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I am new to the photography field, there are few things 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 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. Can some one help me to understand this?

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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.

  • 3
    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
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    @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
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    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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The focal ratio of a lens (sometimes referred to as Aperture value or Av, but more commonly as a focal ratio or f-stop and written using the short-hand f/__) is really the focal length of the lens divided by the diameter of clear aperture.

In other words if a lens had a physical aperture opening with a 25mm diameter and was had a focal length of 100mm then the the focal ratio would be f/4 because 100 ÷ 25 = 4. If you increase the focal length to 200mm but do not change the physical aperture size then it becomes 200 ÷ 4 = 8 ... so now it's f/8. In this example the only thing you deliberately changed was the focal length but the focal ratio changes as a side-effect of the math.

Some lenses employ optics that are able to maintain the focal ratio even while you adjust the focal length (and these tend to be more expensive lenses.)

Knowing that focal ratio is the focal length divided by the clear aperture diameter, this also means that "long" lenses that have "low" focal ratios will probably be very heavy because the low focal ratio requires a large physical diameter (relative to the focal length of the lens). That means each glass element inside the lens has a much larger diameter ... which also means they are thicker and that means they are are heavier.

You might wonder why focal ratios are used instead of just stating the physical diameter of the aperture. It turns out that for purposes of determining how much light will be delivered to the sensor, it is the ratio which matters. e.g. if a lens has a 25mm aperture diameter you really don't know how much light will be delivered to the sensor unless you also know the focal length.

I use a thought experiment of a tunnel in the side of a mountain. If the tunnel diameter is 20' across and you stand at the entrance to the tunnel it will be quite bright because light from many different angles can reach you while you are at the tunnel entrance. As you got deeper into the tunnel, the angle of light necessary to reach deep inside becomes narrower and narrower and the consequence of this is that it gets darker and darker the farther you go. Focal ratios work like this.

This means when you use a light meter to take a meter reading you do not have to tell the meter anything about the focal length of your lens... it can recommend exposure settings based on the focal ratio regardless of the actual focal length.

One other thing to notice... those numbers used in the f-stops... are actually powers of the square root of 2. (the square root of 2 being approximately 1.4 when liberally rounded)

This is because each time you increase the diameter of a circle by that factor (1.4 ... actually by the square root of 2 if you want to be precise) then you will exactly double the area of that circle. That means twice as many photons can pass through that area. The area of the circle is π * radius^2. If you increase the radius by 1.4 (or √2 to be precise) then you will exactly double the area of that circle.

Here is a table I created showing the powers of the square root of 2 ... from 0 to 9. Notice that only the power is changed on the left and on the right you get list of whole f-stops. Each whole f-stop decreases the amount of light by exactly half. f/1.4 allows half as much light to pass through the lens as compared to f/1.0. f/2 is half as much light as compared to f/1.4... and so on.

Powers of the Square Root of 2

Camera manufacturers round values used in photography because use of precise (non-rounded) values wont change the exposure in a noticeable way (i.e. hundredths of an f-stop wont be noticed) and it makes the values easier to remember.

protected by Philip Kendall Jun 18 at 17:39

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