近摄的时候通光孔径的利用率低，所以实际使用的光圈值会变大（比如从2.8变成5.6）。这是镜头的一种光学特性。Canon的微距镜头也是这样，但是EOS机身好像不会告诉你光圈值改变了。曾经有一帖子探讨过此事，还有图。