Why does my aperture setting change as I zoom the lens on my Nikon D90?

Question

I am new to the photography field and this something which I am not able to understand. I am playing around with my Nikon D90 along with its kit lens (18-105mm).

While reading about Depth of preview, the author asks to do some basic exercise to better understand DOF, like this:

• set your aperture to smallest number f/2.8, f/3.5, f/4 with 70mm or longer lens.

When I tried to set aperture to f/3.5, and tried to change focal length, my camera is setting aperture automatically in every every possible mode (which I know of). It is changing aperture in the following fashion:

1. 18-24 ----> 3.4 to 4
2. 18-35 ----> 4.5
3. 18-50 ----> 5
4. 18-105 ---> 5.6

But if i set my aperture to 5.6 or higher, it does not change when I change the focal length of my camera. I know I am not doing some basic thing right, but I'm still not sure why this is happening. Might some one help me to understand this?

Answer

It is happening because you have a variable aperture zoom lens. The solution is to get a quality lens, otherwise you have to live with the limitations which are actually marked on the barrel of your lens.

It says 18-105mm 1 : 3.5 - 5.6G which means your maximum aperture is F/3.5 at the widest focal-length (18mm) and F/5.6 at the longest (105mm). It changes in increments between that. So, if you are set to F/5.6 then you can zoom with whole focal-length without aperture changing. If you set your aperture to F/3.5 then after a short increase in focal-length, the lens has to diminish its aperture.

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