Aperture fixed when 35mm lenses used on small CCD's??

Discussion in 'Digital Photography' started by Dave, Jan 4, 2005.

  1. Dave

    Dave Guest

    I've often heard it said that a 100mm f/2.8 lens (for example) designed
    for a 35mm camera if put on a smaller CCD sensor will become a 160mm
    f/2.8 i.e. the 'effective' focal length gets multiplied by some factor
    (1.6) in my example and the aperture remains constant. I'm not so
    convinced the latter is true.

    I make a few observations.

    1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm
    f/2.8 lens. The lens remains the same.

    2) The smaller CCD sensor means the focal length (as compared to 35mm)
    is longer, as everyone agrees.

    3) The aperture whilst still f/2.8 is "effectively" larger, as much of
    the light is thrown away, missing the sides of the sensor. So the
    viewfinder will be darker than if fitted with a f/2.8 lens which filled
    the sensor and no more.

    At first the digital format would seem to allow long focal, fast
    telephotos. i.e. my 70-200 f/2.8 would become a 112-320 f/2.8, which
    would be a very nice fast lens indeed. But I'm not so sure the lens
    would have the light gathering power of a real f/2.8 lens, but instead
    be effectively an f3.5 (I think). I suspect if the focal length is
    multipled by 1.6, the apeture will be multipled by sqrt(1.6), although I
    might be wrong on the exact calculation.


    PS,
    Does anyone know if Nikon are developing a full frame (35mm) digital SLR
    like Canon and Kodak?? It seems such a move would have a lot of
    technical advantages (lower noise) and people with expensive 35mm lenses
    would get the full benefit, and not throw much of the light away, which
    is what I think would happen now.
     
    Dave, Jan 4, 2005
    #1
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  2. Dave <> wrote in news:41d9de83@212.67.96.135:

    > 1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm
    > f/2.8 lens. The lens remains the same.


    Yes

    > 2) The smaller CCD sensor means the focal length (as compared to 35mm)
    > is longer, as everyone agrees.


    That contradicts 1.

    The smaller CCD equals cropping. Sometimes this is referred
    to as a longer equivalent focal length. But it is still the same
    focal length.

    > 3) The aperture whilst still f/2.8 is "effectively" larger, as much of
    > the light is thrown away, missing the sides of the sensor. So the
    > viewfinder will be darker than if fitted with a f/2.8 lens which filled
    > the sensor and no more.


    The aperture is still f/2.8.

    But you are right with the finder being darker. Moreover,
    if you want the same amount of pixels, the actual sensors
    must be smaller, thus collecting less light, thus you cannot
    have as high ISO - making the system less sensitive.


    /Roland
     
    Roland Karlsson, Jan 4, 2005
    #2
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  3. On Tue, 04 Jan 2005 00:08:34 +0000, Dave <> wrote:

    >I've often heard it said that a 100mm f/2.8 lens (for example) designed
    >for a 35mm camera if put on a smaller CCD sensor will become a 160mm
    >f/2.8 i.e. the 'effective' focal length gets multiplied by some factor
    >(1.6) in my example and the aperture remains constant. I'm not so
    >convinced the latter is true.
    >
    >I make a few observations.
    >
    >1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm
    >f/2.8 lens. The lens remains the same.
    >
    >2) The smaller CCD sensor means the focal length (as compared to 35mm)
    >is longer, as everyone agrees.
    >
    >3) The aperture whilst still f/2.8 is "effectively" larger, as much of
    >the light is thrown away, missing the sides of the sensor. So the
    >viewfinder will be darker than if fitted with a f/2.8 lens which filled
    >the sensor and no more.


    http://www.lonestardigital.com/multipler.htm i

    "Just as "zooming in" is a result of increased real focal length,
    cropped area created by the multiplier factor increases the perceived
    focal length of the lens."

    Note "preceived" , the focal length is not changed but the image is
    cropped down.

