Converting f-stops to digital 35mm equivalent?

Discussion in 'Digital Photography' started by Tim Tar, Sep 11, 2004.

  1. Tim Tar

    Tim Tar Guest

    When converting focal length from 35mm to digital, does any change in f-stop
    occur?


    For pentax, it seems like the conversion factor is about 1.5 ... so my 100mm
    f2.8 lens becomes a 150mm (35mm equivalent) lens on the digital body.... but
    do I still retain the f2.8 or does the lens become "slower" by a factor of
    1.5 f-stops as well?

    Bottom line, does a (for example) 200mm f5.6 lens become a 300mm f 9.5 (and
    therefore way too slow) lens?


    TIA

    Tim
     
    Tim Tar, Sep 11, 2004
    #1
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  2. Tim Tar

    Jim Guest

    "Tim Tar" <> wrote in message
    news:SfF0d.96005$S55.43733@clgrps12...
    > When converting focal length from 35mm to digital, does any change in

    f-stop
    > occur?

    No, because the conversion factor really just applies to the angle of view.
    The conversion factor does not change the focal length of the lens nor does
    it change the lens aperature. Hence, the stops are not affected by the
    change in format.
    Jim
     
    Jim, Sep 11, 2004
    #2
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  3. Tim Tar

    [BnH] Guest

    Its actuallly "cropping factor".
    Imagine you are in a darkroom and compose your 35mm film and zoom it by 1.5x
    factor.

    =bob=

    "Tim Tar" <> wrote in message
    news:SfF0d.96005$S55.43733@clgrps12...
    > When converting focal length from 35mm to digital, does any change in
    > f-stop
    > occur?
    >
    >
    > For pentax, it seems like the conversion factor is about 1.5 ... so my
    > 100mm
    > f2.8 lens becomes a 150mm (35mm equivalent) lens on the digital body....
    > but
    > do I still retain the f2.8 or does the lens become "slower" by a factor of
    > 1.5 f-stops as well?
    >
    > Bottom line, does a (for example) 200mm f5.6 lens become a 300mm f 9.5
    > (and
    > therefore way too slow) lens?
    >
    >
    > TIA
    >
    > Tim
    >
    >
     
    [BnH], Sep 12, 2004
    #3
  4. Tim Tar

    Colin D Guest

    Tim Tar wrote:
    >
    > When converting focal length from 35mm to digital, does any change in f-stop
    > occur?
    >
    > For pentax, it seems like the conversion factor is about 1.5 ... so my 100mm
    > f2.8 lens becomes a 150mm (35mm equivalent) lens on the digital body.... but
    > do I still retain the f2.8 or does the lens become "slower" by a factor of
    > 1.5 f-stops as well?
    >
    > Bottom line, does a (for example) 200mm f5.6 lens become a 300mm f 9.5 (and
    > therefore way too slow) lens?
    >
    > TIA
    >
    > Tim


    Any lens projects a circular image field, the size of which depends on
    the lens design. 35mm lenses generally have a field circle of about
    50mm, or 2 inches. A 35mm film frame sits comfortably within that
    circle, and so does a somewhat smaller digital sensor. Because the
    sensor is smaller thsn a 35mm frame, it 'sees' a lesser area of the
    image thrown by the lens. Therefore, you can see that neither the focal
    length or the aperture changes only the area seen by the receptor
    varies.

    Colin D.
     
    Colin D, Sep 12, 2004
    #4
  5. Tim Tar

    Nick C Guest

    "[BnH]" <b18[at]ii[dot]net> wrote in message
    news:4143afff$0$22806$...
    > Its actuallly "cropping factor".
    > Imagine you are in a darkroom and compose your 35mm film and zoom it by
    > 1.5x factor.
    >
    > =bob=
    >
    >


    Lets say that's a valid glass half empty approach (for it is a valid example
    to describe a condition). Another way of looking at it is there's a small
    sensor that utilizes a portion of the lens area that relates to that portion
    of the lens angle of view used to capture an image which when viewed appears
    as if it's been enlarged or its image captured by a stronger lens with a
    similar narrow angle of view. Hence, it could be seen as a 1.5X multiplier.
    The glass full approach.

    Either way, it's the sensor size that counts and when there's reference made
    as to a camera being 1.3X, 1.5X, 1.6X, or 2.0X what should readily come to
    mind is not so much as being seen as a multiplier or image cropper, but an
    indication of the size of the sensor.

