Why "constant-aperture" zooms?

Discussion in 'Digital Photography' started by Bryan Olson, Nov 7, 2006.

  1. Bryan Olson

    Bryan Olson Guest

    The top-quality, high-priced, "professional" zoom lenses from
    the major manufacturers offer constant maximum f-number over
    their focal length range. The property is called "constant
    speed", but also often incorrectly called "constant aperture"
    or "fixed aperture". So-called "variable aperture" zooms come
    closer to offering constant aperture over their focal length
    range.

    My question is: given the f-number they offer at their longest
    focal length, why are best zooms so slow at the short end of
    their focal-length range? Fast is an advantage. That advantage
    necessarily imposes a cost: reaching a desired speed at a
    given focal length forces lens element size to be at least a
    certain proportion of the focal length. I'm not suggesting
    lenses should get slower at their longest focus; I'm asking
    what stops professional zooms from getting faster at their
    shorter focal lengths.

    Is there some optical principle of image quality that limits
    the speed at the lower end to the speed at the longer end? If
    so, can anyone cite the tech stuff? If not, doesn't that mean
    that professional zoom lenses attain constant f-number by
    artificially and detrimentally reducing their speed at shorter
    focal lengths?


    --
    --Bryan
    Bryan Olson, Nov 7, 2006
    #1
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  2. Bryan Olson wrote:
    > The top-quality, high-priced, "professional" zoom lenses from
    > the major manufacturers offer constant maximum f-number over
    > their focal length range. The property is called "constant
    > speed", but also often incorrectly called "constant aperture"
    > or "fixed aperture". So-called "variable aperture" zooms come
    > closer to offering constant aperture over their focal length
    > range.
    >
    > My question is: given the f-number they offer at their longest
    > focal length, why are best zooms so slow at the short end of
    > their focal-length range?


    If you do videorecording and vary the focal length, the illumination of
    the sensor will be kept constant . This provides the best-quality
    illuminated picture.
    =?iso-8859-1?B?bWlubmVz+HR0aQ==?=, Nov 7, 2006
    #2
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  3. Bryan Olson

    Mark² Guest

    minnesøtti wrote:
    > Bryan Olson wrote:
    >> The top-quality, high-priced, "professional" zoom lenses from
    >> the major manufacturers offer constant maximum f-number over
    >> their focal length range. The property is called "constant
    >> speed", but also often incorrectly called "constant aperture"
    >> or "fixed aperture". So-called "variable aperture" zooms come
    >> closer to offering constant aperture over their focal length
    >> range.
    >>
    >> My question is: given the f-number they offer at their longest
    >> focal length, why are best zooms so slow at the short end of
    >> their focal-length range?

    >
    > If you do videorecording and vary the focal length, the illumination
    > of the sensor will be kept constant . This provides the best-quality
    > illuminated picture.


    Who's talking about video??

    --
    Images (Plus Snaps & Grabs) by Mark² at:
    www.pbase.com/markuson
    Mark², Nov 7, 2006
    #3
  4. Bryan Olson

    Paul Rubin Guest

    Bryan Olson <> writes:
    > Is there some optical principle of image quality that limits
    > the speed at the lower end to the speed at the longer end? If
    > so, can anyone cite the tech stuff? If not, doesn't that mean
    > that professional zoom lenses attain constant f-number by
    > artificially and detrimentally reducing their speed at shorter
    > focal lengths?


    Well, I think making lenses with extremely low f-numbers is difficult
    in general. For example, if an 80-200/2.8 lens instead stayed at
    constant aperture (rather than constant f-number), it would be f/2.8
    at the 200mm end and f/1.12 at the 80mm end. An 80/1.4 prime is
    already a complicated lens, so an 80-200 f/1.12-2.8 zoom would be
    astonishing.