    As for the smaller size of the sensor that can be a good thing in
    that it would be only using that part of a 35mm with the "sweet spot"
    for it's optics if a lens has problems at the edges.




    ********************************************************

    "The fox knows many things, but
    the hedgehog knows one big thing."

    Archilochus
    675 - 635 B.C.
     
    John A. Stovall, Jan 4, 2005
    #3
  4. Dave

    Nick Beard Guest

    lets put it this way. My Nikon 70-200 on my D70 remains at 2.8 all the way
    through. My exposures show this to be true. If your theory is true then the
    shutter has to lie or the daylight must intensify as I throw the switch!
    "Dave" <> wrote in message news:41d9de83@212.67.96.135...
    > I've often heard it said that a 100mm f/2.8 lens (for example) designed
    > for a 35mm camera if put on a smaller CCD sensor will become a 160mm f/2.8
    > i.e. the 'effective' focal length gets multiplied by some factor (1.6) in
    > my example and the aperture remains constant. I'm not so convinced the
    > latter is true.
    >
    > I make a few observations.
    >
    > 1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm f/2.8
    > lens. The lens remains the same.
    >
    > 2) The smaller CCD sensor means the focal length (as compared to 35mm) is
    > longer, as everyone agrees.
    >
    > 3) The aperture whilst still f/2.8 is "effectively" larger, as much of the
    > light is thrown away, missing the sides of the sensor. So the viewfinder
    > will be darker than if fitted with a f/2.8 lens which filled the sensor
    > and no more.
    >
    > At first the digital format would seem to allow long focal, fast
    > telephotos. i.e. my 70-200 f/2.8 would become a 112-320 f/2.8, which would
    > be a very nice fast lens indeed. But I'm not so sure the lens would have
    > the light gathering power of a real f/2.8 lens, but instead be effectively
    > an f3.5 (I think). I suspect if the focal length is multipled by 1.6, the
    > apeture will be multipled by sqrt(1.6), although I might be wrong on the
    > exact calculation.
    >
    >
    > PS,
    > Does anyone know if Nikon are developing a full frame (35mm) digital SLR
    > like Canon and Kodak?? It seems such a move would have a lot of technical
    > advantages (lower noise) and people with expensive 35mm lenses would get
    > the full benefit, and not throw much of the light away, which is what I
    > think would happen now.
    >
    >
    >
    >
     
    Nick Beard, Jan 4, 2005
    #4
  5. Dave

    Guest

    Dave <> writes:

    > I've often heard it said that a 100mm f/2.8 lens (for example)
    > designed for a 35mm camera if put on a smaller CCD sensor will become
    > a 160mm f/2.8 i.e. the 'effective' focal length gets multiplied by
    > some factor (1.6) in my example and the aperture remains constant. I'm
    > not so convinced the latter is true.
    >
    > I make a few observations.
    >
    > 1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm
    > f/2.8 lens. The lens remains the same.
    >
    > 2) The smaller CCD sensor means the focal length (as compared to 35mm)
    > is longer, as everyone agrees.
    >
    > 3) The aperture whilst still f/2.8 is "effectively" larger, as much of
    > the light is thrown away, missing the sides of the sensor. So the
    > viewfinder will be darker than if fitted with a f/2.8 lens which
    > filled the sensor and no more.


    So if you crop the center out of a 35mm shot, does it get darker?

    Really, the F number correlates to what irradiance will hit the image
    plane for a given light field entering the lens. Irradiance is the
    power density in watts per square meter; varying the number of square
    meters you catch doesn't change this at all.

    <snip>

    --
    -Stephen H. Westin
    Any information or opinions in this message are mine: they do not
    represent the position of Cornell University or any of its sponsors.
     
