Are primes brighter and sharper than wide open zooms

Discussion in 'Digital Photography' started by Siddhartha Jain, Sep 28, 2005.

  1. Hi,

    Given two lenses, one a prime (say 28mm) and the other a zoom (say
    28-75mm) and both with an aperture of f2.8 -
    - Will the prime be brighter than the zoom because it has fewer lens
    - Will the prime be sharper wide open than the zoom at 28mm?

    Ofcourse, a lot will depend on the particular makes and models being
    compared but is there a rule of thumb?

    - Siddhartha
    Siddhartha Jain, Sep 28, 2005
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  2. In a word NO. At least it is no if you are talking about real life

    However ---- In general primes will be sharper unless the zoom is a
    much better quality lens. Of course that is possible and a really good zoom
    can outperform a poor prime any day.
    Joseph Meehan, Sep 28, 2005
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  3. Yes, more light will be transmitted through the lens with fewe elements
    (everything else being equal).
    It would probably be, yes, as the optical design makes fewer compromises
    (but having fewer elements gives the optical designer less room to
    Oh, as in, a really good zoom may be better than a cheap and nasty prime?

    David J Taylor, Sep 28, 2005
  4. A good prime should be sharper and exhibit fewer optical aberrations
    compared to a zoom, with the aperture set wide open. Stopping the zoom down
    should improve sharpness, but then you may lose the desirable effect of
    limited depth of field, if that's what you wanted. Zooms typically have
    different geometric distortion throughout the zoom range, usually barrell at
    the wide end and pincushion at the tele end, being neutral somewhere in the
    middle. A good prime should be able to combine better sharpness, contrast
    and distortion characteristics compared to a zoom.

    But there are some outstanding zooms out there these days and the advantage
    of primes has been lessened as a result.

    As the apertures are the same, there should be no difference in brightness.
    The glass doesn't lose enough light in the way you fear to be a major


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    Digital Photography Now, Sep 28, 2005
  5. Siddhartha Jain

    dj_nme Guest

    The prime may be slightly brighter than the zoom, something which may
    be more important in the motion-picture industry
    They use a term called the "t stop" of a lens and it is a measure of
    the light loss through a lens.
    The difference could be as little as a third or as great as (or greater
    than) a 2 stop difference between what the f-stop is and the t-stop,
    while the DoF will be the same and sharpnes is likely to be less with a
    zoom than a prime.
    In real life still photography I don't believe it would be noticable
    (the difference in brightness), because of a variety of factors.
    I personally would not lose any sleep over it.
    dj_nme, Sep 28, 2005
  6. How much light will pure glass stop that not having extra one or two will
    change exposure about a fstop and you can notice diference? ;-)


    Krystian Polak, Sep 28, 2005
  7. Other things being equal, yes to both, though with well-designed modern
    lenses the difference may be small.


    The conventional f-stop designation of aperture is a purely geometric
    measure and takes no account of the actual transmission properties of
    the lens. In reality, not all of the light going into a lens will come
    out at the other end, and some of the light that does come out will be
    scattered, and hence will reduce image quality.

    Attenuation takes two forms: absorption and reflection/scattering.
    Absorption is purely proportional to the depth of glass; a typical
    figure for normal optical glass would be 10% for a total glass path
    length of 100mm. Most photographic lenses would fall far short of this,
    though some big lenses may get there. This attenuation is entirely
    proportional to the length of the light path through glass; thus a zoom
    with 12-15 elements is likely to experience more absorption than a fixed
    focal length lens with 5-10 elements.

    The other form is reflection from glass-air interfaces. This is
    unavoidable, but can be reduced very greatly by coating. The percentage
    of reflection depends on the refractive index of the glass, but for
    typical n=1.50 optical glass (uncoated) the percentage is about 4%.
    This, remember, is at each glass air interface, two per lens element.
    Thus a compound lens with 15 elements will have 30 interfaces, and will
    only transmit (0.96)^30 or about 20% of the light. (In fact another
    20-40% will get to the film or sensor as scattered light - giving an
    image of appallingly bad contrast). A single layer coating will reduce
    the reflection to about 1-1.5%, and modern multi-coating reduces it to
    around 0.3-0.5%.