    There have been some variable f-number zooms that were pretty fast at
    both ends by current "consumer zoom" standards, for example the old
    Vivitar Series 1 28-90/2.8-4.0. There was also an Angenieux 35-70
    f/2.5-3.5.
    Paul Rubin, Nov 7, 2006
    #4
  5. Bryan Olson

    Bryan Olson Guest

    Paul Rubin wrote:
    > Bryan Olson <> writes:
    >> Is there some optical principle of image quality that limits
    >> the speed at the lower end to the speed at the longer end? If
    >> so, can anyone cite the tech stuff? If not, doesn't that mean
    >> that professional zoom lenses attain constant f-number by
    >> artificially and detrimentally reducing their speed at shorter
    >> focal lengths?

    >
    > Well, I think making lenses with extremely low f-numbers is difficult
    > in general. For example, if an 80-200/2.8 lens instead stayed at
    > constant aperture (rather than constant f-number), it would be f/2.8
    > at the 200mm end and f/1.12 at the 80mm end. An 80/1.4 prime is
    > already a complicated lens, so an 80-200 f/1.12-2.8 zoom would be
    > astonishing.


    That's a reasonable point. Still, it doesn't really resolve the issue.
    It suggests that constant aperture might be unreachable in many popular
    lenses, but does not indicate that the lenses are somehow limited to
    constant f-number. If the lens offer f/2.8 at 200mm, can it really
    be no faster than f/2.8 at 80mm?

    And what about the lenses with a similar focal-length range that
    offer constant f/4? What limits their speed to f/4 at the short end?


    --
    --Bryan
    Bryan Olson, Nov 7, 2006
    #5
  6. Bryan Olson

    Paul Rubin Guest

    Bryan Olson <> writes:
    > And what about the lenses with a similar focal-length range that
    > offer constant f/4? What limits their speed to f/4 at the short end?


    I think there may have been some 80-200/2.8-4 lenses but my memory is
    hazy.

    It also seems to me that maybe users didn't care very much. In the
    heyday of those zooms, the users also had primes. The Nikon
    75-150/3.5E was a favorite among fashion photographers. They liked to
    move around and shoot quickly, zooming without wanting to have to
    readjust exposure (especially with studio strbes), so they wanted
    constant f-number, and if they wanted speed at the short end they
    tended to have 85mm or 105mm primes.
    Paul Rubin, Nov 7, 2006
    #6
  7. Bryan Olson wrote:
    > The top-quality, high-priced, "professional" zoom lenses from
    > the major manufacturers offer constant maximum f-number over
    > their focal length range. The property is called "constant
    > speed", but also often incorrectly called "constant aperture"
    > or "fixed aperture". So-called "variable aperture" zooms come
    > closer to offering constant aperture over their focal length
    > range.

    ....

    As I recall, and my memory is fuzzy, there are some zoom designs that by
    their nature have or require a constant maximum f number.


    --
    Joseph Meehan

    Dia 's Muire duit
    Joseph Meehan, Nov 7, 2006
    #7
  8. In article <hNW3h.6135$>, Bryan
    Olson <> wrote:

    > My question is: given the f-number they offer at their longest
    > focal length, why are best zooms so slow at the short end of
    > their focal-length range?


    Glass half full:

    Wow! they took my f2.8-4.5 lens and made it f2.8-2.8!

    Glass half empty:

    Then why didn't they make it f1.8-2.8?

    I've always looked at the half full side and been happy. Now you're
    gonna make me think of the half empty side and be bitter. heh.

    The answer may lie in that it is easier to move a many-element complex
    zoom lens from 4.5 to 2.8 with increases in diameter and other internal
    mods than it is to move a similarly complex lens from 2.8 to 1.8. At
    wide angle, it sees more light, but it still has all that internal
    complexity.
    Steve Cutchen, Nov 7, 2006
    #8
  9. Bryan Olson