    , Jan 4, 2005
    #5
  6. Dave

    Crownfield Guest

    wrote:
    >
    > Dave <> writes:
    >
    > > I've often heard it said that a 100mm f/2.8 lens (for example)
    > > designed for a 35mm camera if put on a smaller CCD sensor will become
    > > a 160mm f/2.8 i.e. the 'effective' focal length gets multiplied by
    > > some factor (1.6) in my example and the aperture remains constant. I'm
    > > not so convinced the latter is true.
    > >
    > > I make a few observations.
    > >
    > > 1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm
    > > f/2.8 lens. The lens remains the same.
    > >
    > > 2) The smaller CCD sensor means the focal length (as compared to 35mm)
    > > is longer, as everyone agrees.
    > >
    > > 3) The aperture whilst still f/2.8 is "effectively" larger, as much of
    > > the light is thrown away, missing the sides of the sensor. So the
    > > viewfinder will be darker than if fitted with a f/2.8 lens which
    > > filled the sensor and no more.


    without any factual basis,
    flys in the face of physics and optics...

    >
    > So if you crop the center out of a 35mm shot, does it get darker?
    >
    > Really, the F number correlates to what irradiance will hit the image
    > plane for a given light field entering the lens. Irradiance is the
    > power density in watts per square meter; varying the number of square
    > meters you catch doesn't change this at all.
    >
    > <snip>
    >
    > --
    > -Stephen H. Westin
    > Any information or opinions in this message are mine: they do not
    > represent the position of Cornell University or any of its sponsors.
     
    Crownfield, Jan 4, 2005
    #6
  7. Dave <> writes:
    >I've often heard it said that a 100mm f/2.8 lens (for example) designed
    >for a 35mm camera if put on a smaller CCD sensor will become a 160mm
    >f/2.8 i.e. the 'effective' focal length gets multiplied by some factor
    >(1.6) in my example and the aperture remains constant. I'm not so
    >convinced the latter is true.


    What actually happens is that the *field of view* is smaller, equivalent
    to the FOV you'd get with a 160 mm lens on a full-frame camera.

    >1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm
    >f/2.8 lens. The lens remains the same.


    Certainly

    >2) The smaller CCD sensor means the focal length (as compared to 35mm)
    >is longer, as everyone agrees.


    No. The smaller sensor means that the field of view with the same lens
    is narrower. This is sometimes *described* as having the same effect as
    using a longer focal length on the full-frame camera.

    >3) The aperture whilst still f/2.8 is "effectively" larger, as much of
    >the light is thrown away, missing the sides of the sensor. So the
    >viewfinder will be darker than if fitted with a f/2.8 lens which filled
    >the sensor and no more.


    This is nonsense. You should try tracing some ray diagrams through a
    lens. A f/2.8 lens collects a certain amount of light from *a single
    point* in the subject and focuses all that light at one point in the
    image, if the subject is in focus. At the same time, and entirely in
    parallel, the lens is also collecting light from all of the other points
    in the subject, and transferring it to corresponding points in the
    image.

    Now, when you reduce the sensor size, some of the points that used to be
    in the image no longer are. And in a sense the light that used to go to
    those points is "wasted". But for all of the points that remain in the
    smaller image, they are just as bright as they always were. The
    f/number of the lens does not describe total light throughput, because
    that depends on angle as well as f/number. The f/number determines
    brightness per point, or per unit area in the image, and that is
    unchanged when some portion of the sensor area is removed.

    >At first the digital format would seem to allow long focal, fast
    >telephotos. i.e. my 70-200 f/2.8 would become a 112-320 f/2.8, which
    >would be a very nice fast lens indeed. But I'm not so sure the lens
    >would have the light gathering power of a real f/2.8 lens, but instead
    >be effectively an f3.5 (I think). I suspect if the focal length is
    >multipled by 1.6, the apeture will be multipled by sqrt(1.6), although I
    >might be wrong on the exact calculation.


    No, you're wrong. It remains a f/2.8 lens. However, there's no free
    lunch. To produce the same quality of image from the smaller sensor, a
    given size print needs to be enlarged a factor of 1.6X more. That means
    that the lens needs to deliver 1.6 times the resolution to the sensor to
    get the same quality print. Thus, lenses that are marginal in sharpness
    for full-frame use may look just plain unsharp with the smaller sensor,
    while very sharp lenses can stand the extra magnification without
    strain.

