Forums


Go Back   The TrekEarth Forums >

Notices

Reply
 
Thread Tools Display Modes
  #1  
Old 03-26-2007, 10:48 PM
broglia broglia is offline
TE Senior Member
Thread Starter
 
Join Date: Dec 2005
Posts: 571
Default What is meant by the term "fast lens" ?

I've heard this term used a lot and would like some clarification. How can I tell how fast a lens is? Thanks in advance.
Reply With Quote
  #2  
Old 03-26-2007, 11:03 PM
ronners ronners is offline
TE Junior Member
 
Join Date: Dec 2002
Posts: 350
Default Re: What is meant by the term "fast lens" ?

A fast lens is one that is able to handle fast shutter speeds. This generally means that the maximum aperture possible is quite large (f/1.8 for example). At sporting events you may see pros with very large zooms with large lens diameters - these are also fast lenses because they are designed to allow in more light, and hence allow you to get an exposure in less time (i.e. faster shutter speed).

Hope that helps.

Ron.

PS Fast lenses also tend to be more expensive, so they also drain your wallet faster...
Reply With Quote
  #3  
Old 03-26-2007, 11:10 PM
Floydian Floydian is offline
TE Senior Member
 
Join Date: Feb 2006
Posts: 972
Default Re: What is meant by the term "fast lens" ?

Roberto,

With a fast lens they mostly mean fast in the way he acts in autofocus and tracking moving objects like motorsports.
They are fast because the builder has put all his know-how in designing and enginering such a lens. I see you are using Canon equipment and with Canon you can consider all L lenses (the L stands for Canon high quality glass with a red ring around the lens) as fast and superb build in quality and speed.
Those long white lenses you see during sportevents are top notch for that kind of work. They are very fast in autofocusing and tracking using USM (Ultra Sonor Motor) technology in following and tracking the object. Bad thing is the price...the better build quility and speed he has the more moeny it will cost you to get that beautiful L glass.
Hope this will do...

Henk
Reply With Quote
  #4  
Old 03-26-2007, 11:12 PM
broglia broglia is offline
TE Senior Member
Thread Starter
 
Join Date: Dec 2005
Posts: 571
Default Re: What is meant by the term "fast lens" ?

Thanks Ron, I'd sort of spotted the most expensive bit for myself. So a fast lens is better in low light situations too ?
Reply With Quote
  #5  
Old 03-26-2007, 11:14 PM
broglia broglia is offline
TE Senior Member
Thread Starter
 
Join Date: Dec 2005
Posts: 571
Default Re: What is meant by the term "fast lens" ?

Thanks for the advice Henk. My wallet is very small at the moment so there's little chance of me splashing out on that L series just yet.
Reply With Quote
  #6  
Old 03-26-2007, 11:33 PM
AdrianW AdrianW is offline
Moderator
 
Join Date: Mar 2003
Posts: 1,596
Default Re: What is meant by the term "fast lens" ?

A fast lens is one with a wide maximum aperture - i.e. f2.8 or less. That widest value is often referred to as "wide open". A wide maximum aperture means that more light gets through the lens, so focussing will be easier in low light.

Whilst it can make for better night shooting too, a wide open lens will have very limited DOF (depth of field), so if you want the foreground and background in focus at once, you may need to try something else - like Image Stabilization, or higher ISO values.

Basically the aperture (f number) is calculated as the focal length/aperture diameter, so a 50mm f1.4 has a widest aperture of 50/1.4 = 35.7mm. A 50/2.5 = 20mm.

You can also calculate the DOF, but that's rather more complex. First you need to define "acceptable sharpness" - and that's where things get fun. Enjoy!
Reply With Quote
  #7  
Old 03-26-2007, 11:41 PM
broglia broglia is offline
TE Senior Member
Thread Starter
 
Join Date: Dec 2005
Posts: 571
Default Re: What is meant by the term "fast lens" ?

Thanks for your help Adrian and the very useful link. Cheers!
Reply With Quote
  #8  
Old 03-28-2007, 04:26 PM
mlopes mlopes is offline
TE Expert
 
Join Date: Feb 2005
Posts: 1,070
Default Re: What is meant by the term "fast lens" ?

Henk,
When people talk about "fast lens" they are not talking about what you are saying here ... there's nothing to do with autofocus and tracking moving objects.

There are "L" lens slower than non "L" following your ideas... you have USM lens without beeing in the L family... wrong idea you caught about "fast lens"

Fast lens refer to lens with appertures of f2.8 or greater (1.8; 1.4; 1.2 ...) ... because this lens can give you faster shutter speads when stoped down to this appertures

Imagine this exercice:

--->>> Canon 17-40 f4 L USM versus Canon 16-35 f2.8 L USM

They are both L and both USM ... what makes one faster than another? the apperture is the anwer

Hope i was clear enough.
Mário
Reply With Quote
  #9  
Old 03-28-2007, 04:27 PM
mlopes mlopes is offline
TE Expert
 
Join Date: Feb 2005
Posts: 1,070
Default Re: What is meant by the term "fast lens" ?

Just checked you equipment.
You sure have a nice collection of "fast lens" :D
Reply With Quote
  #10  
Old 03-29-2007, 08:26 PM
kevinos kevinos is offline
TE Senior Member
 
Join Date: Mar 2005
Posts: 692
Default Re: What is meant by the term "fast lens" ?

Good. Very interesting information! Now I want to buy a Nikon D40 and I want a fast lens for good low light work. What lens would members recommend?
Reply With Quote
Reply


Thread Tools
Display Modes

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. The time now is 06:47 AM.



Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.
explore TREKEARTH