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Thread: Constant f2.8 lenses

  1. #1

    Question Constant f2.8 lenses

    Hi

    I am a newbie here. I would like to ask a question and hope to receive some replies.

    I am looking forward to buy a new len (80-200mm) and I was told that the len has a constant f2.8. What does this mean?

    Does this mean at any focal length from 80-200mm I can fix my F-stop to 2.8? but I still have a choice to close up my aperture if I want to? or it is only at 80mm then I can get f2.8 and when the focal length increases the f number gets bigger?

    Thanks for answering my queries... hope i can have some reply soon as I am getting the lens in a couple of days.


  2. #2
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    Default Re: Constant f2.8 lenses

    constant aperture means that regardless of the focal length, the aperture can be kept at a constant size, e.g. f/2.8, f/4, etc

    Some lenses do not offer this capability, e.g. 10-22mm f/3.5-5.6 which means that at 10mm the widest it can go to is f/3.5 but once you reach the upper end of 22mm the widest is only f/5.6

    When you shoot under constant lighting conditions, having a constant aperture is good because it will save you the time and trouble having to re-adjust the exposure setting everytime you zoom in and out - and instead concentrate on capturing the moment.

    hope this helps! =)
    Last edited by RudyTT; 9th October 2009 at 02:03 PM.

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    Default Re: Constant f2.8 lenses

    Hi, just to share.....

    "On constant aperture lenses, the maximum aperture is the same, regardless of how much the lens is zoomed. How can you tell which lenses have constant maximum apertures?

    The answer is simple: if you see a zoom lens with only ONE aperture number then it has a constant maximum aperture.

    Example: 28-70mm f/2.8 (2.8 is the max aperture at both 28mm and 70mm and everywhere in between)"
    Canon EOS 40D | EF-S 15-85mm f/3.5-5.6 IS USM

  4. #4

    Default Re: Constant f2.8 lenses

    Quote Originally Posted by RudyTT View Post
    constant aperture means that regardless of the focal length, the aperture can be kept at a constant size, e.g. f/2.8, f/4, etc

    Some lenses do not offer this capability, e.g. 10-22mm f/3.5-5.6 which means that at 10mm the widest it can go to is f/3.5 but once you reach the upper end of 22mm the widest is only f/5.6

    When you shoot under constant lighting conditions, having a constant aperture is good because it will save you the time and trouble having to re-adjust the exposure setting everytime you zoom in and out - and instead concentrate on capturing the moment.

    hope this helps! =)
    Thanks Rudy,
    In other word, for this constant f/2.8 len. I just set my camera to Aperture Priority mode and select f2.8 on my camera and no matter what zoom length i am at the camera aperture will still be at f/2.8 right? (sorry just to cfm before committing to the len)

    Thanks.

  5. #5
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    Default Re: Constant f2.8 lenses

    Quote Originally Posted by fotos View Post
    Thanks Rudy,
    In other word, for this constant f/2.8 len. I just set my camera to Aperture Priority mode and select f2.8 on my camera and no matter what zoom length i am at the camera aperture will still be at f/2.8 right? (sorry just to cfm before committing to the len)

    Thanks.
    Thats right.
    You can choose your aperture as wide as f/2.8 no matter what zoom length you are at.
    You can also choose smaller apertures as f/4.0, f/8.0 etc.etc. regardless of your zoom.

    but those 2.8 zoom lenses dont come cheap though.
    And beware, they will turn you into an Lchoholic.
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  6. #6

    Default Re: Constant f2.8 lenses

    Other CSer have answered your questions.
    Constant aperture (of F2.8) means you can chose to have f2.8 from focal length of 80 mm, 85 .... 100....until 200. At any focal length you can chose to change to other aperture too, like f3.5, f4, f5.6....

  7. #7

    Default Re: Constant f2.8 lenses

    constant aperture lenses definitely come at a premium
    other than the convenience of not having to worry too much about exposure settings when you zoom in and out, i think the more important reason for getting one is to consider under what kind of conditions you are going to use this lens for, whether do you need a fast lens because of lighting or subject issues
    of course, if you have the extra $$, then go for constant aperture but they are also generally heavier and bigger than similar focal range lenses without the constant aperture

  8. #8

    Default Re: Constant f2.8 lenses

    Quote Originally Posted by infinity View Post
    Hi, just to share.....

    "On constant aperture lenses, the maximum aperture is the same, regardless of how much the lens is zoomed. How can you tell which lenses have constant maximum apertures?

    The answer is simple: if you see a zoom lens with only ONE aperture number then it has a constant maximum aperture.

    Example: 28-70mm f/2.8 (2.8 is the max aperture at both 28mm and 70mm and everywhere in between)"
    Thanks infinity. I am a little confused now with the above.

    I was also testing the Tamron SP AF90mm f/2.8 Di Macro 1:1 (max aperture 2.8)
    It has only one aperture number (f/2.8) but when I use it on my camera (aperture priority mode) and camera aperture setting is at f2.8, the F number on the camera changes when I focus on different subject. Is this correct?

    Or I should set to Shuttle Priority? playing with my shuttle speed and the aperture will be fixed to f2.8 constant?

    sorry if i sound a little lost... newbie here, trying to learn...

  9. #9
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    Default Re: Constant f2.8 lenses

    Quote Originally Posted by fotos View Post
    Thanks infinity. I am a little confused now with the above.

