f/8 is the magic aperture for sharpness

Discussion in 'Digital SLR' started by paul, Jan 21, 2005.

  1. paul

    paul Guest

    According to this page f/8 is much sharper than higher or lower
    apertures: http://www.photodo.com/art/Impr13.shtml.
    Scroll to the bottom for test shots at f/1,8-f/22

    I had no idea. It seems too simple, surely it varies depending on the
    lens? The writing is a bit confusing, I think these are the relevant
    parts explaining this chart: http://www.photodo.com/pix/art/best_at_8.gif

    "In the diagram above with the two curves, the descending curve
    symbolizes diffraction. At wide apertures the diffraction is minimal.
    The part of the light beam that turns fuzzy is very small. But when you
    stop down and the hole gets smaller the diffraction gets worse."

    "A lens improves optically when stopped down. At large apertures most of
    the glass in the lens is used, resulting in a slight blur caused by
    unavoidable imperfections in the lenses. When stopping down you screen
    off parts of the lens and use only the central area of it. The optical
    picture is more correct and the resolution improves. The ascending
    curve, starting at 40, symbolizes this "optical" and theoretical sharpness."

    "It's only the light passing closest to the aperture edge that turns off
    and becomes fuzzy, so when you are using a wider aperture the percentage
    of light that is fuzzy decreases. Small holes result in a lot of
    diffraction."

    "When a wave passes an edge, it turns slightly. It's called diffraction.
    The diameter if the light passing a lens is limited by the diameter of
    the aperture. The aperture forms a sharp edge and the light closest to
    the edges turns off slightly. The edges of the aperture cause a certain
    fuzziness."
     
    paul, Jan 21, 2005
    #1
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  2. paul

    Jay Beckman Guest

    Hence the phrase: "f8 and be there..."
     
    Jay Beckman, Jan 21, 2005
    #2
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  3. paul

    Alan Browne Guest

    It's part right. The real deal is that for most lenses, peak sharpness begins
    at about 2 to 3 stops closed from max aperture. So the story is slightly
    different for an f/1.4 lens v. an f/3.5 lens or a zoom that starts at f/5.6.
    Most lenses are fairly sharp through about f/16.

    OTOH, if you don't use a tripod, it is all a very moot point.

    It's not a bad rule of thumb to assume f/8 and the differences between f/8, f/11
    and f/16 in sharpness are not very discernible in an image that is worth pondering.

    There are diffraction limited lenses that are sharp at their widest apertures
    and remain sharp for 4 or 5 stops down from there before becoming softer due to
    difraction at very small apertures.

    Having said all that, composition is usually more important than sharpness, so
    choose the aperture for the composition.

    Cheers,
    Alan
     
    Alan Browne, Jan 21, 2005
    #3
  4. Hmmmm ... I am just as surprised as you. If you have a perfect
    circular aperture, then the diffraction is very easy to calculate.
    So - thats not a problem. But - it sounds very strange that the
    unsharpness due to lens faults should follow some kind of curve
    that is similar for different lenses. Are you sure that it is not
    first of April? :)


    /Roland
     
    Roland Karlsson, Jan 21, 2005
    #4
  5. paul

    RichA Guest

    For crummy, cheap regular lenses, stopping down always improves
    sharpness. This is a shame in a way because the larger the aperture
    of the lens, the more true resolution it should have.
    The diffraction limit, which imposes the limit on detail seen is
    roughly 4.48 arc seconds (smallest detail resolvable) per inch of
    lens aperture. So a 1" lens can show detail 4.48 arc seconds across
    a 2" wide lens will show detail half that size, 2.24" across, etc.
    For instance, given the same quality, a lens with a 4" wide aperture
    should yield details twice as small as a lens of 2" aperture.
    However, this only applies to lenses with enough focal length to take
    advantage of the resolution afforded by aperture. This would
    (unfortunately) mean having a focal length = 2500mm for a 4" aperture
    lens or 1200mm for a lens with a 2" maximum aperture. This is one way
    to reach the diffraction limit for a lens. The only way to provide
    that kind of focal lenght for those aperture is to use a telescope
    which consists of a "prime" lens (called the objective) and a series
    of positive lenses interposed between the camera the the prime lens.
    Camera lenses are not designed to approach the diffraction limit, none
    of them are made to a high enough standard (1.8th wave accurate or so)
    to get to the diffraction limit. However, there have been
    (apparently) camera lenses designed to provide maximum sharpness when
    at full aperture, some of the "Noct" lenses from Nikon or Leica are
    such lenses. However, I don't know if the claims for their sharpness
    are true or not.
    But getting back to the original concept of stopping a lens down.
    Suppose you stop a lens with a 2" wide front lens down to f8. That
    means that the focal length versus the aperture is now 8:1. For
    a 100mm prime lens with a 2" (50mm) front lens, this means the lens
    opening is only 12.5mm across and it's theoretical maximum resolution
    is now only 9 arc seconds, far less than if the original 2" aperture
    could be used. What you have done by stopping the lens down is
    minimize the inherent aberrations in that lens. Sperical aberration
    (the inability to focus light entering different areas of the front
    lens to the same point) chromatic aberration, astigmatism, coma, etc.
    This means that the residual aberrations of the camera lens (on
    average) are so bad that you must drastically reduce the aperture to
    obtain maximum sharpness. However, because of the limit on focal
    length verus aperture in a camera lens, the lens's true resolving
    power (even at a small 12.5mm aperture) is never reached.
    -Rich
    -Rich
     
    RichA, Jan 21, 2005
    #5
  6. paul

    Stacey Guest

    It does. A good lens is best at much larger openings.

