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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?

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3  
Great question! You are right on track and correct with your findings, that is a variable aperture zoom lens, and as such, as you zoom in, the maximum aperture(smallest number) becomes increases up to f/5.6. So to do what the author asks, you will have to be at 70mm and f/5.6! The answer above has more details if you are interested as well. – dpollitt Jan 4 '12 at 3:08
Also a duplicate of photo.stackexchange.com/questions/14492/… – dpollitt Jan 4 '12 at 3:09
Full name of D90's kit lens is: Nikkor AF-S DX 18-105mm f/3.5-5.6G ED VR. It is a variable aperture zoom lens. – rockacola Jan 4 '12 at 3:52
@dpollitt: thanks for the inputs :) – Umesh Awasthi Jan 6 '12 at 1:57

1 Answer

up vote 6 down vote accepted

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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Are all "variable aperture zoom lenses" low quality? – mattdm Jan 4 '12 at 3:19
@mattdm - No, but in the consumer range they are all low quality. Professional grade variable aperture zooms that are high quality on the Canon mount include the 100-400mm L, 70-300mm L, 28-300 L etc. The 10-22mm is also high quality, but not "L". – dpollitt Jan 4 '12 at 3:24
@Itai - You make it sound so bad! – dpollitt Jan 4 '12 at 3:26
The 18-105 is not a bad lens at all. It is a good beginners tool for everyone who is new to Nikon photography or works on a budget. I started with this kit-lens, too and I found it very useful to find out what kind of lenses I need to go on in the future. The price / performance ratio is very good on this one, but you will recognize for your self what kind of high quality lenses you will need in advance. – Michael K Jan 4 '12 at 11:55
1  
Did you look at your images? ;) I had that lens too and it really crippled the D90 it came with. – Itai Jan 4 '12 at 15:46
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