    IMO, newbie's to DSLR's would relate much easier to seeing the lens factor
    as a multiplier rather than a photo cropper.

    nick
     
    Nick C, Sep 12, 2004
    #5
  6. Tim Tar

    JeffTaite Guest

    On Sun, 12 Sep 2004 14:55:55 +1200, Colin D
    <> wrote:

    >Tim Tar wrote:
    >>
    >> When converting focal length from 35mm to digital, does any change in f-stop
    >> occur?
    >>
    >> For pentax, it seems like the conversion factor is about 1.5 ... so my 100mm
    >> f2.8 lens becomes a 150mm (35mm equivalent) lens on the digital body.... but
    >> do I still retain the f2.8 or does the lens become "slower" by a factor of
    >> 1.5 f-stops as well?
    >>
    >> Bottom line, does a (for example) 200mm f5.6 lens become a 300mm f 9.5 (and
    >> therefore way too slow) lens?
    >>
    >> TIA
    >>
    >> Tim

    >
    >Any lens projects a circular image field, the size of which depends on
    >the lens design. 35mm lenses generally have a field circle of about
    >50mm, or 2 inches. A 35mm film frame sits comfortably within that
    >circle, and so does a somewhat smaller digital sensor. Because the
    >sensor is smaller thsn a 35mm frame, it 'sees' a lesser area of the
    >image thrown by the lens. Therefore, you can see that neither the focal
    >length or the aperture changes only the area seen by the receptor
    >varies.
    >
    >Colin D.


    While this is probably not directly related to the OP's question, I
    think it would be important to also mention the available
    depth-of-field from the same f/stop for an SLR or digital camera
    will alter drastically, even though they are the same f/stop number
    and focal length number on the two cameras. (This qualifies as a
    "change", in the OP's question.) Losing a shallow depth-of-field at
    familiar f/stops was disheartening when I moved to digital. You only
    have 2 axes to frame any image (horizontal and vertical), unless you
    include the 3rd, that being DOF. I never realized I relied on this
    third axis of framing so much until I had lost most of it from the
    smaller sensor sizes in digital cameras. This is something not often
    stated by others when talking about digital cameras to new-comers,
    but it should be revealed as one of the significant changes they
    will face. I've had to find new ways to compensate for the loss of
    shallow DOF by using more telephoto or zoomed macro shots with
    close-up lenses, which brings some shallower depth-of-field back
    into the picture. (pun intended)
     
    JeffTaite, Sep 12, 2004
    #6
  7. Tim Tar

    Colin D Guest

    JeffTaite wrote:
    >
    > On Sun, 12 Sep 2004 14:55:55 +1200, Colin D
    > <> wrote:
    >
    > >Tim Tar wrote:
    > >>
    > >> When converting focal length from 35mm to digital, does any change in f-stop
    > >> occur?
    > >>
    > >> For pentax, it seems like the conversion factor is about 1.5 ... so my 100mm
    > >> f2.8 lens becomes a 150mm (35mm equivalent) lens on the digital body.... but
    > >> do I still retain the f2.8 or does the lens become "slower" by a factor of
    > >> 1.5 f-stops as well?
    > >>
    > >> Bottom line, does a (for example) 200mm f5.6 lens become a 300mm f 9.5 (and
    > >> therefore way too slow) lens?
    > >>
    > >> TIA
    > >>
    > >> Tim

    > >
    > >Any lens projects a circular image field, the size of which depends on
    > >the lens design. 35mm lenses generally have a field circle of about
    > >50mm, or 2 inches. A 35mm film frame sits comfortably within that
    > >circle, and so does a somewhat smaller digital sensor. Because the
    > >sensor is smaller thsn a 35mm frame, it 'sees' a lesser area of the
    > >image thrown by the lens. Therefore, you can see that neither the focal
    > >length or the aperture changes only the area seen by the receptor
    > >varies.
    > >
    > >Colin D.

