Sharpest part of zoom lens?

Discussion in 'Photography' started by D.R., May 2, 2004.

  1. D.R.

    D.R. Guest

    Is the sharpest spot generally in the middle of the zoom?
    ie at around 50 on a 28-70? Aperture? I always though
    that the smaller the aperture that sharper the image. But
    my local shop reckon that F11-16 is the sharpest for many
    lenses. I have seen plenty of sharp landscapes taken at
    F11-16. Or is this because no tripod available to have a
    smaller aperture? Would F22 be more sharp? Or am I
    confusing Depth Of Field with sharpness?

    A little confused.
    D.R.
     
    D.R., May 2, 2004
    #1
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  2. D.R.

    n.t. Guest

    Zoom lenses perform differently from model to model. There is no general
    rule for quality in a zoom range. Some lenses might be good at the short end
    but suck at the long end, and vice-versa. You need to test your lens by
    shooting a resolution chart or some newsprint and find it's 'sweet spot'

    As for aperture, most lenses will generally perform best at about 1-2 stops
    closed from their widest aperture. ( eg: an f2.8 lens will perform best at
    about f5.6, and a f5.6 lens will perform best at f8 or f11)

    Shooting a lansdscape at such a small aperture yeilds excellent results,
    because a) lenses perform better when closed down b) landscapes look better
    with deep-focus. Depth of field and sharpness are two different things. But
    the benefits in this situation are two-fold.
    hope this helps
    n.t.
     
    n.t., May 2, 2004
    #2
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  3. D.R.

    D.R. Guest

    Thanks n.t. for the reply. More questions....

    Can please elaborate a little here? I am not sure
    what you mean by this process. So you mean there
    is a way to test this without wasting rolls of film?
    I often use the DOP preview button, but at smaller
    apertures it gets very dark.
    So even though greater depth of field is at say F22,
    it pays to never shoot landscapes at that if your
    lens is sharpest at say F11? I got the feeling from
    reading books that F22 means max depth of field,
    ie more area in focus, thus sharper looking image.
    Yet, it would seem that this is totally dependant
    on having a lens that is sharp at F22. Just when
    I though I was getting the hang of the basics, they
    throw a curve ball. ;-)
    So if not stopping down the lens all the way, thus
    not having max DOP, where would one focus to ensure
    a sharp looking image? Obviously not the horizon,
    right? Everything in foregroup would not be sharp.
    That is given that there is no major subject in
    foreground, but there are some details in between
    foreground and horizon - and not using a telephoto
    lens.

    Many thanks. :)
    D.R.
     
    D.R., May 2, 2004
    #3
  4. D.R.

    n.t. Guest

    You need to shoot pictures of newsprint at the same distance with different
    lenses (but the same focal length: eg; if you want to test the sharpness of
    your zoom at 50mm, shoot the same test with another zoom at 50mm- and all
    the same f-stop settings) get prints from the lab and it will become clear
    which lens is sharper

    When shooting a wide angle landscape there is little difference in
    depth-of-field at f11, f16 or f22. Due to the fact that the lens is so
    wide. Focus your lens at Infinity or a little closer. With a typical wide
    angle lanscape lens like a 20mm, depth of field is rarely an issue is you
    shoot with a decent f-stop.

    Lenses SHARPNESS (different to Depth-Of-Field) only tend to suffer poor
    performance at 'wide-open' f stops.

    I think you might be confusing Depth of field and lens sharpness. Depth of
    field is only a measure of ' the amount of the subject, foreground to
    background which is in focus' where as lens sharpness is ONLY an indication
    of the resolving power of the glass.

    hope this clears up a few things.
    n.t.
     
    n.t., May 2, 2004
    #4
  5. D.R.

    Graham Cluer Guest

    Although, in general, the smaller the aperture the greater the depth of
    field, there does come a time when diffraction comes into the equation.

    Now, in theory, this shouldn't be a problem until the aperture gets
    close to the wavelength of light. However, I have heard it said that
    small apertures can lead to lack of sharpness because of diffraction
    effects.