    And this part of the article shows me it's BS..

    "The 50/1,8 lens was focused using the cameras superb auto-focus."

    You NEVER use auto focus for lens testing.


    Bottom line, test your own lenses. Some medium format lenses need to be
    stopped down to f11 to be "their best" some of the newest digital lenes are
    best at f4.
     
    Stacey, Jan 22, 2005
    #6
  7. paul

    Sheldon Guest

    I think it depends on the lens. I've used my 85 1.8 to take some awesome
    and very sharp 35mm photos taking advantage of the limited depth of field
    the lens provides. Looking forward to getting my new D70 and using the lens
    with it. The only thing I'll miss is the up-close-and-personal touch the 85
    gave me when I used it for portraits. I'll have to back up a bit due to the
    magnification factor.
     
    Sheldon, Jan 22, 2005
    #7
  8. paul

    JPS Guest

    In message <css1tk$mua$>,
    That would depend on your shutter speed, and angle of view. A tripod
    can only improve on a 15mm lens at 1/2000 in terms of steadily
    maintaining composition and rotation. It doesn't improve sharpness.
    --
     
    JPS, Jan 22, 2005
    #8
  9. On 22-Jan-05 20:09:32, JPS said
    Just to muddy the waters.... at my last place of work, I foolishly went to
    take a group photo without a tripod, this was a cloudy December afternoon.

    I was using a D1X with I think a 35-70(?) Nikkor zoom. I usually keep it
    at 200 ISO but went to 400 for another stop and ended up using the lens
    wide open. The results were pretty horrible, people out of the centre of
    the picture were really blurry - and it wasn't a big group ( a frontage of
    only 10 people).. I thought it was a "rotating" camera movement but my
    colleague (who uses identical kit" said after a similar experience, he
    never goes below f8 with a group. I went outside and photographed the wall
    at a similar distance to the group shot - on a tripod -same aperture. The
    result was the same yukky sharpness (forgive the technical term) off the
    centre. I was surprised that a new and well looked-after lens could not
    yield an acceptable result in these conditions.

    Therefore, in practice I would avoid large apertures like the plague where
    definiton off the centre of the image was a factor.




    All the best,
    Angus Manwaring. (for e-mail remove ANTISPEM)

    I need your memories for the Amiga Games Database: A collection of Amiga
    Game reviews by Amiga players http://www.angusm.demon.co.uk/AGDB/AGDB.html
     
    Angus Manwaring, Jan 23, 2005
    #9
  10. paul

    paul Guest

    That makes sense that the wide apertures are used for single portraits
    for an extra blurry background & a fairly soft face. Hmm, now that I
    think of it, the people at the edges would be further away than the
    middle unless they stood in a semi-circle with the camera at the center.
    What's a bit contradictory though is that page says narrow apertures
    cause more edge blur due to diffraction across the edge of the narrow
    opening. Their diagram indicates this should be much worsse at the edges
    and negligible in the center (except at very very small apertures).

    Larger apertures (according to them) cause blur due to the larger area
    of glass being used introducing more imperfections in the glass,
    especially with zoom lenses having more glass in them (is that
    correct?). I don't know, if there were slight imperfections in the
    center of the glass, that could be badly emphasized too I would think.
    Higher quality lenses should have little loss at wide apertures but even
    expensive lenses should suffer at small apertures (except in the center).

    I'm not very experienced but the f/36 on my D70 is unusually tiny isn't
    it? I did this hasty test leaning the camera on a doorway (no tripod):
    <http://www.edgehill.net/1/?SC=go.php&DIR=Misc/photography/f-stop-test>
    it's a 28-200 nikor which is rated well considering the huge zoom range
    but obviously not the most extraordinary lens. (you will also notice the
    hideous sensor dist at f/36 which I pointed out in another thread.
     
    paul, Jan 23, 2005
    #10
  11. paul

    Stacey Guest

    If you're using a cheapo zoom lens. A good lens =IS= sharp enough at the
    edges wide open, a poor one isn't. But then again a lens that is sharp at
    the edges wide open isn't going to be cheap either.
     
    Stacey, Jan 23, 2005
    #11
  12. paul

    Lionel Guest

    You can get cheap primes that are that good, though.
     
    Lionel, Jan 24, 2005
    #12
  13. paul

    Stacey Guest

    =SOME= cheap primes are, many are not. Very few of the 'old skool' fast
    lenses are very good wide open on the edges either.
     
    Stacey, Jan 24, 2005
    #13
  14. paul

    Lionel Guest

    Very true - that's why I said "can get". ;)
     
    Lionel, Jan 24, 2005
    #14
  15. Isn't DoF also an issue? I mean at wider apertures you have shallow DoF
    making it difficult to focus accurately as I discovered with a Pentax
    Super-Takumar 50mm f/1.4.

    The limited testing I did with my Canon dRebel kit lens and the pentax
    lens told me the same thing as the photodo test - that the lens is
    sharpest from f8-f11. From then on, given enough light I use the lenses
    at f8.

    - Siddhartha
     
    Siddhartha Jain, Jan 24, 2005
    #15
  16. On 23-Jan-05 18:36:31, Stacey said

    Well I'd have expected better performance from a Nikor, and it wasn't a
    one-off, as I said my colleague's lens behaved in the same way.

    All the best,
    Angus Manwaring. (for e-mail remove ANTISPEM)

    I need your memories for the Amiga Games Database: A collection of Amiga
    Game reviews by Amiga players http://www.angusm.demon.co.uk/AGDB/AGDB.html
     
    Angus Manwaring, Jan 25, 2005
    #16
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