One for the camera and/or maths buffs

Mark · 07-11-2007, 19:03

An object approximately 75 feet long by 50 feet wide is visible 81,000 feet away and occupies approximately 1/16th of the total frame on a TV camera (16:9 aspect ratio).

Approximately what zoom lens would this require?

PS - I realise this information is incomplete - your best guess will be fine as this is only to satisfy my curiosity.

petemc · 07-11-2007, 19:58

Get close up and use a 10-20

Dr. Z · 07-11-2007, 19:59

I have had a think and I don't think its possible to calculate it without knowing more information. I shall ponder it a bit more - the thing is you need to know the size of the projected image.

If you assume a 24mm frame height (quick google turned up that) and you assume that the object is exactly vertical in the frame, you get a projected image size of 1.5mm for a 22.9m tall object.

A quick look at the lens equation should allow you to do the rest - draw a ray diagram! I need to go out else I will be late but if you havnt worked it out by the time I get back I will do it for you.

Mark · 07-11-2007, 20:58

Actually 1/16th is wrong - it's lower than that, but let's say 1/4 of the frame height. 22.9m is fair enough, we'll stick with that one. Can't help with the projected image size I'm afraid as the camera used is most definitely not 'off-the-shelf'. It's also entirely possible the camera wasn't on the ground, though that would only introduce a 25% error and I'm not really bothered about that.

Don't spend too much time on it as it's not important, but I'd still like to know. Thanks.

Pumpkinstew · 08-11-2007, 00:12

81,000 feet?

Have you hacked into a spy satellite or something?

Darrin · 08-11-2007, 03:53

Quote:

"81,000 feet?

Have you hacked into a spy satellite or something?"

Or at least a spy plane picture?

07-11-2007, 19:03	#1
Mark Screaming Orgasm Join Date: Jul 2006 Location: Newbury Posts: 15,194	One for the camera and/or maths buffs An object approximately 75 feet long by 50 feet wide is visible 81,000 feet away and occupies approximately 1/16th of the total frame on a TV camera (16:9 aspect ratio). Approximately what zoom lens would this require? PS - I realise this information is incomplete - your best guess will be fine as this is only to satisfy my curiosity.

07-11-2007, 19:58	#2
petemc Moonshine Join Date: Jul 2006 Location: Nr Liverpewl Posts: 4,371	Get close up and use a 10-20 __________________ Thats no hamster, its a space station!

07-11-2007, 19:59	#3
Dr. Z I'm going for a scuttle... Join Date: Jul 2006 Posts: 2,021	I have had a think and I don't think its possible to calculate it without knowing more information. I shall ponder it a bit more - the thing is you need to know the size of the projected image. If you assume a 24mm frame height (quick google turned up that) and you assume that the object is exactly vertical in the frame, you get a projected image size of 1.5mm for a 22.9m tall object. A quick look at the lens equation should allow you to do the rest - draw a ray diagram! I need to go out else I will be late but if you havnt worked it out by the time I get back I will do it for you. __________________

07-11-2007, 20:58	#4
Mark Screaming Orgasm Join Date: Jul 2006 Location: Newbury Posts: 15,194	Actually 1/16th is wrong - it's lower than that, but let's say 1/4 of the frame height. 22.9m is fair enough, we'll stick with that one. Can't help with the projected image size I'm afraid as the camera used is most definitely not 'off-the-shelf'. It's also entirely possible the camera wasn't on the ground, though that would only introduce a 25% error and I'm not really bothered about that. Don't spend too much time on it as it's not important, but I'd still like to know. Thanks. Last edited by Mark; 07-11-2007 at 21:00.

08-11-2007, 00:12	#5
Pumpkinstew Absinthe Join Date: Oct 2007 Location: Mostly Oxford, Sometimes Bristol Posts: 1,156	81,000 feet? Have you hacked into a spy satellite or something? __________________ Get old, or die tryin' PSTEWREVIEWS - Chunks of Meaty Reviews, Mixed with Your Five a Day of News, Comment and Opinion, Floating in a Broth of Suspect Grammar and Seasoned Liberally with Mixed Metaphor. Tasty.