    Dave
     
    Dave Martindale, Jan 4, 2005
    #7
  8. Dave

    Dps Guest

    Hi Dave,

    AFAIK "magnification factor" is a marketing term rather than an optical
    term. As has been already stated, it is a cropping factor, which results in
    a reduction of the FOV. If I cut off a tiny square of a 35mm film, I get a
    "magnification", in terms of modern digicam marketing ;-) As far as the
    f-stop is concerned, this will remain the same. Focal length and focal ratio
    are characteristics of the lens - not the focal plane or whatever lies on
    that. BTW the f-number is not defined through terms of power denity or
    whatever interpretation anyone could come up with. In classical optics, the
    f-number=focal length/diaphragm aperture, nothing more, nothing less. Try
    http://en.wikipedia.org/wiki/F-stop and http://www.celestron.com/tb-trms.htm
    for some terms.

    I hope this helps :)

    dimitris

    P.S. I remember well that f-number used to be the inverse of F-number, i.e.
    f=1/F, I couldn't find any information on this on the web...
     
    Dps, Jan 4, 2005
    #8
  9. Dave

    Justin Thyme Guest

    "Dave" <> wrote in message news:41d9de83@212.67.96.135...
    > I've often heard it said that a 100mm f/2.8 lens (for example) designed
    > for a 35mm camera if put on a smaller CCD sensor will become a 160mm f/2.8
    > i.e. the 'effective' focal length gets multiplied by some factor (1.6) in
    > my example and the aperture remains constant. I'm not so convinced the
    > latter is true.
    >
    > I make a few observations.
    >
    > 1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm f/2.8
    > lens. The lens remains the same.

    Yes it is still a 100mm lens - but it's field of view becomes the same as a
    160mm. What this means is that the image you get from a 100mm lens on a 1.6x
    digital will be pretty much the same as a 160mm lens on a 35mm cam.
    >
    > 2) The smaller CCD sensor means the focal length (as compared to 35mm) is
    > longer, as everyone agrees.

    uhh... isn't this the opposite of what you said in 1? - the _equivalent_
    focal length is longer. the lens is still 100mm. The behaviour of a lens in
    terms of field of view is only relevant when considered in conjunction with
    the sensor/film in use. a 50mm lens on a digital SLR is a mild telephoto,
    on a 35mm cam it becomes a standard lens, yet on a medium format camera it
    is a wide angle lens. on a 8x10 large format camera it would be a fisheye or
    extreme wide angle, while on a compact digital it is an extreme telephoto.
    >
    > 3) The aperture whilst still f/2.8 is "effectively" larger, as much of the
    > light is thrown away, missing the sides of the sensor. So the viewfinder
    > will be darker than if fitted with a f/2.8 lens which filled the sensor
    > and no more.

    The light is not thrown away, it is just that you are not using all of the
    projected image circle. To have the equivalent of changing aperture, you
    would actually have to throw away some of the light that would otherwise
    have hit the sensor. Take one of your 35mm negatives or slides and cut it
    down to 24x16mm. Did the negative/slide just become darker? of course it
    didn't, it is still correctly exposed. This is because the light that hit
    what's left of the negative was still F2.8.
    >
    > At first the digital format would seem to allow long focal, fast
    > telephotos. i.e. my 70-200 f/2.8 would become a 112-320 f/2.8, which would
    > be a very nice fast lens indeed.

    Yep.
    >But I'm not so sure the lens would have the light gathering power of a real
    >f/2.8 lens, but instead be effectively an f3.5 (I think). I suspect if the
    >focal length is multipled by 1.6, the apeture will be multipled by
    >sqrt(1.6), although I might be wrong on the exact calculation.