    >
    > While this is probably not directly related to the OP's question, I
    > think it would be important to also mention the available
    > depth-of-field from the same f/stop for an SLR or digital camera
    > will alter drastically, even though they are the same f/stop number
    > and focal length number on the two cameras. (This qualifies as a
    > "change", in the OP's question.) Losing a shallow depth-of-field at
    > familiar f/stops was disheartening when I moved to digital. You only
    > have 2 axes to frame any image (horizontal and vertical), unless you
    > include the 3rd, that being DOF. I never realized I relied on this
    > third axis of framing so much until I had lost most of it from the
    > smaller sensor sizes in digital cameras. This is something not often
    > stated by others when talking about digital cameras to new-comers,
    > but it should be revealed as one of the significant changes they
    > will face. I've had to find new ways to compensate for the loss of
    > shallow DOF by using more telephoto or zoomed macro shots with
    > close-up lenses, which brings some shallower depth-of-field back
    > into the picture. (pun intended)


    Actually, using the same (slr) lens designed for film on a smaller
    sensor results in a shallower depth of field for a given subject
    distance/f-stop setting. Since greater magnification is needed from the
    sensor to equal the print size from film, you will have less apparent
    dof.

    The greater dof from small-sensor digital point-and-shoots, or zlrs
    comes about because their lenses generally zoom from about 5 or 6 mm to
    about 30 or so, very much shorter than slr-type lenses.

    Fundamentally, dof is related directly to aperture (the bigger the
    aperture the less the dof) and object size/image size ratio (the greater
    the object/image ratio, the greater the dof). Some confusion arises
    with different focal lengths, longer lenses appearing to have less dof,
    but in fact this effect is entirely due to object/image ratio, which is
    less with longer lenses, and conversely greater with short lenses, which
    is why digicams have comparatively great dof.

    However, dof is also related to the degree of enlargement when printing
    from the original image, where small images have to be magnified more
    than larger images, which reduces the visual dof in the print.

    The above is a skeleton description only. The full subject is too
    complicated to be the subject of a NG post.

    Colin D.
     
    Colin D, Sep 12, 2004
    #7
  8. "Tim Tar" <> writes:
    > When converting focal length from 35mm to digital, does any change
    > in f-stop occur?


    No.

    > For pentax, it seems like the conversion factor is about 1.5 ... so
    > my 100mm f2.8 lens becomes a 150mm (35mm equivalent) lens on the
    > digital body....


    No, it is still a 100mm f/2.8 lens.

    However, because the sensor is smaller than the 35 mm film frame, you
    get a image that is /cropped/ with respect to film. The 1.5x crop
    factor of the Pentax gives you an image with the same /angle of view/
    as a 150mm lens would have given you on a 35 mm film camera.

    Just putting it on a digital body does not change the focal length
    of a lens. But because 35 mm SLR-photographers are familiar with
    the angle of view a certain focal length will give them on a film
    body - the conversion to "35mm equivalent" focal lengths is presently
    done when talking about lenses on digital bodies.

    In time, this will change. In a few years, when the 1.5-1.6x crop
    factor has become the "standard" for prosumer digicams, we will
    mentally think about a "normal" lens as something with a focal
    length around 32 mm and anyting below as WA and anything above as
    tele - and nobody except old fogies will use a term such as
    "35 mm equivalent".

    You can simulate the effect of the smaller digital sensor on
    film by using scirrors to «crop» the film negative by removing
    the edges until the negative has the same size as the sensor in
    your Pentax. Cutting away the edges changes the angle of view,
    not the f-stop,
    --
    - gisle hannemyr [ gisle{at}hannemyr.no - http://folk.uio.no/gisle/ ]
    ========================================================================
    «To live outside the law, you must be honest.» (Bob Dylan)
     
    Gisle Hannemyr, Sep 12, 2004
    #8
  9. JeffTaite <> writes:
    > While this is probably not directly related to the OP's question, I
    > think it would be important to also mention the available
    > depth-of-field from the same f/stop for an SLR or digital camera
    > will alter drastically, even though they are the same f/stop number
    > and focal length number on the two cameras.


    For a DSLR with a crop factor of 1.5x (Nikon, Pentax), the shift in
    DOF is equal to 1.2 stops. E.g. an f/2.0 on a Pentax or Nikon DSLR
    will give you the same DOF as f/3.0 on a 35 mm film camera.

    I would not call a 1.2 shift drastic, but I guess it all depends on
    your outlook.

    (The "DOF-shift" for Canon D20 is 1.3 stops, and for the 1DMk2,
    its 0.7 stops.)

