[joviphoto]

Hi all,
i'm a new beginner.
Here i come across a question.
i'm using Nikon D70s.
But my problem is relating to the lens.
I'm using Nikon AF-S DX Nikkor 18-70 mm f/3.5-4.5G IF-ED.

Q1: I don't know why there is a range of apecture size "f/3.5-4.5"
i know it's the maximum apecture, but why the maximum size will change? Is it related to the 焦距? If yes, why?

Q2: What is the meaning of the words "f/" and "G" in "f/3.5-4.5G". Any other special term relating to the apecture?

Thanks a lot.

I really hope to learn more from you guys.

Jovi

[tam_dominic]

There would be less light fall on the sensor when you zoom to the long end, thus the max. aperture would change from F3.5 (large) to F4.5(small)

Aperture is the hole letting light to go onto the sensor. The large the hole the smaller the number.

'G' does not relate to the aperture, I guess it is Nikkon specific.

Happy shooting !

[micros]

"f/" 係光圈值o既表示。
如 f/2 .... f/4 ......

鏡頭焦距 / 光圈直徑 = 光圈值
由於直徑有可能有小數﹐使用上唔方便﹐所以用光圈值表示光圈o既大細。


"g鏡"
nikon o既 g鏡同傳統o既 nikon 鏡唔同之處﹐就係無o左鏡身光圈環。
部份舊機只可以用鏡身光圈環轉光圈﹐咁佢地就用唔到 g 鏡﹐或者只可以係某o的曝光模式下用。
如果係用 d70 s﹐g 鏡無問題。

[joviphoto]

Thanks you guys very much!!

But i'm still confused about the sentences:

"There would be less light fall on the sensor when you zoom to the long end, thus the max. aperture would change from F3.5 (large) to F4.5(small)"

I know this is a phisics theory, but could you please explain and eleborate a little bit more so that i can make it more clear?
i know the fact that long focal length cause the smaller apecture, but why? If there's really less light coming in, why should the specture opens wider to collect more light?

i think it's a causal relationship, but i really hope to understand.

Thanks again.

[diwinger]

The "F-Numbers" describe the amount of light falling to the sensor instead of the physical size of the aperture. Since light is a spherical wave, its intensity followings the usual inverse square law.

Suppose the focal lenght is double but the physical size of the aperture is keep constant, the intensity of light following on the sensor will be only 1/(2x2) = 1/4 of the original intensity. Therefore, the area of the aperture must be four times of the original one for the amount of light to be the same as the original one. That is, the diameter of the aperture should be doubled.

As we define the f-number to be (Focal Length) / (Aperture Diameter), the light intensity will be the same for different focal length given the apeture is opened to have the same f-number.

For the lens 18-70mm 3.5-4.5, the diameter of the aperture at the widest end is 18mm/3.5 = 5.14mm while the diameter of the aperture at the highest zoomed end is 70mm/4.5 = 15.56mm. We can see that the aperture is in fact wider in highest zoomed end instead of the widest end. (Although the f-number, i,e, the ability to collect light is smaller.)

In fact, at almost all lens. It is designed that the aperture at the highest zoomed end to be larger in order to compensate for the loss of light. For most highest standard lens, the degree of increase of aperture is the same as the increase in focal length when we zooms. Those lenses have a constant f-number under their full ranges.

[joviphoto]

I UNDERSTAND NOW!!!

I MAKE ALL CLEAR NOW!!!

THANKS A LOT!!!

Thanks you guys : tam_dominic, micros and diwinger.

Hope to learn more from you guys.