Should 4/3 lenses be half the size of full frame lenses?

Discussion in 'Digital Cameras' started by bob, Jun 13, 2011.

  1. bob

    bob Guest

    4/3 camera has a sensor that is about half the length and height of a full
    frame sensor (1/4 the area).

    Does that mean a 4/3 camera lense would be half the diameter of an
    equivalent full frame lense?
    bob, Jun 13, 2011
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  2. ... and half the length, and one eighth of the weight! Doesn't seem to be
    so in practice, though. You also use the word "equivalent". To be
    equivalent in light gathering power an f/2.8 full-frame lens would need to
    be replaced by an f/1.4 half-frame lens. Somewhat more costly to make, if
    possible at all.

    David J Taylor, Jun 13, 2011
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  3. []
    Yes, perhaps the OP might like to consult the archives.
    Which is why I asked the OP to clarify what they meant by "equivalent".
    For the same number of photons on the sensor, what I say is correct.

    I haven't done the depth of field sums - that was likely Roger Clarkson.

    David J Taylor, Jun 13, 2011
  4. []
    Lenses with the same f/number (more strictly, T/number) will deliver the
    same number of photons per unit area, but as the full-frame sensor has
    four times the sensitive area of the half-frame sensor, it gathers four
    times as much light - four times as many photons. To get the same number
    of photons, you need a lens with twice the physical opening, four times
    the aperture area, and one half the numeric f/number.

    It's why small sensor cameras need a higher light level to get the same
    signal-to-noise ratio or, put another way, why they are noisier at higher
    ISOs. Their smaller sensor captures fewer photons for a given f/number
    and light level.
    David J Taylor, Jun 13, 2011
  5. You're assuming the same pixel count, which is regrettably frequently
    true, but is not an inherent aspect of sensor size. I think a lot of
    confusion arises from your relating the issue to sensor size
    rather than pixel size (okay, sensel size).

    Same number of photos per unit area is what matters for exposure.
    "Everybody knows" this and it's true, as true as anything is anyway.
    Your statements always start out looking like they're trying to
    deny this, which gets everybody up in arms, because this really
    *is* true.
    Hmmm; this feels like a back-door way of coming at the problem, and
    doesn't give me any help in minimzing noise -- both my cameras have
    a base ISO of 200, so I can't turn down the ISO in either case.

    But one of them is a Nikon D700, and one is an Olympus E-PL2. I know
    which one is noisier at high ISO! It's the one with the smaller
    which in this case is the one with the smaller sensor. (They're also
    same number of pixels, so that makes all sorts of comparisons easy.)

    Signal-to-noise ratio isn't a stated spec or anything we have a
    for measuring in cameras (that's widely used and understood, anyway).

    It's generally true that, for any given technology, smaller pixels
    will be
    noisier at ANY ISO (than larger pixels at that same ISO). Won't
    against that for a moment.

    But despite having been around this course before, I still wasn't
    able to read that out of your initial statements. It'd maybe be good
    to work on how you present this issue, to avoid the initial confusion
    and opposition.
    David Dyer-Bennet, Jun 13, 2011
  6. []
    Pixel count actually has nothing to do with it. It's the sensor area
    which matters. Pixel count sets how you trade-off spatial resolution for
    signal-to-noise ratio at the pixel level.
    Yes, it is true.
    More sensitive area reduces noise at the same ISO when more photons are
    captured, other things being equal.
    But a newer 4/3 sensor, one with higher quantum efficiency, might just
    beat an older APS-C sensor. That's when all things /aren't/ equal.
    It takes time to think about these issues, as they are not initially

    David J Taylor, Jun 14, 2011
  7. bob

    bob Guest

    To me, an 4/3 lense that is equivalent to a 35mm lense should have the same
    maximum f number and angle of view.

    This way, a 35mm camera and a 4/3 camera can shoot the same scene using the
    same exposure (shutter speed and aperture) and end up with more or less
    identical photos (except DOF, noise, and maybe resolution).
    bob, Jun 14, 2011
  8. Yes, that's a perfectly fair way of looking at it - depending on your
    definition of "identical"! Given that, why aren't the lenses half the
    linear dimension, a quarter of the area, and an eighth of the weight,
    particularly for micro-4/3 where back-focus requirements are less?

    David J Taylor, Jun 14, 2011
  9. bob

    Neil Ellwood Guest

    The glass might scale but the mounting bits containing the glassware do
    not scale, look at scale models of loco engines, wagons and carriages and
    aircraft for an instance or more.
    Neil Ellwood, Jun 14, 2011
  10. bob

    Paul Furman Guest

    With 1/4 the sensor size so 1/4 the print size.
    Paul Furman, Jun 14, 2011
  11. bob

    Bruce Guest

    That's only true if the Four Thirds sensor has a quarter as many
    pixels as the full frame sensor.

    If the number of pixels is the same, the print size will be the same
    for the same ppi at the printing stage. All that matters at the
    printing stage is the number of pixels. The printer has no idea
    whether those pixels came from a P&S, Four Thirds, APS-C, full frame
    or medium format.
    Bruce, Jun 14, 2011
  12. That makes no sense whatever.