    This still gives a transmission factor of about 83.5% for a 20-surface
    (10-element) system, against 91.4% for a 10-surface (5-element) system.


    There is not the same direct relationship between complexity and
    resolution as that above between complexity and transmission. However,
    the compromises required to balance zoom ratio, overall size, mechanical
    complexity and cost at the same time as controlling the seven distinct
    varieties of lens aberrations mean that in almost every case the zoom
    lens will have lower resolution than the fixed focal length lens of
    similar quality of design and manufacture. You can see this from the MTF
    function curves published by most lens manufacturers. Having just
    checked some of these myself to answer your question, I am actually
    quite impressed by how small these differences are; a couple of decades
    ago the differences would have been much greater. (Be aware when
    comparing MTF curves that they usually show wide open and f/8 data; as a
    zoom will usually have a smaller maximum aperture than the comparable
    fixed focal length lenses, you should avoid comparing these - best look
    at the f/8 curves for a fair comparison.)

    David Littlewood, Sep 28, 2005
  8. Rule of Thumb: A prime at any focal length and wide open is better
    than a zoom at any focal length wide open.


    "In general, the art of government consists in taking as
    much money as possible from one party of the citizens
    to give to the other."

    Voltaire (1764)
    John A. Stovall, Sep 28, 2005
  9. Although my understanding of lens construction is limited but I believe
    not all lenses are all glass. They use different material in a lens
    alongwith some glass elements to keep the weight lower than an all
    glass lens.

    And as David Littlewood explains so lucidly below in his post, there
    are factors other than an individual glass element's transmittance
    that can effect the total transmittance of a lens.

    - Siddhartha
    Siddhartha Jain, Sep 28, 2005
  10. Siddhartha Jain

    Nostrobino Guest

    Zoom lenses ARE prime lenses, notwithstanding the now-popular misusage of

    A prime lens is the camera lens as distinct from some other lens or
    lenticular device (close-up lens, tele converter, etc.) used with it. It has
    meant that since long before zoom lenses became commonplace, and therefore
    no need to use another term to mean "non-zoom."

    "Prime" is properly used in the sense of primary, main, chief or
    original--all dictionary definitions for "prime."

    There is NO dictionary definition for "prime" which means fixed focal length
    or single focal length, or fixed or single anything else.

    It would be nice if this nonsensical misusage, which obviously is based on
    someone's misunderstanding of the term some years ago (and then spread like
    cancer through the power of the Internet) could be stamped out. Surely "FFL"
    is at least as easy to type as "prime" anyway, and there never was any
    reason other than shortness to replace "fixed focal length" with the
    incorrect term.

    Nostrobino, Sep 28, 2005
  11. Siddhartha Jain

    SimonLW Guest

    Good information. In virtually any lens, many of the elements are cemented
    together into groups. There is typically much less glass to air boundaries
    than 2x the element count. At the cemented boundaries, the reflection loss
    is much lower than glass to air loss, even with coatings.
    SimonLW, Sep 28, 2005
  12. I am aware of the mis-usage of the term *prime* and so guilty of
    propogating the mis-usage but I feel its time the FFL camp realised
    that there is no turning back.

    - Siddhartha
    Siddhartha Jain, Sep 28, 2005
  13. Yes, some elements can be made of exotic plastics and even other materials,
    like fluorite. Indeed there have been discussions about glass lenses bonded
    to plastic ones and the potential problem of different rates of expansion
    and contraction in extremes of temperature...


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  14. Siddhartha Jain

    Nostrobino Guest

    Well, not necessarily, though of course the more people who misuse the term,
    the harder it will be to correct it.

    Most people do not want to use wrong terminology since it makes them look
    ignorant. In the case of "prime" being used to mean FFL, this has only
    spread because readers who have not seen the term before, and then see it
    used by people they assume are knowledgeable, naturally adopt it themselves.
    Thus newbies are caught up in the misusage and (perhaps partly because they
    feel using jargon will make them look knowledgeable too), contribute to the

    Some will continue to use it anyway, but others will drop it (and some have
    dropped it) when the error is pointed out to them.

    Nostrobino, Sep 28, 2005
  15. True; the explanation was however already getting rather long....

