#1  
Old 20-12-2007, 01:18 PM
ving's Avatar
ving (David)
~Dust bunny breeder~

ving is offline
 
Join Date: Nov 2004
Location: The town of campbells
Posts: 12,359
silly noob question...

... and one that i should by rights know the answer to...

200mm lens with a front lens size of 52mm @ f/9

600mm lens with a front lens size of 72mm @ f/9


are they going to offer the same brightness at the same shutter speed or will the greater sixed front lens offer more light gathering ability?

I really should know the answer!

i know how it works for telescopes, but how does it work for camera lenses?
Reply With Quote
  #2  
Old 20-12-2007, 02:42 PM
higginsdj's Avatar
higginsdj
A Lazy Astronomer

higginsdj is offline
 
Join Date: Mar 2006
Location: Canberra
Posts: 613
The greater the aperture the fainter you can detect in a given exposure.

Cheers
Reply With Quote
  #3  
Old 20-12-2007, 03:08 PM
ving's Avatar
ving (David)
~Dust bunny breeder~

ving is offline
 
Join Date: Nov 2004
Location: The town of campbells
Posts: 12,359
like i said, i know how it works in telescopes... say a refractor where a aperture of 80mm has better light grabbing ability than a 60mm. but the aperture of a camera lens is controlled at the back of the lens (and yes still controls the light handling), but how about the front of said lens?

is a 300mm f4 lens set at f4 going to capture as much light as a 300mm f2.8 set at f4? if so then whats all that extra glass at the front doing exactly? they take different filter sizes because the front of the f2.8 is soooooooooooooo much bigger.

my head is going to explode i think.

taken the above 2 300mm lenses as an example, we know the aperture which we control and which controls the light is at the back of the lens right? why not then make the f2.8 the same physical size as the f4?
Reply With Quote
  #4  
Old 20-12-2007, 08:51 PM
Outbackmanyep's Avatar
Outbackmanyep
Registered User

Outbackmanyep is offline
 
Join Date: Oct 2006
Location: Walcha , NSW
Posts: 1,652
Inverse Square law have something to do with it??

To get f/9 @ 600mm you need a wider aperture to have same light gathering ability as a 200mm f/9, but at a higher magnification. Higher quality components are needed to make the field sharp and free of vignetting @ f/2.8.
My guess is that they use a much bigger lens so that the manufacturer can control the vignetting in the lens, much like a field stop in an eyepiece.

I'd have to look into that a bit more i think to make sure!
Reply With Quote
  #5  
Old 20-12-2007, 09:06 PM
acropolite's Avatar
acropolite (Phil)
Registered User

acropolite is offline
 
Join Date: Feb 2005
Location: Launceston Tasmania
Posts: 8,988
Light wise, F9 is identical on both lenses, otherwise your exposures would be all over the place. An image with two different size lenses @ say f8 will have identical exposure (i.e. same amount of light) at a nominated shutter speed, only the FOV and DOF will differ.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 07:11 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
SkyWatcher Australia
Advertisement
Bintel
Advertisement
Meade Australia
Advertisement
OzScopes Authorised Dealer
Advertisement
Lunatico Astronomical
Advertisement
Astromechanics
Advertisement
Celestron Australia
Advertisement
Astronomy and Electronics Centre
Advertisement