optimum aperture ratio

Discussion in 'Photography Beginners' Forum' started by t00sl0w, Dec 16, 2009.

  1. t00sl0w

    t00sl0w TPF Noob!

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    i have been wondering this for a few days....
    is there is a optimum ratio between the lens aperture, and the aperture you set on the camera?
     
  2. Goontz

    Goontz TPF Noob!

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    Some lenses will be sharpest at a certain aperture. Most lenses are not sharpest when wide open. However, there's not really an "optimum" aperture across the board; every lens is different.

    I think this is what you're asking. As far as a "ratio," I'm not sure what you mean. Whatever you set the aperture to on your camera is what the "lens aperture" is going to be. The physical aperture itself is only in the lens, you only set what it will adjust to (open or close as the picture is taken) on the camera.
     
  3. robbie_vlad

    robbie_vlad TPF Noob!

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    Do you mean is there an optimum "max aperture" to "actual aperture" ratio? Such as:

    Max Aperture of my 70-200 : 2.8
    "Sharpest" Aperture: 8
    so the optimum aperture ratio would be 2.8:8?

    If so, then no. I believe for MOST, not all, lenses the "sharpest" apertures will be between f/8 and f/11, regardless of MAX aperture.
     
  4. rufus5150

    rufus5150 TPF Noob!

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    Actually, based on my reading, the sharpest apertures will almost invariably be *at* 8 or 11 due to optics. What you end up paying more for in a lens is sharpness at places other than that.

    That being said, I read that in 'understanding exposure' or a similar book that I loaned out a while ago and haven't gotten back but there was quite the explanation about it having to due simply with the nature of optics. I've yet to discover an online resource that explains it as well but I've not really bothered to do the searching.

    When I did some anecdotal testing when I had only an 18-55 kit lens for an XTi, I could get surprisingly sharp pictures at F8. It was almost magical over wider apertures where it was hit and miss on sharpness.
     
  5. IgsEMT

    IgsEMT No longer a newbie, moving up!

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    I read it few times and still confused about what you mean.
    Do you mean ratio in terms of lighting, depth of field, best sharpness?
     
  6. Inst!nct

    Inst!nct TPF Noob!

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    generally i believe it is f / 11
     
  7. t00sl0w

    t00sl0w TPF Noob!

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    i am looking for the best comprimise between sharpness and DOF.....
    basically what robbie said....

    if your lens is a 2.8 then for best general results, you need to set your camera at 8, or whatever....is there a numerical correlation between that being the best setup?

    say, i can always, use a 1:2.85 for the best setup....(example)
     
  8. o hey tyler

    o hey tyler Been spending a lot of time on here!

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    If your lens is an f/2.8, that's the largest aperture the lens can handle. The way the glass is though, most are the sharpest at f/8 - f/11. But it really depends on the manufacturer.
     
  9. robbie_vlad

    robbie_vlad TPF Noob!

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    With my 70-200 2.8 (lens I use the most) it depends on what/where I'm shooting. Sports, I normally stay in f/2.8-f/4. When I'm shooting portraits I stay between f/8-f/11. f/2.8 is plenty sharp though, so it's not like your everyday viewer will notice a difference in sharpness.
     
  10. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    That doesn't really make sense either. :scratch:

    DOF is a result of the physics involved. Aperture size, focal length, distance to subject/focus point etc. It is what it is. You could be making a compromise to get more DOF, or less DOF...it's up to you and what you want to do with the shot.

    What some people are saying (also what I've been told by some photo instructors) is that F8 is usually going to be the best aperture for image quality.

    Also, many people will tell you that most lenses are poorest at their extremes. Meaning the maximum aperture or maximum zoom. There is also a quality drop off as you near the smallest aperture as well.

    The F number is already a ratio. F8 is 1:8 (lens entrance pupil to focal length).
     
  11. rufus5150

    rufus5150 TPF Noob!

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    And 'sharpness' is also somewhat of an ambiguous definition. The 24-70L is tack sharp wide open in the center but a shade soft at the extents, even on a crop sensor. However opened to F8, that sharpness is the same across the entire frame.
     
  12. Stosh

    Stosh TPF Noob!

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    According to physics and optics, assuming your optics are perfect, a lower f-ratio (faster lens) is ALWAYS sharper. That also means that with any given lens, the more you open it up, the sharper it is. The reason for this is because larger apertures can resolve finer details.

    But that is theory. Welcome to reality. The "imperfections" of the optics usually degrade the fastest stop or 2 (or 3) of your lens so that most lenses are sharpest across the board somewhere around 1-2 stops below max. But if you're using pro lenses that are extremely sharp at f/2.8 or f/4, it's very possible the optics are out-resolving your sensor. If that's the case, then everything from that sharpest point to the limits of your sensor will appear equally sharp to you. The more you stop down a lens, the worse the sharpness gets because of diffraction. So depending on the pixel size of your sensor, the diffraction limit is reached somewhere around f/8 plus or minus a stop, hence everyone's answer of f/8 or f/11 being the sharpest. Just wanted to give a larger piece of the answer why.

    Here is a great page on diffraction:
    Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks
     

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