[kenanet]

想問下2.8L同4L除價錢同光圈不同外其他會有好大分別4L嗎？見4有瑩石晶片而2.8L就無，唔知瑩石晶片有什麼好處呢？

[zwei]

聽說過1片瑩石=2片ED鏡片.....

[kenanet]

有什麼用呢?

[ACanonGuy]

The weight and size? F2.8 is heavier and bigger than F4. Some said F4 is a sharper lens than F2.8 but I can't really tell the different. I do like both.

[Obi-Wan]

kenanet
I have used all these four lenses, now have f2.8 IS, f2.8 non IS and f4 non IS
Sharpness:
70-200/f4L IS sharpest > f2.8 IS, non IS > f4L non IS
but the difference is very very small

瑩石 (fluorite) can be artificially made, but since it is quite brittle, it cannot be easily made into large size (Some lenses have fluorite lens of rather size, but quite expensive). Fluorite has rather high refractive index and rather low dispersion, can be used in telephoto lens to reduce the chromatic aberration.

ED lenses element is an alternative to fluorite, usually cheaper & can be made into larger size than fluorite.

If u just want to but one lenses, go buy 70-200/f4L IS

[SleepingIII]

据说小小白锐度较好。
重量方面小小白轻得多。
对于消除色散的效果，一片萤石的效果约等于两片UD，约等于一片sUD。