    Aperture is not effected at all. This is one benefit of digital - you can
    have fast telephoto lenses at lower cost. Instead of having to pay for a
    300mm F2, you can get a 200mm F2 for the same amount of zoom and light
    gathering capability. 200mm F2 is a lot lighter and a lot cheaper to produce
    than a 300mm F2. To take this to extremes, the Panasonic Lumix FZ10 has a
    lens that is the equivalent of 420mm F2.8. A 420mm F2.8 lens for film would
    be very heavy and would cost a fortune, however for the very small sensor in
    a compact digicam, it is lightweight and compact.
    >
    >
    > PS,
    > Does anyone know if Nikon are developing a full frame (35mm) digital SLR
    > like Canon and Kodak?? It seems such a move would have a lot of technical
    > advantages (lower noise) and people with expensive 35mm lenses would get
    > the full benefit, and not throw much of the light away, which is what I
    > think would happen now.

    The aps sized sensors are a compromise between production cost, image
    quality and usability. 35mm is an arbitrary size - there is no rule that
    says 35mm is "full frame". If 645 cameras were just as common as 35mm I
    could just as easily ask when Nikon would bring out a camera with a 6cm x
    4.5cm imaging sensor, so that I could use my old 645 lenses without having a
    crop factor. It is a fact of life that every format change requires a
    change in other equipment. by having APS sized sensors, people who use
    their 35mm lenses benefit by having the equivalent of long focal length fast
    lenses, but lose out because their 24mm extreme wide angle lens is now only
    a 35mm mild wide angle. It is only at the extreme wide angles that the
    smaller sensor becomes a big disadvantage, but at the extreme wide angles,
    due to the different characteristics of digital and film, it is best to have
    a lens designed specifically for digital anyway. Film doesn't care if light
    hits it at an angle, digital does.
     
    Justin Thyme, Jan 4, 2005
    #9
  10. Dave

    Chris Brown Guest

    In article <41d9de83@212.67.96.135>, Dave <> wrote:
    >I've often heard it said that a 100mm f/2.8 lens (for example) designed
    >for a 35mm camera if put on a smaller CCD sensor will become a 160mm
    >f/2.8 i.e. the 'effective' focal length gets multiplied by some factor
    >(1.6) in my example and the aperture remains constant. I'm not so
    >convinced the latter is true.
    >
    >I make a few observations.
    >
    >1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm
    >f/2.8 lens. The lens remains the same.
    >
    >2) The smaller CCD sensor means the focal length (as compared to 35mm)
    >is longer, as everyone agrees.


    No, the focal length is the same, but the field of view changes, as the CCD
    is a smaller format. Think of 645 vs 35mm for good film analogy.
     
    Chris Brown, Jan 4, 2005
    #10
  11. Dave

    Lourens Smak Guest

    In article <41d9de83@212.67.96.135>, Dave <> wrote:

    > 1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm
    > f/2.8 lens. The lens remains the same.


    correct

    > 2) The smaller CCD sensor means the focal length (as compared to 35mm)
    > is longer, as everyone agrees.


    I don't agree; the angle of view changes, not the focal length.

    > 3) The aperture whilst still f/2.8 is "effectively" larger, as much of
    > the light is thrown away, missing the sides of the sensor. So the
    > viewfinder will be darker than if fitted with a f/2.8 lens which filled
    > the sensor and no more.


    You are missing the fact that film (or FF digital) NEEDS more light in
    total to get the same exposure, because the same amount of light is
    spread out over a much larger area. You have to look at exposure *per
    square mm*, which remains the same of course, regardless of format.

    > Does anyone know if Nikon are developing a full frame (35mm) digital SLR
    > like Canon and Kodak??


    They have said numerous times they aren't. It has no advantages. BTW the
    bodies with the most noise are the Contax N-digital and the Kodak 14n.
    Now what does that tell you?

    > It seems such a move would have a lot of
    > technical advantages (lower noise) and people with expensive 35mm lenses
    > would get the full benefit.


