Change Forum... Civil Aviation Travel, Polls & Prefs Tech/Ops Aviation Hobby Aviation Photography Photography Feedback Trip Reports Military Av & Space Non-Aviation Site Related LIVE Chat My Starred Topics | Profile | New Topic | Forum Index | Help | Search
 Apperture?
 Wietse From Netherlands, joined Oct 2001, 3809 posts, RR: 53Posted Sat Jun 8 2002 11:23:42 UTC (13 years 8 months 1 week 4 days 14 hours ago) and read 962 times:

 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 From Netherlands, joined Apr 2001, 10213 posts, RR: 17 Reply 1, posted Sat Jun 8 2002 11:45:50 UTC (13 years 8 months 1 week 4 days 14 hours ago) and read 949 times:

 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 From Netherlands, joined Oct 2001, 3809 posts, RR: 53 Reply 2, posted Sat Jun 8 2002 11:56:33 UTC (13 years 8 months 1 week 4 days 13 hours ago) and read 946 times:

 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 From Slovenia, joined May 1999, 836 posts, RR: 3 Reply 3, posted Mon Jun 10 2002 11:05:38 UTC (13 years 8 months 1 week 2 days 14 hours ago) and read 875 times:

 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 From Netherlands, joined Apr 2001, 10213 posts, RR: 17 Reply 4, posted Mon Jun 10 2002 20:50:57 UTC (13 years 8 months 1 week 2 days 5 hours ago) and read 836 times:

 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 From Netherlands, joined Oct 2001, 3809 posts, RR: 53 Reply 5, posted Mon Jun 10 2002 21:07:45 UTC (13 years 8 months 1 week 2 days 4 hours ago) and read 825 times:

 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 From Netherlands, joined Apr 2002, 3674 posts, RR: 60 Reply 6, posted Mon Jun 10 2002 21:13:49 UTC (13 years 8 months 1 week 2 days 4 hours ago) and read 822 times:

 I think that's about right. It costs 60.000 pounds!!!
 Alderman Exit
 Top Of Page Change Forum... Civil Aviation Travel, Polls & Prefs Tech/Ops Aviation Hobby Aviation Photography Photography Feedback Trip Reports Military Av & Space Non-Aviation Site Related LIVE Chat Forum Index

This topic is archived and can not be replied to any more.

Printer friendly format

 Similar topics: More similar topics...