Why not use anything other than 2.8f?

Discussion in 'Photography Beginners' Forum' started by BLD_007, Feb 21, 2010.

  1. BLD_007

    BLD_007 TPF Noob!

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    I have the cannon 70-200 2.8 and I am about to buy the cannon 23-70 2.8 L. I use a cannon 50d.

    Light is key, right? So why not keep your Fstop at 2.8 and lower your ISO to 100 and increase your shutter speed to compensate?

    Unless your shooting the sun, there should be no reason to have anything greater than 2.8, right.

    From my understanding, the lower the fstop the greater depth of field you have. IE background is blurry, right?

    so why would you want to use a higher fstop, say f16?



    Granted, the ONLY time I have used higher Fstops is when I have been playing with "painting with light".

    so why?
     
  2. ann

    ann No longer a newbie, moving up!

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    no, the higher the fstop the greater the depth of field. high meaning f11, etc.

    the smaller the number the less depth of field, the larger the number equals more dof.

    the subject matter should dicated the DOF, of course the focal length of the lens and what your focusing on will influence DOF as well.

    or, perhaps i miss understand your wording.
     
  3. chammer

    chammer TPF Noob!

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    no lens is sharpest at its widest aperture either. it's usually 2 to 3 stops down - around 5.6 - 8.0.
     
  4. NateWagner

    NateWagner TPF Noob!

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    Typically you stop down (use something other than 2.8) because lenses are typically sharper at f/8 or so. Also, if you want a deeper DOF you will typically use an aperture higher than 2.8. There are a number of other reasons as well, such as if you want to knock down the ambient lighting so that your flash controls the exposure more.

    There are also effects that one might use f/16 for... for example, if you want a star shaped pattern you might use something like that.
     
  5. BLD_007

    BLD_007 TPF Noob!

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    ahh I have so many ideas now.

    I thought 2.8=best and in order to get good DOF you had to have the background further back form your subject and zoomed in.

    So you can have goon DOF with a wide angle at a higher fstop "f8" ?

    im going to go get cleaned up and go outside lol

    just to make sure, greater DOF means your subject is clear but everything else behind it is blurred out or is DOF meaning your subject is clear and everything behind it is visible?
     
  6. anm90

    anm90 TPF Noob!

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    I recommend that you read the book Understanding Exposure by Bryan Peterson. This will greatly improve your understanding of aperture, shutter speed, and ISO as well as help you become more creative in your photographs.

    The aperture number of f/2.8 is the size of the opening in the lens that lets light through. The size of the opening is the focal length (f) divided by 2.8. The smaller the number in the denominator of this fraction, the larger your aperture. So, a large aperture produces a small depth of field and a small aperture produces a large depth of field. You can experiment with this by looking through the viewfinder in your camera when you have the lens wide open (largest aperture available) and pressing the depth of field preview button. Notice that the image in the viewfinder will not change - that is because the lens is already wide open while you look through the viewfinder, and that is the depth of field that will be in the photograph if you were to take an image at that aperture. Now try stopping the lens down to something around f/16 or so and press the DOF preview button. The image in your viewfinder will become significantly darker (the opening in the lens just got smaller, so now you're not letting as much light in) but you will also notice that your depth of field increased greatly.

    Now, to answer your original question... sometimes we use small depth of fields (apertures like f/2.8, f/3.5, or maybe even f/5.6) to produce an effect that isolates the subject from the background. This can be used for portraits, macro photography, wildlife photography, etc. But what if you're in an expansive field of flowers and you want everything from 2 feet in front of you to infinity to be in focus while using a relatively wide focal length (say 18mm)? In this case you would use a small aperture (like f/22) in order to create what Bryan Peterson calls a "story telling aperture."

    To sum it up, the use of different apertures in camera is just a tool that allows the photographer to be creative in different situations.
     
  7. KmH

    KmH Helping photographers learn to fish Supporting Member

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    Greater depth-of-field (DOF) means the range of focus is deeper, or more of the scene is in focus.

    Less DOF is used to help isolate a subject from the background by having a shallow DOF so the background is not in focus.


    How much DOF there is in an image is a function of 4 things:
    • the aperture of the lens
    • the focal length of the lens
    • the lens to subject distance
    • and subject to background distance.
    One of the really handy features to have on a camera is a DOF Preview button.
     
  8. BLD_007

    BLD_007 TPF Noob!

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    even say the canon 24-70L and 70-200L is better quality at f8 and not 2.8?
     
  9. rufus5150

    rufus5150 TPF Noob!

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    The difference between wide-open and the 'sweet spots' in lenses is less pronounced with L-quality (or equivalent) glass. 2.8 on the 24-70 is extremely sharp and quite usable, but f8 is even more so. Also across the board high-grade lenses tend to be sharp in the middle at all apertures, but as you stop-down you get increased corner sharpness.

    Contrast that with say a kit lens wide open at 3.5 (or whatever) vs it at f11 or f8, is like night and day. Even crappy kit lenses can produce some stellar sharp images between f8 and f11.
     
  10. chammer

    chammer TPF Noob!

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    all lenses. that includes the L series lenses.
     
  11. JPooh

    JPooh TPF Noob!

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    this is actually a funny thing i was just talking about... i bought the 17-55mm f2.8 from nikon but it i rarely use 2.8

    usually i'm shooting at about f4 and higher... i find it to be sharper at the higher stops
     
  12. Sw1tchFX

    Sw1tchFX TPF Noob!

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    I have a 50mm f/1.4 that's OK at f/1.4. not a whole lot of contrast, tons of vignetting, and definetly not as sharp as it could be. Stop it down to f/5.6 and it could cut glass, and is completely even across the frame.
     

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