    An f/2.8 lens is an f/2.8 lens, regardless of what format it's meant to
    cover. Its "light gathering power" is the same as any other lens of similar
    f-number as far as photographic purposes are concerned, disregarding
    differences in coating, etc.

    Even if you were right about that light-gathering business you'd be wrong in
    your conclusion: a half-frame format does not have half the diagonal of full
    frame. (It's 30mm vs about 43mm.)
    Neil Harrington, Jun 14, 2011
  13. bob

    Guest Guest

    yes it does.
    it's light gathering power per unit area is the same (exposure) but a
    larger sensor has a larger area so the total amount of light collected
    is higher.
    4/3rds is approximately 1/4 the area of full frame, or two stops, thus
    an f/2.8 lens on full frame is equivalent to an f/1.4 lens on 4/3rds.
    Guest, Jun 14, 2011
  14. Which is all that matters, so you could stop right there.
    Why would you need or want four times as many photons if you're going to
    throw 3/4 of them off the sensor? If instead you concentrate them all on the
    quarter-size sensor, then you've gained two full stops of exposure. So how
    on earth do you make that "equivalent" to the different f-stop.

    Regardless of format size, f/2.8 is f/2.8. If we needed to concern ourselves
    with physical aperture size, we wouldn't need f-stops in the first place.
    It's the *ratio* of focal length to aperture size that matters, hence the
    Regardless of format size, *per unit area* the sensor captures the same
    number of photons at the same f-number.
    Neil Harrington, Jun 14, 2011
  15. bob

    Guest Guest

    for exposure yes, for total light no.
    but the area is bigger, so the total is higher.
    Guest, Jun 14, 2011
  16. Yes. Stop right there.
    You didn't stop where I told you. Larger sensor = larger area = more light
    collected = *same exposure*, no difference as long as the f-number is the
    *Whoa!* where do you get the "or two stops" business?
    Absolutely not. f/2.8 on full frame is equivalent to *f/2.8* on 4/3, on
    medium format, on Minox, on roll film, on 16mm, on Super 8, or on anything
    else. f/2.8 is f/2.8.
    Neil Harrington, Jun 14, 2011
  17. bob

    Guest Guest

    that's because there's more to say.
    however, more light means better signal/noise ratio.

    if you match the noise to obtain the *same* image quality (by raising
    the iso) you can use smaller f/stops on the larger sensor. that's where
    the difference is.

    a good explanation is here:
    1/4 the area is 1/4 the light.

    one stop is half the light and two stops is 1/4. therefore, a sensor
    that has 1/4 the area is a two stop difference.
    Guest, Jun 14, 2011
  18. bob

    dj_nme Guest

    You really should have stopped with "For exposure, yes."
    That is "total light" as far as taking photographs is concerned.
    So what?
    The same f-stop setting gives the same exposure at the same shutter
    speed regardless of format size.

    You seem to have not grasped the reason why some people use the term
    "equivalent f-stop" when taking 4/3, APS and FF.

    Using FF as the baseline, all the others have greater Depth of Field
    (DoF) with lenses of the same angle of view (IE: 35mm Equivalent Focal
    Length [35mm EFL]) with the same f-stop setting.
    "equivalent f-stop" is using a aperture (f-stop) setting that gives an
    equivalent DoF to 35mm at their 35mm EFL, not the same exposure value at
    the same shutter speed.
    To get the same DoF on a 4/3 as a FF the f-stop setting must be twice
    the size (two f-stops); IE: set the 4/3 to f/1.4 @ 25mm FL (50mm in 35mm
    EFL) to get the same Dof as FF set to f/2.8 @ 50mm FL.
    For the same DoF on APS as FF the f-stop setting must be 50 percent
    bigger (one f-stop); IE: set the APS to f/2.0 @ 35mm FL (50mm in 35mm
    EFL) to get the same Dof as FF set to f/2.8 @ 50mm FL.
    It gives a much greater exposure setting at the same shutter setting:
    the shutter speed must be set higher (shorter time) to compensate for
    more light let in by the wider (lower f-stop setting) aperture.
    dj_nme, Jun 14, 2011
  19. bob

    dj_nme Guest

    Of course it makes no sense: it is dead wrong.
    If "bob" wrote that for the same 35mm equivalent FL that he was after
    the same DoF, then two stops wider aperture (for 4/3, in this case) will
    get the same DoF as FF (35mm full frame) at the same angle of view lens
    (35mm equiv focal length).
    It does not work for exposure, only DoF.
    In this case (for 4/3), it is really a quarter-frame format (about 22mm
    Vs about 43mm diagonal).
    Not that I really believe that the facts will "get in the way" of "bob"s
    dj_nme, Jun 14, 2011
  20. bob

    Guest Guest

    nope. exposure is light per unit area. if you have more area (a larger
    sensor), you have more total light distributed over the larger area,
    which ends up being the same for a given area.
    but not the same result. the noise and depth of field are different.
    actually you haven't grasped it, since you contradict yourself below.
    true, but smaller formats will have more noise (all other things being
    equal) so you don't get the same image.

    right, which for 4/3rds, is 2 stops wider.
    you can also raise the iso, which has the effect of an equivalent s/n
    ratio, and then you can use the same shutter speed with the equivalent

    that contradicts what you said earlier.
    Guest, Jun 14, 2011
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