    tomm42 Guest

    On Nov 7, 2:42 am, Bryan Olson <> wrote:
    > The top-quality, high-priced, "professional" zoom lenses from
    > the major manufacturers offer constant maximum f-number over
    > their focal length range. The property is called "constant
    > speed", but also often incorrectly called "constant aperture"
    > or "fixed aperture". So-called "variable aperture" zooms come
    > closer to offering constant aperture over their focal length
    > range.
    >
    > My question is: given the f-number they offer at their longest
    > focal length, why are best zooms so slow at the short end of
    > their focal-length range? Fast is an advantage. That advantage
    > necessarily imposes a cost: reaching a desired speed at a
    > given focal length forces lens element size to be at least a
    > certain proportion of the focal length. I'm not suggesting
    > lenses should get slower at their longest focus; I'm asking
    > what stops professional zooms from getting faster at their
    > shorter focal lengths.
    >
    > Is there some optical principle of image quality that limits
    > the speed at the lower end to the speed at the longer end? If
    > so, can anyone cite the tech stuff? If not, doesn't that mean
    > that professional zoom lenses attain constant f-number by
    > artificially and detrimentally reducing their speed at shorter
    > focal lengths?
    >
    > --
    > --Bryan


    It is a photography thing, if you are shooting manual, you want to know
    4 clicks is f5.6 on a f2.8 lens. Good photographers can get a light
    reading and work from there is the light doesn't change substantially.
    This is called knowing your equipment. Now with a variable focal length
    the focal lenngth muddies the exposure. So if you are using automatic
    the variable fstop doesn't mean squat, if you are working manually it
    really helps. Haven't decided yet since fstops are controled by the
    front wheel on my camera, makes my life a little difficult and I'm not
    as facile as I am with my Leicas (just about 40 years of practice).
    tomm42, Nov 7, 2006
    #9
  10. "tomm42" <> wrote in message news:...
    > It is a photography thing, if you are shooting manual, you want to know
    > 4 clicks is f5.6 on a f2.8 lens. Good photographers can get a light
    > reading and work from there is the light doesn't change substantially.
    > This is called knowing your equipment. Now with a variable focal length
    > the focal lenngth muddies the exposure. So if you are using automatic
    > the variable fstop doesn't mean squat, if you are working manually it
    > really helps. Haven't decided yet since fstops are controled by the
    > front wheel on my camera, makes my life a little difficult and I'm not
    > as facile as I am with my Leicas (just about 40 years of practice).


    Many cameras permit using variable f-stop lenses as if they were
    constant f-stop (it is a custom setting on my Nikons) - so, for
    instance, an f3.5-4.5 zoom can be used (with constant resultant
    shutter speed with zooming) as if it were an f4.5 lens.
    --
    David Ruether


    http://www.ferrario.com/ruether
    David Ruether, Nov 7, 2006
    #10
  11. Bryan Olson

    Ben Brugman Guest


    >
    > Many cameras permit using variable f-stop lenses as if they were
    > constant f-stop (it is a custom setting on my Nikons) - so, for
    > instance, an f3.5-4.5 zoom can be used (with constant resultant
    > shutter speed with zooming) as if it were an f4.5 lens.
    > --

    Yes Nikon does that, but I do not consider that a feature in all
    cercumstances. I often like to use an aperature as large as
    possible but one stop down. You lose one stop, but gain a lot
    of quality, this is not possible anymore with Nikon.

    Offcourse there is an upside as well, taking flash pictures with
    non TTL flashes it is a huge advantage that the aperature stays
    constant over the complete zoomrange. (As Nikon does with
    all aperatures except the 'largest' and probably the smallest).

