wietse
Topic Author
Posts: 3630
Joined: Thu Oct 18, 2001 12:49 am

### Apperture?

Hi, just wondering what the 2.8, 5.6, etc. stand for. It can't be cm, cause 2.8 has a larger opening than the 5.6, but what is it then? Lux or something?

Wietse
Wietse de Graaf

jwenting
Posts: 9973
Joined: Mon Apr 23, 2001 10:12 pm

### RE: Apperture?

According to my old university optics course the f/number is equal to f/D' where D is the diameter of the entrance pupil of the lens and f is the focal length.
It goes on with an example, a 50mm lens with a 25mm aperture has an f/number of 2.0.
Then it goes into photographic lenses, stating that one f-stop in those is equal to a step equal to the square root of 2 (rounded), yielding the set we know of 1.0, 1.4, 1.8, 2, 2.8, 4, 5.6, 8, and so on.
Thus closing down 1 f-stop decreases the lens opening by 1 divided by the square root of 2 and the total light reaching the focal point of the lens by a factor of 2 (thus halving it). (it assumes you know that the area of a circle is defined as pi * the square of half the diameter of that circle, therefore the square root in the formula).

Hope this explains the mathematics behind it somewhat.
I wish I were flying

wietse
Topic Author
Posts: 3630
Joined: Thu Oct 18, 2001 12:49 am

### RE: Apperture?

Ah, ok, that explains it. (seriously! it does!  ) So that's why 2.8 lenses grow in diameter when the focal length increases.
thanks,

Wietse
Wietse de Graaf

Dufo
Posts: 800
Joined: Tue May 18, 1999 2:41 am

### RE: Apperture?

In February there were some rumors about Canon developing a 500mm/F2.8 lens.. imagine the size of it!

Jernej
I seriously think I just creamed my pants without any influence from any outside variables.

jwenting
Posts: 9973
Joined: Mon Apr 23, 2001 10:12 pm

### RE: Apperture?

Not too bad, front lens in the area of 18cm diameter or more. Add the tube and you're up to a diameter of maybe 20cm.
Length of 50cm or more.
Weight a trivial 20kg approximately (not counting the dual tripod mounts it would need).
Price? €1 per gram like most lenses in that class for an inconsequential €20000,-- estimated
I wish I were flying

wietse
Topic Author
Posts: 3630
Joined: Thu Oct 18, 2001 12:49 am

### RE: Apperture?

I saw a price listing for a 1200mm f5.6 L lense for 119000€. Is that a typo or is it really that expensive?

Wietse
Wietse de Graaf

timdegroot
Posts: 3258
Joined: Wed Apr 24, 2002 10:37 pm

### RE: Apperture?

I think that's about right. It costs 60.000 pounds!!!
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