    Does the group have an opinion on whether this is a genuine problem or
    an old wives tale?
    --

    -- -------------------------------------------------------------------- --
    Graham Cluer | Email: news1 <at> cluer <dot> com

    (If replying by e-mail please make the above into a legal email address)
    ==========================================================================
     
    Graham Cluer, May 2, 2004
    #5
  6. D.R.

    n.t. Guest

    I cant say i have heard of this anomoly (even after some study of optics)
    but there are many proven situations where shooting at absolute minimum
    aperture, particularly on a bright day can markedly increase (and in fact
    create un-realistic) contrast. Non-professional equipment, has many,many
    trade-offs, including not being able to use the equipments to its full range
    (ie wide-open or closed right down) without there being some abberations.
    But the original poster shouldn't concern himself too much with these
    follies, as getting a full understanding of the basics of depth of field is
    what should come first.

    n.t.
     
    n.t., May 2, 2004
    #6
  7. Huh? My Physics courses are pretty far away, but I don't see why
    this would have to do with the wavelength. I think the diffraction
    pattern is the same for a given edge; it's just that the amount of
    diffracted light is negligible compared to the brutal amount of light
    that passes without diffraction -- when the aperture gets small
    enough, this statement stops being true.
    For large focal distances (i.e., high magnification) or large contrast,
    I think this is a genuine problem -- of course, it all depends on what
    you call "a problem" (a picture that for you is absolutely poor
    quality, for someone else can be indistinguishable from perfect -- ok,
    I'm overstating it, but I'm just trying to say that diffraction may or
    may not be a problem depending on how picky you and your eyes are).

    Carlos
    --
     
    Carlos Moreno, May 2, 2004
    #7
  8. I haven't taken any courses on optics, so I won't claim to be an expert,
    but from what I've read Carlos is correct. I can say from personal
    experience that pictures I have taken at minimum apertures tend not to
    be as sharp overall, so there is a trade-off between depth of field and
    absolute sharpness. This only becomes an issue if you're trying to blow
    up your prints past 8x10 or so.

    If you go to Popular Photography's website and look at some of the lens
    test results, most lenses are sharpest around f/8 and decline from
    there. I don't take their reviews as being all that objective (they
    test manufacturer's samples only), but they might a better idea how
    certain lenses perform at different lengths and apertures.
     
    Brian C. Baird, May 2, 2004
    #8
  9. D.R.

    Travis Smith Guest

    I can't claim to be an expert on a subject, but this is what seems logical
    to me, concerning diffraction.

    I'll just draw some diagrams since I think it makes things easier to
    understand.

    Consider that [ ] indicates your aperture opening. If the distance away
    from the edge of the blades that light becomes diffracted is constant, then
    at a wide aperture it would look like this:

    [ i i ]

    Where the 'i's indicated the edge of the light that becomes diffracted
    around the blades of the lens. Now with a wide aperture like above, not
    much of your light becomes diffracted. But if you had a smaller aperture,
    like this:

    [ i i ]

    More of your light is being effected by diffraction, therefore causing your
    picture to be less sharp. The only question is, how much of the light is
    actually effected by diffraction.

    Travis Smith
     
    Travis Smith, May 3, 2004
    #9
  10. Careful with the phrasing -- there is actually less amount of light
    being diffracted (since the circumference is less), but diffracted
    light accounts for a higher fraction of the light that passes through
    (since the area goes with r^2 whereas the amount of diffraction goes
    linearly with r).

    But the above is not even the whole story -- when you project that
    the the image plane, the diffracted light "cone" (it's not actually
    a cone, but we can view it as a cone for the purpose of this reasoning)
    expands as you get further; so, for high focal distances, the effect
    of diffracted light is amplified. Same thing if you zoom in too
    much (the otherwise micrometric projection of the diffracted light
    becomes noticeable if you enlarge a lot -- e.g., when you print at
    30 by 20, or when you crop the middle 5% of your image and expand
    it, etc.)

    Cheers,

    Carlos
    --
     
    Carlos Moreno, May 3, 2004
    #10
  11. D.R.

    D.R. Guest

    Just taken a look at Pop Photo's site. Great site.
    Seems most lenses do seem sharpest at F8. Is that
    why the old wedding photographers rule/slogan says:
    "F8 and Be there!" ?
     