    The benefit would be they can use their old gear, which is an advantage
    for users but not for Nikon. Their customers aren't the "brand-loyal"
    people who have a dozen old Nikkors and a few bodies, their customers
    are people who actually BUY new stuff.

    Lourens
     
    Lourens Smak, Jan 4, 2005
    #11
  12. Dave

    me Guest

    "Justin Thyme" <> wrote in message
    news:...
    > "Dave" <> wrote in message

    news:41d9de83@212.67.96.135...
    > Aperture is not effected at all. This is one benefit of digital - you can
    > have fast telephoto lenses at lower cost. Instead of having to pay for a
    > 300mm F2, you can get a 200mm F2 for the same amount of zoom and light
    > gathering capability. 200mm F2 is a lot lighter and a lot cheaper to

    produce
    > than a 300mm F2. To take this to extremes, the Panasonic Lumix FZ10 has a
    > lens that is the equivalent of 420mm F2.8. A 420mm F2.8 lens for film

    would
    > be very heavy and would cost a fortune, however for the very small sensor

    in
    > a compact digicam, it is lightweight and compact.
    > >
    > > PS,
    > > Does anyone know if Nikon are developing a full frame (35mm) digital SLR
    > > like Canon and Kodak?? It seems such a move would have a lot of

    technical
    > > advantages (lower noise) and people with expensive 35mm lenses would get
    > > the full benefit, and not throw much of the light away, which is what I
    > > think would happen now.

    >
    > The aps sized sensors are a compromise between production cost, image
    > quality and usability. 35mm is an arbitrary size - there is no rule that
    > says 35mm is "full frame".


    True, except when using a film lens on a digital camera.

    > If 645 cameras were just as common as 35mm I
    > could just as easily ask when Nikon would bring out a camera with a 6cm x
    > 4.5cm imaging sensor, so that I could use my old 645 lenses without having

    a
    > crop factor. It is a fact of life that every format change requires a
    > change in other equipment. by having APS sized sensors, people who use
    > their 35mm lenses benefit by having the equivalent of long focal length

    fast
    > lenses, but lose out because their 24mm extreme wide angle lens is now

    only
    > a 35mm mild wide angle. It is only at the extreme wide angles that the
    > smaller sensor becomes a big disadvantage, but at the extreme wide angles,
    > due to the different characteristics of digital and film, it is best to

    have
    > a lens designed specifically for digital anyway. Film doesn't care if

    light
    > hits it at an angle, digital does.


    It would be misleading to say that the crop factor of digital capture
    devices which simulates a longer lens length doesn't come without a price.

    As Dave Martindale pointed out: "However, there's no free lunch. To produce
    the same quality of image from the smaller sensor, a given size print needs
    to be enlarged a factor of 1.6X more. That means that the lens needs to
    deliver 1.6 times the resolution to the sensor to get the same quality
    print. Thus, lenses that are marginal in sharpness for full-frame use may
    look just plain unsharp with the smaller sensor, while very sharp lenses can
    stand the extra magnification without strain."

    Film best,
    me
     
    me, Jan 4, 2005
    #12
  13. Dave

    Lourens Smak Guest

    In article <crdh33$sv4$>,
    (Dave Martindale) wrote:

    > No. The smaller sensor means that the field of view with the same lens
    > is narrower. This is sometimes *described* as having the same effect as
    > using a longer focal length on the full-frame camera.


    In fact a small sensor increases DoF. What decreases DoF is the fact
    that you need a shorter FL for the same image.

    Lourens
     
    Lourens Smak, Jan 4, 2005
    #13
  14. Dave

    Lourens Smak Guest

    In article <crdh33$sv4$>,
    (Dave Martindale) wrote:

    > No, you're wrong. It remains a f/2.8 lens. However, there's no free
    > lunch. To produce the same quality of image from the smaller sensor, a
    > given size print needs to be enlarged a factor of 1.6X more. That means
    > that the lens needs to deliver 1.6 times the resolution to the sensor to
    > get the same quality print. Thus, lenses that are marginal in sharpness
    > for full-frame use may look just plain unsharp with the smaller sensor,
    > while very sharp lenses can stand the extra magnification without
    > strain.


    addendum to my other reaction:
    This exact effect in fact *decreases* DoF when a smaller sensor is used.

