[onlyken]

各位C兄, 點解細機既鏡頭光圈很少細過 f8? 即使較prosumer機, 如 P6000, G10最細光圈都係 f8, 沒有 f11, f16,....

[Obi-Wan]

This involve some physics

In short, the f no. is in fact a fraction, f11 = the diameter of the clear part of the lens is 1/11 of the focal length, e.g. for a lens with focal length 11 mm, the clear part which allow the light to pass is 1 mm.

Since small DC has a small sensor, the telephoto lens on a small DC is just 5 to 10 mm, which mean the size of the clear part is just 0.5 mm or so

BUT if the size of the clear part of the aperture is TOO SMALL, a physics phenomenon called "diffraction" also occurs which lower the quality of the picture taken.

Hope my poor English can make u understand.

[barracudar3]

Hi Obi-Wan,

What do you meant by "the clear part"? Does it mean the size the aperture?

[Obi-Wan]

barracudar3

in short: YES
the clear part refers to the approximate size of the aperture assuming it is a circle. [But in fact the opening of the aperture is seldom a circle]

[onlyken]

Obi-Wan兄, 小弟不太同意, 在鏡頭光學定義, 鏡頭光圈每細一級 (e.g. f1.4 => f2.0), 能投影到sensor 或 菲林的光度會暗了一倍 (參看以下數表), 而成像圈size是不變的, 而光圈數值是跟focal length 沒直接關係, 以一些wide angle 鏡(e.g. 12-24mm)為例, 其focal length 在12mm時, 其光圈值仍能設為f16 或更少......

f1.0 = 1=100%光線能投射到sensor 或菲林
f1.4= 1/2=50%, 如此類推.....
f2.0= 1/4
f2.8= 1/8
f4.0= 1/16
f5.6= 1/32
f8= 1/64
f11= 1/128

[Obi-Wan]

onlyken, which part of my writing is wrong?

in short: assuming 50mm lens focal length is made of one lens element of the diameter 50mm, without any aperture, the f no. is 1

If an aperture of diameter of 25mm is put to cover the lens element, the f no. is 50mm/25mm = 2, the light intensity on the sensor will be 1/4 of the original.

PLEASE point out which part is wrong?

[jacobchu]

Onlyken,
your definition of aperature is absolutely wrong. The definition of f number for example 4 , in a lens of focal length of 50mm, means that the diameter of the aperture is is 50mm/4 = 12.5mm. For the same f number 4 in a lens of focal length of 200mm, the the diameter size should be 200mm/4 = 50mm.
the question of the top is: why there are no f number less than 8. I don't know the reasons. But there may be some suggested answers. if I am wrong, please tell me.
1. The manufacture will not produce so small f number fo DC as there are usually not the requirement of general purpose users.
2. The technique to produce too large or too small f number is so complicated that will need more investment and hence the production cost will become too high for most dc users. Just like the difference of f2.8 and f4 in the range of 16-55mm in the dslar, the price will be 2x or more. even in dslr not all user will buy large f number lens.
3. As mension by other ching in the previous reply, the diameter will be too small that it will require a very long exposure time. Do u think that general will like to use b shutter?
4. As you have mentioned, the formation of image is the same for large or small f number of the lens, but there are a lot of problems for large and small f number. only the mid range is easy to handle.

Just something to share. Thanks.

[王迦南]

Ｘ我一下吧！

[Obi-Wan]

jacobchu

u are smarter in Physics! U should be studying science!

The major reason for not using a smaller aperture in DC is "diffraction" (or more correctly "scattering", when light pass through a small hole with the size similar to the wavelength of the light, diffraction and "scattering" occur. As different colour of light has different wavelength, the different colour will produces different degree of scattering.

The result, image formed on the sensor will be extremely poor quality.

I don't want to explain more physics in details as this may not be the interest of most brothers in this forum.

Thanks for reading.

[jacobchu]

Obi-Wan

I think, the term diffraction is not suitable to be used here. The scattering of color during transmission in medium is color dispersion (色散). This is one of the problem to be solved in large aperture or small aperture. The effect of diffraction of course should be consider here, but not so important. Why? As we know, the wavelength of visible light is of the order of 0.0000001m and the aperture size is only as small as 0.001m (i.e. ~mm), the effect of diffraction is not significant in the range. Diffraction is dominant if the aperture size is comparable to the wavelength.

Thanks for your reply.

[春田花]

物理因素只係其中一環，但不要忽略實用因素，拍照手機鏡頭又細，
感光原件面積又細，但係質素不斷改進，一般DC鏡頭約只有8光圈，質素絕對無問題，只係實用性太低，無必要做到咁細。

[Obi-Wan]

jacobchu

dispersion occurs in the medium of the lens element

diffraction occurs in the edge the aperture

please do go to http://www.kenrockwell.com/tech/focus.htm check

[onlyken]

C Hing,

1. 在鏡頭光學定義, 鏡頭光圈每細一級 (e.g. f1.4 => f2.0, 光圈面積細了一半), 能投影到sensor 或 菲林的光度會暗了一倍, 而成像圈size是不變的 (我沒說光圈size不變), <===這定義有錯嗎!?

2. The definition of f number for example 4 , in a lens of focal length of 50mm, means that the diameter of the aperture is is 50mm/4 = 12.5mm. For the same f number 4 in a lens of focal length of 200mm, the the diameter size should be 200mm/4 = 50mm.
如你所說, 如果一支變焦鏡 70-200mm f2.8 為例, 70mm/2.8=25mm, 200mm/2.8=71mm, 你有見過這麼大的光圈在你鏡頭內出現嗎?

3. 以FX 為例, 36mm x 24mm, 其成像圈直徑大約等於43.2mm=[square root of(36^2+24^2)], 所以如果是200mm f2.8的鏡頭, 其光圈size應為(43.2mm/200mm) x 71mm=15.3mm; 如果是200mm f4.0, 其光圈size應為(43.2mm/200mm) x 50mm=10.8mm

[onlyken]

jacobchu 兄,

1. 在鏡頭光學定義, 鏡頭光圈每細一級 (e.g. f1.4 => f2.0, 光圈面積細了一半), 能投影到sensor 或 菲林的光度會暗了一倍, 而成像圈size是不變的 (我沒說光圈size不變), <===這定義有錯嗎!?

2. The definition of f number for example 4 , in a lens of focal length of 50mm, means that the diameter of the aperture is is 50mm/4 = 12.5mm. For the same f number 4 in a lens of focal length of 200mm, the the diameter size should be 200mm/4 = 50mm.
如你所說, 如果一支變焦鏡 70-200mm f2.8 為例, 70mm/2.8=25mm, 200mm/2.8=71mm, 你有見過這麼大的光圈在你鏡頭內出現嗎?

3. 以FX 為例, 36mm x 24mm, 其成像圈直徑大約等於43.2mm=[square root of(36^2+24^2)], 所以如果是200mm f2.8的鏡頭, 其光圈size應為(43.2mm/200mm) x 71mm=15.3mm; 如果是200mm f4.0, 其光圈size應為(43.2mm/200mm) x 50mm=10.8mm

[onlyken]

Obi-Wan 兄,

The major reason for not using a smaller aperture in DC is "diffraction" (or more correctly "scattering", when light pass through a small hole with the size similar to the wavelength of the light, diffraction and "scattering" occur. As different colour of light has different wavelength, the different colour will produces different degree of scattering.

100% 同意!