    Ben



    > David Ruether
    >
    >
    > http://www.ferrario.com/ruether
    >
    >
    Ben Brugman, Nov 7, 2006
    #11
  12. Bryan Olson

    Ben Brugman Guest

    "Bryan Olson" <> schreef in bericht
    news:hNW3h.6135$...
    >
    > The top-quality, high-priced, "professional" zoom lenses from
    > the major manufacturers offer constant maximum f-number over
    > their focal length range. The property is called "constant
    > speed", but also often incorrectly called "constant aperture"
    > or "fixed aperture". So-called "variable aperture" zooms come
    > closer to offering constant aperture over their focal length
    > range.
    >
    > My question is: given the f-number they offer at their longest
    > focal length, why are best zooms so slow at the short end of
    > their focal-length range? Fast is an advantage. That advantage
    > necessarily imposes a cost: reaching a desired speed at a
    > given focal length forces lens element size to be at least a
    > certain proportion of the focal length. I'm not suggesting
    > lenses should get slower at their longest focus; I'm asking
    > what stops professional zooms from getting faster at their
    > shorter focal lengths.
    >
    > Is there some optical principle of image quality that limits
    > the speed at the lower end to the speed at the longer end? If
    > so, can anyone cite the tech stuff? If not, doesn't that mean
    > that professional zoom lenses attain constant f-number by
    > artificially and detrimentally reducing their speed at shorter
    > focal lengths?
    >


    It could be that the lens design is such that the aperature
    (the fysical blades) are in the back of the lens and that
    all zooming/moving and focusing goes on in the front of the
    lens, this would give you a fairly constant aperature.
    This is conjecture of my side, but a setup like that could
    have advantages. (And very probably disadvantages as well).

    Ben






    >
    > --
    > --Bryan
    Ben Brugman, Nov 7, 2006
    #12
  13. "Ben Brugman" <> wrote in message news:...
    David Ruether wrote...

    >> Many cameras permit using variable f-stop lenses as if they were
    >> constant f-stop (it is a custom setting on my Nikons) - so, for
    >> instance, an f3.5-4.5 zoom can be used (with constant resultant
    >> shutter speed with zooming) as if it were an f4.5 lens.


    > Yes Nikon does that, but I do not consider that a feature in all
    > cercumstances. I often like to use an aperature as large as
    > possible but one stop down. You lose one stop, but gain a lot
    > of quality, this is not possible anymore with Nikon.


    Then you set the Nikon custom control the other way, and using
    aperture priority and setting f5 on an f3.5-X lens at the short
    end, the f-stop will slip smaller and the shutter speed slower with
    zooming longer (assuming you have set the custom controls
    for 1/3rd stop shift intervals). I prefer to select constant f-stop,
    though, since most zooms perform better at their wider stops
    toward the long end, with more stopping down required at the
    shorter end for good performance. Also, it is useful to not have
    the shutter speed slowing and becoming more difficult to hand
    hold as the lens is zoomed longer (which is harder to hand hold
    in any case).

    > Offcourse there is an upside as well, taking flash pictures with
    > non TTL flashes it is a huge advantage that the aperature stays
    > constant over the complete zoomrange. (As Nikon does with
    > all aperatures except the 'largest' and probably the smallest).
    >
    > Ben


    Yes - it can make us realize again how useful "Auto" flash mode
    can be now that some of our fancy TTL flashes no longer work
    in TTL mode with our fancy new digital SLR bodies...;-(
    --
    David Ruether


    http://www.ferrario.com/ruether
    David Ruether, Nov 8, 2006
    #13
  14. "David Ruether" <> wrote in message
    news:Ecn4h.9698$...

    > Then you set the Nikon custom control the other way, and using
    > aperture priority and setting f5 on an f3.5-X lens at the short
    > end, the f-stop will slip smaller and the shutter speed slower with
    > zooming longer (assuming you have set the custom controls
    > for 1/3rd stop shift intervals). I prefer to select constant f-stop,
    > though, since most zooms perform better at their wider stops
    > toward the long end...