    For compacts, with smaller sensors, the shift is more dramatic.
    For the 1/1.8" sensors used in Canons Powershot G-series, the
    "DOF-hift" is 4.5 stops. The maximum aperture of f/2.0 on the
    Canon G5 gives you the same DOF as f/9.7 on a 35 mm film camera.
    --
    - gisle hannemyr [ gisle{at}hannemyr.no - http://folk.uio.no/gisle/ ]
    ========================================================================
    «To live outside the law, you must be honest.» (Bob Dylan)
     
    Gisle Hannemyr, Sep 12, 2004
    #9
  10. JeffTaite <> writes:
    > While this is probably not directly related to the OP's question,
    > I think it would be important to also mention the available
    > depth-of-field from the same f/stop for an SLR or digital camera
    > will alter drastically, even though they are the same f/stop
    > number and focal length number on the two cameras.


    > Losing a shallow depth-of-field at familiar f/stops was
    > disheartening when I moved to digital.


    You've got this slightly wrong. The same f-stop and the same focal
    length will actully give you a /more shallow/ DOF on the camera with
    a smaller sensor.

    What you probably meant to say was that with same f-stop and the same
    /angle of view/ - the camera with a smaller sensor will have a greater
    DOF. True.

    In that case - for a DSLR with a crop factor of 1.5x (Nikon, Pentax),
    the shift in DOF is equal to 1.2 stops. E.g. an f/2.0 on a Pentax or
    Nikon DSLR will give you the same DOF as f/3.0 on a 35 mm film camera.

    I would not call a "DOF-shift" of 1.2 stops drastic, but I guess it
    all depends on your outlook.

    (The "DOF-shift" for Canon D20 with a 1.6x crop is 1.3 stops, and
    for the canon 1DMk2 with 1.3x crop, it is 0.7 stops.)

    For compacts, with smaller sensors, the shift is more dramatic.
    For the 1/1.8" sensors used in Canons Powershot G-series, the
    "DOF-hift" is 4.5 stops. E.g.: the maximum aperture of f/2.0 on
    the Canon G5 gives you the same DOF as f/9.7 on a 35 mm film camera.
    --
    - gisle hannemyr [ gisle{at}hannemyr.no - http://folk.uio.no/gisle/ ]
    ========================================================================
    «To live outside the law, you must be honest.» (Bob Dylan)
     
    Gisle Hannemyr, Sep 12, 2004
    #10
  11. Tim Tar

    JeffTaite Guest

    On Sun, 12 Sep 2004 09:20:20 +0200, Gisle Hannemyr
    <> wrote:

    >You've got this slightly wrong. The same f-stop and the same focal
    >length will actully give you a /more shallow/ DOF on the camera with
    >a smaller sensor.
    >
    >What you probably meant to say was that with same f-stop and the same
    >/angle of view/ - the camera with a smaller sensor will have a greater
    >DOF. True.
    >


    While reading the replies I couldn't understand why others couldn't
    see the loss of shallow depths-of-field with their digital cameras,
    but your statements now answers the confusion, and clarifies what I
    had intended but did not state clearly enough.

    When typing it I was (in my mind) making a reference to the 35mm
    equivalent focal-lengths in digital cameras. My mind automatically
    makes the conversion. So true, that's not the actual focal-length.
    But I don't think yet that most people think in terms of true focal-
    length with their digital cameras and lenses and more often
    reference focal-lengths in 35mm equivalents. A person familiar with
    35mm equipment and buying a new digital camera, trying to get the
    same familiar focal-length effects with their new lenses, will
    wonder what happened to their shallow depths-of-field at the same
    f/stops at the same angle of view (35mm eq. focal-length).

    It will take time before that mental convention passes. 'Til then
    there will be some confusion.

    Replace "focal-length" with "35mm equivalent focal-length" in my
    statements and they will hold true. Your changing it to "angle of
    view" adequately corrects what I had intended, and was thinking, but
    left out in print. You were answering from a purely technical point
    of view, whereas I was trying to convey the problem from, and to,
    the average consumer point of view -- which is what the OP sounded
    like in his questions.

    Thanks for clarifying this.


    [In my defense: from years of speaking experiences I've learned that
    I have to very quickly assess the audience from subtle clues (not so
    easy in text-only communications) and speak in their terms, lest I
    be misunderstood by them at their level of interests and concerns.
    So I consider the questioner first and attempt to put my replies in
    terms that they will find meaningful. If this thread was started by
    an optician I would have raised the issue using other terms. i.e.
    "Know your audience." If you are ever misunderstood, it is your
    fault, not the listeners fault. Just as I was at fault here. I was
    inadvertently trying to address two completely different audiences
    but didn't realize it at the time.]
     
    JeffTaite, Sep 12, 2004
    #11
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