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Category: New Questions

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Photography Question 
Diana Burrows

member since: 12/17/2003
 

What Does Fast Lens Mean?


I've heard reference to "fast" lenses. What does this mean and why is it important? I have a Canon 20D with a Canon EF-S17-85mm f/4-5.6 IS USM lens and also a Canon EF75-300mm f4-5.9 IS lens. My primary "focus" (no pun intended) is taking portraits (outside a studio) and wedding photography. Thanks!

2/21/2006 10:22:23 AM

 
Christopher A. Vedros
BetterPhoto Member

member since: 3/14/2005
  Diana,
A "fast" lens is one with a large maximum aperture (small f-number). This allows you to use a faster shutter speed for a given scene. Both of your lenses have a maximum aperture of f/4. In comparison, the Canon 70-200mm f/2.8L lens has a maximum aperture of f/2.8, so it is a "faster" lens. In a given scene, the faster lens would allow you to use a faster shutter speed, since the larger aperture lets in more light.

2/21/2006 10:36:43 AM

 

BetterPhoto Member
  To add to what Chris was saying, The larger the opening, the smaller the f-stop number. The smaller the f-stop number, the more light that gets to the film/sensor. The more light that gets to the film/sensor, the faster the shutter speed needed to regulate the light. Smaller f-stop number means faster shutter speeed, thus, faster lens. That work for ya, Chris?

2/21/2006 10:52:02 AM

 
Diana Burrows

member since: 12/17/2003
  I see now. So with my "slower" lens, the speed I use needs to be slower, thus there is a potential for blurred pictures or "ghost-like" figures in lower light unless I use a tripod, right? ...Thank you both so much!

2/21/2006 10:56:13 AM

 
Jon Close
BetterPhoto Member

member since: 5/18/2000
  A tripod (or using the IS feature of your lenses instead) will keep camera movement from blurring your photos with slow shutter speeds, but will do nothing to freeze motion in your subject. To freeze subject motion you have to have wider aperture lenses, and/or shoot at higher ISO.

2/21/2006 11:05:54 AM

 
Diana Burrows

member since: 12/17/2003
  Wow; so much to learn, but I guess I have to start somewhere. Thanks so much for all of the help!

2/21/2006 11:11:15 AM

 
Bob Fately

member since: 4/11/2001
  One more point, Diana - at a faster f-stop, depth of field is reduced. That is, the thickness of the sharply focused area is less at f2.8 than at f4. This can be a useful factor in portraiture, where you could use it to your advantage to keep the subject in focus while any distractions in the background would become blurred out and less noticeable.

2/21/2006 12:00:46 PM

 
Forrest C. Wilkinson
BetterPhoto Member

member since: 5/15/2005
  Generally speaking, fast lenses will have the maximum aperture of f/2.8 or wider--you will see most portraits taken on f/stops 5.6 and faster. As far as portraiture goes, it's a little less important to have the faster f/stop, the stops are more important for a sports photographer shooting at night perse, than a portrait shooter with a flash in controlled conditions with a tripod.

On the other hand, bokeh is produced by a faster f/stop, this is essential for any type of photographer. Bokeh, if you didn't already know, refers to the background blur amount. Many portraits try to have the subject appear to be, "on an island" where the background is extremely blurred in a beautiful mix of colors. You can only expect to get great bokeh when shooting 2.8 or lower, depending on how close the subject is and how far away the background is.

Many portrait shooters enjoy being able to create such bokeh very easily but shooting with a lens that is faster than 1.8, say the Canon 50mm f/1.4 for example. By shooting at f/1.4, the photographer can get strong bokeh without having to separate the subject and the background as much as with a "slower" or smaller aperture.

2/21/2006 1:30:58 PM

 
Justin G.
BetterPhoto Member

member since: 7/13/2004
  I always thought that 'selective focus' was the term to describe the amount of background blur and 'bokeh' was termed to describe the quality of the out-of-focus highlights. A lens with a 5 blade diaphram will have "uglier" out of focus areas as to where a 9 blade diaphram will have more of a "buttery" smooth look on the backgrounds. maybe i'm just getting my terms mixed up. just a thought.