    I used to get a bit annoyed when I set aperture at f3.5 on my 18-70 kit lens
    and it would not return to that setting when I zoomed out then back in
    again, so I was never really sure which f stop I was using without having to
    contantly check the reading in the viewfinder! I'd much rather have a
    constant f4 than a 3.5-4.5 lens, but I expect this would push the price up a
    fair bit.
    Adrian Boliston, Nov 8, 2006
    #14
  15. On Wed, 8 Nov 2006 19:32:05 -0000, in rec.photo.digital "Adrian Boliston"
    <> wrote:

    >I used to get a bit annoyed when I set aperture at f3.5 on my 18-70 kit lens
    >and it would not return to that setting when I zoomed out then back in
    >again, so I was never really sure which f stop I was using without having to
    >contantly check the reading in the viewfinder! I'd much rather have a
    >constant f4 than a 3.5-4.5 lens, but I expect this would push the price up a
    >fair bit.


    FWIW, I have no such issues on my either my D70 or D200 with two different
    kit lenses.

    --
    Ed Ruf ()
    http://edwardgruf.com/Digital_Photography/General/index.html
    Ed Ruf (REPLY to E-MAIL IN SIG!), Nov 8, 2006
    #15
  16. Bryan Olson

    jpc Guest

    On Tue, 07 Nov 2006 07:42:05 GMT, Bryan Olson
    <> wrote:

    >
    >The top-quality, high-priced, "professional" zoom lenses from
    >the major manufacturers offer constant maximum f-number over
    >their focal length range. The property is called "constant
    >speed", but also often incorrectly called "constant aperture"
    >or "fixed aperture". So-called "variable aperture" zooms come
    >closer to offering constant aperture over their focal length
    >range.
    >
    >My question is: given the f-number they offer at their longest
    >focal length, why are best zooms so slow at the short end of
    >their focal-length range? Fast is an advantage. That advantage
    >necessarily imposes a cost: reaching a desired speed at a
    >given focal length forces lens element size to be at least a
    >certain proportion of the focal length. I'm not suggesting
    >lenses should get slower at their longest focus; I'm asking
    >what stops professional zooms from getting faster at their
    >shorter focal lengths.
    >
    >Is there some optical principle of image quality that limits
    >the speed at the lower end to the speed at the longer end? If
    >so, can anyone cite the tech stuff? If not, doesn't that mean
    >that professional zoom lenses attain constant f-number by
    >artificially and detrimentally reducing their speed at shorter
    >focal lengths?



    There is nothing artifical about the limiting aperture or field stop
    in zoom lens. It's there to control the aberations. IF there were
    another cost effective way to get around the aberation problems I
    assure you that lens mamufactures would be using it.

    The other problems are cost, size, weight and the need to have a low
    aberation image big enough to cover a large area sensor .For example I
    own a 35-350 zoom lens for a studio size orthicon TV camera. The
    front lens element is 7 inches in diameter and the complete lens
    weighs 19 pounds.

    jpc
    jpc, Nov 9, 2006
    #16
  17. Bryan Olson

    ben brugman Guest

    "Adrian Boliston" <> wrote in message
    news:...
    > "David Ruether" <> wrote in message
    > news:Ecn4h.9698$...
    >
    >> Then you set the Nikon custom control the other way, and using
    >> aperture priority and setting f5 on an f3.5-X lens at the short
    >> end, the f-stop will slip smaller and the shutter speed slower with
    >> zooming longer (assuming you have set the custom controls
    >> for 1/3rd stop shift intervals). I prefer to select constant f-stop,
    >> though, since most zooms perform better at their wider stops
    >> toward the long end...

    >
    > I used to get a bit annoyed when I set aperture at f3.5 on my 18-70 kit
    > lens and it would not return to that setting when I zoomed out then back
    > in again, so I was never really sure which f stop I was using without
    > having to contantly check the reading in the viewfinder! I'd much rather
    > have a constant f4 than a 3.5-4.5 lens, but I expect this would push the
    > price up a fair bit.

    To prevent this from hapening, zoom in to the 70 mm setting. (Or any other
    zoom to the highest setting). Change your aperature to the setting you
    prefere. For the 3.5-4.5 lens the largest aperature you can set will be 4.5,
    so set it to this value and when zooming in or out it will remain on this
    setting.