    As you suggest, you should look at the number of groups rather than the
    number of elements.

    In practice, the number of cemented pairs in any lens is usually much
    smaller than half the number of elements. 1 or 2 pairs is common in
    fixed focal length lenses (and quite a few have none). Zooms on average
    have a couple more - but even here, some have none (e.g. the EOS 28-80
    f/3.5-5.6II and 35-80 f/4-5.6 III both have 10 elements in 10 groups).

    David Littlewood, Sep 28, 2005
  16. Siddhartha Jain

    Eric Miller Guest

    Many now accepted meanings of words have been created through misusage.
    Perhaps you would prefer a dead language to English?

    Eric Miller
    Eric Miller, Sep 28, 2005
  17. Siddhartha Jain

    Mark Roberts Guest

    The rule of thumb is that it depends on the make and model.
    Mark Roberts, Sep 28, 2005
  18. We're talking a few percent, not whole f/stops. T/stop is a better
    measure of light transmission than f/stop. I wouldn't be surprised if a
    colour cast was the first effect to be noticed rather than the light loss

    David J Taylor, Sep 28, 2005
  19. Siddhartha Jain

    Jeremy Guest

    In theory, zooms will always be somewhat below the quality of prime lenses.
    Zooms typically have barrel distortion at one end of the zoom range, and
    pincushion distortion at the other. Older zooms, especially those that did
    not have decent multicoating, were more prone to flare and ghosting, because
    of the light bouncing back and forth off the air-to-glass surfaces.

    The margin of superiority of primes over zooms has narrowed, and many
    photographers find the convenience and economy of one zoom versus several
    primes to be more important than some slight degree of image degradation. I
    have a couple of Pentax zooms in K-mount that do a credible job, and it
    certainly is easier to carry two zooms than it is to carry 5 or 6 primes.
    Thirty years ago, I bought a couple of third-party zooms for my M43 bodies,
    and the results were just awful, relative to my SMC Takumar prime lens.
    Colors had a grayish cast, saturation was less than on the OEM lens, the
    aperture ring was operated in the reverse direction of my Takumar's (Pentax
    does it "backwards"), the front element turned when the focusing ring was
    moved, making polarizer use difficult, the lens front element was not the
    standard Takumar 49mm or 58mm, making it necessary to buy filters just for
    use in that lens, and the resolution was noticeably less than that of the
    OEM Takumar.

    The build quality was obviously less-good than the OEM lens. The focusing
    was not nearly as smooth, the zoom ring was a bit on the tight side, the
    lens barrel was not as sturdy and the lens lacked multicoating (this was 30
    years ago). So, while I saved a few dollars, I got pretty much what I paid
    for and no more. I ended up putting that zoom lens up on the shelf, where
    it remains to this day, and I bought only OEM lenses after that. They cost
    a bit more, but the level of satisfaction that I derived from them made up
    for the higher price. And not a single one of the OEM lenses has failed, in
    3 decades.

    British landscape photographer and author Brian Bower noted that, while his
    Leica R zoom lenses cost a lot more than non-OEM lenses, he felt that they
    were a good value because they retained their accuracy after over a decade
    of hard use. He noted in one of his books that the cheaper zoom lenses
    might see the elements go out of precise adjustment and the zoom mechanism
    might become very loose after a time, making it necessary to keep checking
    the zoom ring to be certain that the zoom ratio has not changed from
    whatever it was originally set to. Bower valued consistently good results
    more than lower price. He made his living with those tools, and he had
    little tolerance for lens failures.

    My own take on it is that if the proposed use of the lens is of a very
    casual nature, it is probably okay to go for the savings. But if top
    performance and reliability are paramount, one really has to think about
    whether the savings might be offset by potential future loss from
    poorly-performing equipment. I would rather have only a couple of really
    good lenses than a kit full of lenses of questionable reliability and
    performance. It seems that, in my own case, virtually every time I have
    tried to save money by cutting corners I ended up paying double.
    Jeremy, Sep 28, 2005
  20. I've heard it suggested that may high quality zooms are visually
    indistinguishable from their prime equivalents - however both can usually
    out-perform most photographers!
    Cockpit Colin, Sep 28, 2005
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