    D.R., May 3, 2004
    #11
  12. Of all the F stops, I think it's my favorite. :)
     
    Brian C. Baird, May 4, 2004
    #12
  13. D.R.

    D.R. Guest

    Out of interest, what size lens do you favor F8?
     
    D.R., May 4, 2004
    #13
  14. All of them, but it depends what I'm shooting. If I want the absolute
    best center sharpness (product photos, screwing around, etc), f/8 is
    great with my 50mm and my 28-135mm. Otherwise I shoot the widest I can
    for the faster shutter I can manage (I have shaky hands). If I want
    great center sharpness and a decent depth of field with my 20mm, f/8 is
    also great because the depth of field is one m to infinity at that
    point. Stepping down closer to f/22 brings the background more in
    focus, but at the expense of absolute center sharpness.
     
    Brian C. Baird, May 4, 2004
    #14
  15. D.R.

    D.R. Guest

    Primo! Thanks a heap for this.

    I too use high shutter speed and wide aperture.
    I normally use shutter priority if I can.

    I noticed early on with shooting my 70-300 wide
    open that some photos of my dog had his eyes in
    focus, but ears and nose out of focus. Must have
    had virtually 0 DOP, eh? Since then, DOP preview
    is very handy! I guess I should calculate the
    DOP for my lenses at various settings and write
    down a reference card or something until I remember.
     
    D.R., May 4, 2004
    #15
  16. Longer lenses have very small depth of field wide open. Sometimes this
    is useful when you want to blur out the background, but can be a
    hindrance at other times. All said, if you have to choose a focusing
    point and your depth of field is nil, go for the eyes.

    Many fixed-length lenses have a depth of field guide near the focus ring
    for quick reference. For everything else, just experiment until you
    find settings that work best for you.
     
    Brian C. Baird, May 4, 2004
    #16
  17. D.R.

    D.R. Guest

    I'd be happy when I get the hang of that "compressed"
    look using a large lens on tripod on a street/sidewalk
    scene that so many wedding photographers do in B&W.
    Had one done when my wife and I got married, but didn't
    take notice of the cameraman. Does one stand far away
    and zoom in with lens wide open, or stand closer?
    Subjects walking up the sidewalk towards camera and
    camera is on the road a little, to give a slight angle.
    Any tips for that? I am guessing DOP preview is very
    important here.

    ....here's something weird. I noticed a wedding photo-
    grapher the other day arriving at the park just as we
    were leaving. We left because the last of the warm
    late afternoon sun had gone. Bad timing, eh?. But also
    noticed that the only camera used was an F80(N80), same
    as mine. Hah! Not an F5, no spare. One thing I did
    notice though was that a brides vail lights her up
    nicely even in crap lighting. Never noticed that before.
    Learn something new every day. :)
     
    D.R., May 4, 2004
    #17
  18. The rule of thumb is, the closer the focal point is to the camera, the
    narrower the depth of field. For good isolating portraits, shoot as
    wide as you can, the distance won't matter so much unless you're using a
    1000mm lens, at which point I'd be calling you something else other than
    "photographer." ;)
     
    Brian C. Baird, May 5, 2004
    #18
  19. D.R.

    D.R. Guest

    Many thanks again Brian!
     
    D.R., May 5, 2004
    #19
  20. D.R.

    D.R. Guest

    Just figured.... perhaps I am now confusing DOP with perspective.

    Now to see if I have understood, would this be a reasonable summation:

    * DOF is how much of a shot is in focus (before and after subject).
    * The closer to object, the less DOF. Less in focus.
    * Wider lens = more DOP. Longer lens = less DOF
    * Large aperture = less DOP, small aperture = more DOF

    * Lenses are sharpest at F8-F11.
    * Using smallest aperture may still result in soft photo due to
    quality of lens at that aperture.

    * Compressed perspective is achieved with telephoto, but is not DOF.
    * The further from object, but zooming, the more compressed perspective.
    * Compressed perspective + smaller DOF can look cool.

    ????
     
    D.R., May 5, 2004
    #20
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