    Lourens
     
    Lourens Smak, Jan 4, 2005
    #14
  15. Lourens Smak <> writes:

    >> No. The smaller sensor means that the field of view with the same lens
    >> is narrower. This is sometimes *described* as having the same effect as
    >> using a longer focal length on the full-frame camera.


    >In fact a small sensor increases DoF. What decreases DoF is the fact
    >that you need a shorter FL for the same image.


    No, you've got that reversed. A smaller sensor, while keeping the lens
    focal length the same, *decreases* DoF because of the greater
    magnification needed when printing.

    If you want to keep the field of view the same, you need a shorter FL
    lens. The shorter FL *increases* DoF by a factor of the square of the
    sensor size ratio.

    When both of these effects are taken into account, you get a net greater
    DoF from the smaller sensor with the same field of view, by a factor
    equal to the reciprocal of the sensor size ratio.

    Dave
     
    Dave Martindale, Jan 4, 2005
    #15
  16. In article <41d9de83@212.67.96.135>, Dave <> wrote:

    > 1) A 100mm f/2.8 lens put on a small digital sensor remains a 100mm
    > f/2.8 lens. The lens remains the same.


    Correct.

    > 2) The smaller CCD sensor means the focal length (as compared to 35mm)
    > is longer, as everyone agrees.


    Nope.

    > 3) The aperture whilst still f/2.8 is "effectively" larger, as much of
    > the light is thrown away, missing the sides of the sensor. So the
    > viewfinder will be darker than if fitted with a f/2.8 lens which filled
    > the sensor and no more.


    This is a whole lot easier to think about if you're familiar with many
    different formats of film (or sizes of image sensors). I have two 120mm
    lenses, one is a wide angle (for a 4x5" camera) the other is a short
    telephoto (for a 6x4.5cm camera). The 120mm f5.6 Schneider has a 179mm
    image circle, so any format of film that fits inside a 179mm diameter
    circle can use that lens, from APS (which only needs a 35mm image
    circle) on up to 4x5" (which needs a 153mm or so minimum image circle).
    It's just for really small formats, most of the image is wasted, and so
    most folks use lenses that are designed and optimized specifically for
    those smaller formats. My 120mm f4 Mamiya lens probably has only an 80mm
    or so image circle, as it's designed for a 6x4.5cm camera (56x41.5mm
    film size), but they're both the same focal length.

    Lenses designed for 35mm film should have an image circle diameter of
    45mm or so, they can have more, and tilt/shift lenses must have more,
    but there's no real benefit to going much over 45mm. Your dSLR is just
    using a smaller part of that 45mm image circle. Just calculate the
    diagonal (43.3mm for 24x36mm) and round up a bit to figure the minimum
    image circle required for your particular dSLR. The edges of the image
    circle are generally not as sharp, so you want a little bit of excess.

    This same 120mm lens has a horizontal angle of view of 53 degrees on
    4x5" (95.3 x 120.7mm actual image size), 39 degrees on 6x9cm, 32 degrees
    on 6x7cm, 26 degrees (horizontal, remember) for 6x6cm and 6x4.5cm, 17
    degrees for 24x36mm, and a fairly narrow 14 degrees for 16.7 x 30.2, the
    old APS standard which is about what many digital SLR's use.

    It's always f5.6 at maximum aperture, no matter how much of that image
    circle you use. Really wide angle lenses have light falloff at the
    edges, which changed the amount of light that reaches the film at the
    edges. This would be the case if there were a 120mm lens designed for
    11x14" film, for example.

    Less confused or more?

    Drew

    --
    Drew W. Saunders

    dru (at) stanford (dot) eee dee you
     
    Drew Saunders, Jan 4, 2005
    #16
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