    (On the other end of the scale aperature on 22 or 28, this does work
    differently, only in extreme conditions that will matter).

    ben brugman





    >
    ben brugman, Nov 9, 2006
    #17
  18. Bryan Olson

    ben brugman Guest

    "Ed Ruf (REPLY to E-MAIL IN SIG!)" <> wrote in message
    news:...
    > On Wed, 8 Nov 2006 19:32:05 -0000, in rec.photo.digital "Adrian Boliston"
    > <> wrote:
    >
    >>I used to get a bit annoyed when I set aperture at f3.5 on my 18-70 kit
    >>lens
    >>and it would not return to that setting when I zoomed out then back in
    >>again, so I was never really sure which f stop I was using without having
    >>to
    >>contantly check the reading in the viewfinder! I'd much rather have a
    >>constant f4 than a 3.5-4.5 lens, but I expect this would push the price up
    >>a
    >>fair bit.

    >
    > FWIW, I have no such issues on my either my D70 or D200 with two different
    > kit lenses.


    If you do not have these issues, how do you keep the D70 constant on 1 stop
    from the largest aperature. (This was my issue).

    The issue described above can be solved to set the Aperature when completely
    zoomed in, then it does not change when zooming. (Not for the smallest
    aperature).

    Ben


    >
    > --
    > Ed Ruf ()
    > http://edwardgruf.com/Digital_Photography/General/index.html
    ben brugman, Nov 9, 2006
    #18
  19. Bryan Olson

    Ed Ruf Guest

    On Thu, 9 Nov 2006 14:39:22 +0100, in rec.photo.digital "ben brugman"
    <> wrote:

    >
    >"Ed Ruf (REPLY to E-MAIL IN SIG!)" <> wrote in message
    >news:...
    >> On Wed, 8 Nov 2006 19:32:05 -0000, in rec.photo.digital "Adrian Boliston"
    >> <> wrote:
    >>
    >>>I used to get a bit annoyed when I set aperture at f3.5 on my 18-70 kit
    >>>lens
    >>>and it would not return to that setting when I zoomed out then back in
    >>>again, so I was never really sure which f stop I was using without having
    >>>to
    >>>contantly check the reading in the viewfinder! I'd much rather have a
    >>>constant f4 than a 3.5-4.5 lens, but I expect this would push the price up
    >>>a
    >>>fair bit.

    >>
    >> FWIW, I have no such issues on my either my D70 or D200 with two different
    >> kit lenses.

    >
    >If you do not have these issues, how do you keep the D70 constant on 1 stop
    >from the largest aperature. (This was my issue).


    That's not what you originally said. You said "I used to get a bit
    annoyed when I set aperture at f3.5 on my 18-70 kit lens and it would
    not return to that setting when I zoomed out then back in again."

    If you set the lens to 18mm, set the camera to f/3.5, then zoom to
    70mm the camera changes to f/4.5. Then when you zoom back to 18mm the
    camera changes back to f/3.5. Actually with the D200 you can set any
    aperture wider than f/4.5 while at 18mm and it will return to that
    again at 18mm.

    -
    Ed Ruf ()
    http://edwardgruf.com/Digital_Photography/General/index.html
    Ed Ruf, Nov 9, 2006
    #19
  20. Bryan Olson

    ben brugman Guest


    >
    > Then you set the Nikon custom control the other way, and using
    > aperture priority and setting f5 on an f3.5-X lens at the short
    > end, the f-stop will slip smaller and the shutter speed slower with
    > zooming longer (assuming you have set the custom controls


    I am not aware of a Nikon custom control on the D70 to do this.


    >> Ben

    >
    > Yes - it can make us realize again how useful "Auto" flash mode
    > can be now that some of our fancy TTL flashes no longer work
    > in TTL mode with our fancy new digital SLR bodies...;-(


    Yes exactly my sentiments.

    Ben

    > --
    > David Ruether
    >
    >
    > http://www.ferrario.com/ruether
    >
    >
    ben brugman, Nov 9, 2006
    #20
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