2/21/2006 2:17:08 PM

 
Justin G.
BetterPhoto Member

member since: 7/13/2004
  Ken Rockwell's definition.

2/21/2006 2:19:06 PM

 
Glenn E. Urquhart
BetterPhoto Member

member since: 1/3/2006
  Hi Diana - I also have a 20D. Check out your Manual, page 68, "When Lens whose maximum aperture is F2.8 or Larger". Basicaly, what it says, if the lens is F2.8 or faster, the focusing mechanism responds 2x more efficently. Hope this helps. Cheers, Glenn.

2/22/2006 4:19:00 AM

 
Forrest C. Wilkinson
BetterPhoto Member

member since: 5/15/2005
  I may be mistaken, but I believe that selective focus would be when only a selected part of the frame is in focus and rest is blurred. I do not believe that you can have an "amount" of selective focus--it's either selectively focused or it's not. The term 'bokeh' refers to both the quality of the background blur, and the amount that the object is blurred horizontally and vertically, making the circular appearance of out-of-focus elements of the photo.

If I were to selectively focus a picture, my objective would be to separate one item of the photo from the rest of the photo by use of bokeh, instead of using selective focus to determine the bokeh if that makes any sense to you. In other words, something could be selectively focused at f/5.6 if it was close enough to you and the background was far enough away; but with f/1.2, for example, there would be a greater amount of bokeh and depending on the lens quality, the quality of the bokeh could be higher or lower. If it were a bad lens, there would be more bad bokeh, if it was high quality, it would be more good bokeh. If it has a good amount of bokeh, it is selectively focused and if it is selectively focues, there is a good amount of bokeh. Very confusing, I know, but that's how I look at it, perhaps I'm wrong?

2/22/2006 1:10:04 PM

 
Justin G.
BetterPhoto Member

member since: 7/13/2004
  Forrest I think we're both right and it's just hard explaining it on here. I understand your explanation and I hope you understand mine. I guess it's just one of those things where there's different definitions and none are neccessarily wrong.

2/22/2006 2:37:09 PM

 
Forrest C. Wilkinson
BetterPhoto Member

member since: 5/15/2005
  Could very well be, guess it's just your personal photographic dialect.

2/23/2006 5:07:11 PM

 
Justin G.
BetterPhoto Member

member since: 7/13/2004
  haha lol. i'm from the north, are you from the south?

2/23/2006 5:50:09 PM

 

BetterPhoto Member
  SWING YER PARTNER ROUND & ROUND...

2/23/2006 7:02:31 PM

 
Justin G.
BetterPhoto Member

member since: 7/13/2004
  Creek = (krēk) · not (krick)

Iced Tea = (īst tē) · not (ass'd tay)

Pie = (Pī) · not (pah)

2/23/2006 7:13:07 PM

 
Forrest C. Wilkinson
BetterPhoto Member

member since: 5/15/2005
  Born in the north, live in the south.

2/24/2006 1:18:13 PM

 

BetterPhoto Member
  Otay, Otay! Let's GIT ER DUN!!!!!!!

2/24/2006 8:05:46 PM

 
Joe Ciccone
BetterPhoto Member

member since: 3/7/2005
  A lens allows the light to enter the camera to record whatever the camera is 'looking' at.

Two things dertermine exactly if the correct amount of light enters.

How large the lens is (this is your
'f-stop') and how long the camera allows the lens to let the light in
(this is your speed).

Speed is easy and clear :
1/100 lens open for 100 of a second
1/2000 lens open for a 2000 of a second

Arpature which means opening, which is represented by the letter 'f', as you can see is a little confusing.
Mostly because 'bigger = less "
22f = a very small lens opening
8f = a medium lens opening
2.8f = a larger lens opening
1.8f = an even larger opening

There's another main consideration in all this...film speed.
This term is term used to discribe how
much light a film needs to be properly exposed. A control for sensitivity has been carried over into 'digital' and is represented by the letters 'ISO'

And once again in this area some one was looking to make this subject much harder than it really is to understand.

Again things seem to be reversed :
100 speed setting requires more light
400 speed setting needs less light

Hope this helped the newcomers a little.

2/28/2006 4:44:46 AM

 

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