    I was also testing the Tamron SP AF90mm f/2.8 Di Macro 1:1 (max aperture 2.8)
    It has only one aperture number (f/2.8) but when I use it on my camera (aperture priority mode) and camera aperture setting is at f2.8, the F number on the camera changes when I focus on different subject. Is this correct?

    Or I should set to Shuttle Priority? playing with my shuttle speed and the aperture will be fixed to f2.8 constant?

    sorry if i sound a little lost... newbie here, trying to learn...

    Your aperture will change if you're using modes like "P" or "Shutter Priority" mode.
    If you want to keep your aperture at f/2.8 all time, then use "Aperture Priority" or "Manual" mode.
    My Gallery at DeviantART http://waiaung.deviantart.com

  10. #10

    Default Re: Constant f2.8 lenses

    Quote Originally Posted by waiaung View Post
    Your aperture will change if you're using modes like "P" or "Shutter Priority" mode.
    If you want to keep your aperture at f/2.8 all time, then use "Aperture Priority" or "Manual" mode.
    Thanks everyone! (Waiaung, Setsunaze, Shen Siung, Infinity and Rudy)

    I got it now....

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    Default Re: Constant f2.8 lenses

    Quote Originally Posted by fotos View Post
    Thanks infinity. I am a little confused now with the above.

    I was also testing the Tamron SP AF90mm f/2.8 Di Macro 1:1 (max aperture 2.8)
    It has only one aperture number (f/2.8) but when I use it on my camera (aperture priority mode) and camera aperture setting is at f2.8, the F number on the camera changes when I focus on different subject. Is this correct?

    Or I should set to Shuttle Priority? playing with my shuttle speed and the aperture will be fixed to f2.8 constant?

    sorry if i sound a little lost... newbie here, trying to learn...
    f2.8 only for subject with some distance only... when you goes into macro range (no idea what focus distance for Tamron, i'm using other macro lens), the maximum aperture would drop from f3 to f4.8...

    hope this helps...

  12. #12

    Default Re: Constant f2.8 lenses

    Quote Originally Posted by waiaung View Post
    Your aperture will change if you're using modes like "P" or "Shutter Priority" mode.
    If you want to keep your aperture at f/2.8 all time, then use "Aperture Priority" or "Manual" mode.
    This doesn't apply to macro lenses.
    Your aperture is at f2.8 when at infinity, the aperture will become smaller as you focus closer.
    This applies even if you set to A or M mode.

  13. #13
    Moderator catchlights's Avatar
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    Default Re: Constant f2.8 lenses

    for macro lenses, the size (as in diameter) of the aperture remain the same thru out all the distance, however, when come to close focusing range, due to the extension of the lens barrel, the effective aperture will drop, so when is in 1:1, you lost two stops.
    Last edited by catchlights; 9th October 2009 at 03:17 PM.
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  14. #14

    Default Re: Constant f2.8 lenses

    Quote Originally Posted by jaRv1s View Post
    f2.8 only for subject with some distance only... when you goes into macro range (no idea what focus distance for Tamron, i'm using other macro lens), the maximum aperture would drop from f3 to f4.8...

    hope this helps...
    Thanks jaRv1s, Hack, catchlights... now I am getting it and it explains why i can't get constant f/2.8 using the macro len when I focus on closer distance (not infinity).

    OK... many a time i don't really need f2.8 for macro shots, on the other hand the f/2.8 (infinity) can produce very nice bokeh for portraits..

    Thanks again!

  15. #15

    Default Re: Constant f2.8 lenses

    Quote Originally Posted by fotos View Post
    Thanks jaRv1s, Hack, catchlights... now I am getting it and it explains why i can't get constant f/2.8 using the macro len when I focus on closer distance (not infinity).

    OK... many a time i don't really need f2.8 for macro shots, on the other hand the f/2.8 (infinity) can produce very nice bokeh for portraits..

    Thanks again!
    You are right.
    Although its stated f2.8, but in actual fact the aperture decreases as you focus closer and closer.
    Anyway like you said, f2.8 is not needed for macro shots.

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    Default Re: Constant f2.8 lenses

    It means, the widest aperture you can use at all focal range is ƒ2.8. Which is a good thing, because most low-end zooms have variable aperture like ƒ3.5-ƒ4.5 (from the wide end to the tele-end).

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    Default Re: Constant f2.8 lenses

    I just got myself a 17-55mm/f2.8 and I love it! Its great!

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    Default Re: Constant f2.8 lenses

    Quote Originally Posted by siron View Post
    I just got myself a 17-55mm/f2.8 and I love it! Its great!
    yes, i got one and love it too!
    nver leave home without one.
    (sorry for the o/t, TS)

  19. #19

    Default Re: Constant f2.8 lenses

    Quote Originally Posted by waiaung View Post
    Your aperture will change if you're using modes like "P" or "Shutter Priority" mode.
    If you want to keep your aperture at f/2.8 all time, then use "Aperture Priority" or "Manual" mode.
    Tsk tsk tsk..
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  20. #20
    Member Cartman2000's Avatar
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    Default Re: Constant f2.8 lenses

    Quote Originally Posted by siron View Post
    I just got myself a 17-55mm/f2.8 and I love it! Its great!
    I was initially skeptical about that lens due to its rather substantial cost. After having used it for a day, I realised that it is worth it for crop users. No complaints

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