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

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

Wietse de Graaf

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.

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

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

thanks,

Wietse

Wietse de Graaf

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

Jernej

Jernej

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

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

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

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

Wietse de Graaf

- timdegroot
**Posts:**3258**Joined:**

Users browsing this forum: No registered users and 6 guests