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  1. #1
    Join Date
    Sep 2006
    Posts
    200

    Question fast lens apertures?!?!

    I am still looking around at different lenses and hearing everyone talk about fast lenses for low light situations and was wondering (although may be stupid thoughts anyways) If you had two different lenses one being f2.8 and the other being f4.5 would both lenses let in the same amount of light at say f8? Or would the f2.8 let more light in since it starts at a bigger aperture? Are all apertures sizes for any given lens the same when they are smaller than their largest aperture? I am wondering this because if I bought a f2.8 lens would any benefit of having a fast lens be gone once I stopped it down to F3.3 and smaller (would an f2.8 still be brighter at smaller apertures) Like I said kinda retarded question but some insight would be helpful. Let me know if I need to clarify. Sorry for such a noob question and the redundancy.
    Thanks
    Cory
    Last edited by gmtech79; 10-21-2006 at 12:33 PM.
    Nikon D80 with battery grip
    Nikkor 18-200mm AF-S VR DX
    Nikkor 28-80 f3.3-5.6 G
    Nikkor 50mm f1.8 AF
    Nikon SB-800 X2

  2. #2
    Join Date
    Feb 2006
    Location
    Des Plaines, IL
    Posts
    9,560

    Talking To shake or not to shake... that's a question?

    Okay... just to keep it straight, let's define the lens:

    EF 70-200mm f/2.8 USM ($1200)

    What this means is that your WIDEST aperture available will be f/2.8

    If you manually setting it to a smaller aperture, say f/4, it will act identically to light as an

    EF 70-200mm f/4 USM ($600)

    Fair enough?

    Now, where this really comes into play is with shutter speed.

    Scenario:
    • You have an EOS 20D with an EF 70-200mm f/2.8 lens mounted.
    • You are shooting an image in a low light setting.
    • Your camera's light meter is centered at the "0" reading with the aperture set to f/2.8 and the shutter speed at 1/60 sec. and ISO-1600 (highest normal level available)


    Chances are, with these settings, you might or might not see image shake as you snap the shutter. It's a tough shot by anyone's standards.

    If you were to change the lens for the f/4-version of the 70-200mm, your maximum aperture is now f/4... your exposure is off... therefore you have to double the shutter speed setting (set to 1/30 sec)... a longer period of time or chance going to ISO-3200 (a rarely used, highest setting the camera can acheive, done through a control setting in the menu. Using this setting will also almost guaranty your image quality (IQ) will also be in the hopper.). Your chance of introducing image shake has significantly increased to a degree where you may never be able to get the image without a tripod or some other way of stabilizing the camera, other than your paws. Remember: brides don't usually care for pictures where their make-up looks smeared when IT WASN'T!

    One other thing, even if YOU don't move, the chances are the subject will, in a 30th of a second!

    Admittedly, these are border line decisions, but they happen indoors. There is half the light available in an f/4 lens vs a f/2.8. That is the struggle. Where does it f-stop? You can always getter smaller...

    I hope this helps... a little.
    Last edited by DonSchap; 10-21-2006 at 01:07 PM.
    Don Schap - BFA, Digital Photography
    A Photographer Is Forever
    Look, I did not create the optical laws of the Universe ... I simply learned to deal with them.
    Remember: It is usually the GLASS, not the camera (except for moving to Full Frame), that gives you the most improvement in your photography.

    flickr & Sdi

  3. #3
    Join Date
    Jun 2006
    Location
    Washington State
    Posts
    439
    Nice explanation Don. Kinda helped me a bit. The only thing is about the 2.8 is that you will be able to shorten your shutterspeed by 1 full stop because the like Don said a f2.8 @ 1/60th is the same as a f4 @ 1/30. The reason being is that you need to compensate the shutterspeed for the amount of light the aperture lets it. The 2.8 is more capable of "stopping" action with a hotshoe flash attached. It all depends on where you are shooting and what you are shooting. If it is a nighttime football game then you will definitely need that extra stop, but if all of your shooting is done outside during the day then the f4 is a good deal as well.
    Nikon D300
    Nikkor AF-S 17-55mm f/2.8
    Nikkor 80-200 f/2.8 AF-D (70-200mm f/2.8 VR soon)
    Nikon SB600 (SB800 soon)
    Nikon MB-D10 grip
    Sekonic L-358
    Bogan/Manfrotto 055xprob tripod

    Canon AE-1 Program
    Canon FD 50mm f/1.8
    CPC 2x Teleconverter
    CPC Phase 2 CCT 80-200 f/4.5
    CPC Phase 2 CCT 28mm Macro f/2.8

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