So why dont P&S work like dSLR's?

Discussion in 'Digital Photography' started by Jonathan Wilson, May 3, 2004.

  1. Having used my 300D for some while, and reading some posts on how
    dSLR's dont have a "lag" I decided to try some tests....

    With my old G2 (and a lot of P&S's), if you full press the button it
    will take the photo even if it hasnt got a lock on the focus, the full
    press overrides the focus stage and tells the camera to just take the
    damn pic.

    I'd been doing the same with the 300d, half pressing and waiting for
    the "in focus" marker in the viewer... but then reading about the near
    zero lag I wondered what would happen if I full pressed the shutter
    after moving the cameras focus points to where I knew it wouldnt be in
    focus but seeing a blured view through the viewfinder...

    what happended was that there was a buzz as lense moved to focus, then
    it did it... press... focus... phototaken....

    so why dont P&S's do this... surely it is no more hard work for the
    all in one compacts to work in the same way... bringing the dealay
    down to the focus and not any lag, percieved or real.

    My other "mmm strange..." is that with the 300D (I'd like to hear
    about other makes) is that when you bracket a shot, the mirror ends up
    slapping 3 times (in burst mode... in non-burst you have to physically
    press the shoot button 3 times).... surely all that needs to happen is
    pull the mirror once then do 3 different shutter fires, or am I
    looseing something here?


    --
    Jonathan Wilson.
    www.somethingerotic.com
    Jonathan Wilson, May 3, 2004
    #1
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  2. Jonathan Wilson

    JR Guest

    The focusing mechanisms in DSLR's and P&S cameras are very different. A
    DSL has a rather large motor in it to turn big pieces of glass, or in
    some cases the lens itself has an ultra fast motor, in a P&S camera,
    where is the room for that motor?

    JR
    JR, May 3, 2004
    #2
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  3. Jonathan Wilson

    Jan Wagner Guest

    Jonathan Wilson wrote:
    > Having used my 300D for some while, and reading some posts on how
    > dSLR's dont have a "lag" I decided to try some tests....
    >
    > With my old G2 (and a lot of P&S's), if you full press the button it
    > will take the photo even if it hasnt got a lock on the focus, the full
    > press overrides the focus stage and tells the camera to just take the
    > damn pic.
    >
    > I'd been doing the same with the 300d, half pressing and waiting for
    > the "in focus" marker in the viewer... but then reading about the near
    > zero lag I wondered what would happen if I full pressed the shutter
    > after moving the cameras focus points to where I knew it wouldnt be in
    > focus but seeing a blured view through the viewfinder...
    >
    > what happended was that there was a buzz as lense moved to focus, then
    > it did it... press... focus... phototaken....


    Sounds like you should set continuous-AF to take pics even while the
    camera focuses, otherwise there's the extra delay for the AF before it
    takes a pic, and the result isn't comparable to your G2 test with no AF.
    (don't know about precise 300D settings though...)

    > so why dont P&S's do this... surely it is no more hard work for the
    > all in one compacts to work in the same way... bringing the dealay
    > down to the focus and not any lag, percieved or real.


    Maybe if you turn off the LCD live preview ;-) Well actually the new
    "P&S" from Canon have a shutter release lag of <300ms down to <100ms.
    e.g.
    http://www.dpreview.com/reviews/canona70/page9.asp
    (shutter release lag, viewfinder, <100ms)
    vs
    http://www.imaging-resource.com/PRODS/D70/D70A.HTM
    D70/300D/10D 100..150ms shutter release lag.

    > My other "mmm strange..." is that with the 300D (I'd like to hear
    > about other makes) is that when you bracket a shot, the mirror ends up
    > slapping 3 times (in burst mode... in non-burst you have to physically
    > press the shoot button 3 times).... surely all that needs to happen is
    > pull the mirror once then do 3 different shutter fires, or am I
    > looseing something here?


    Probably because of the focusing, with the dSLR (re?)focusing on a
    separate small sensor via the mirror while the mirror is down. The
    non-dSLR would typically AF via the CCD sensor (AFAIK), which is slower.

    cheers,
    - Jan
    Jan Wagner, May 3, 2004
    #3
  4. Jonathan Wilson

    B.A.S. Guest

    Jan Wagner wrote:

    > Maybe if you turn off the LCD live preview ;-) Well actually the new
    > "P&S" from Canon have a shutter release lag of <300ms down to <100ms.
    > e.g.
    > http://www.dpreview.com/reviews/canona70/page9.asp
    > (shutter release lag, viewfinder, <100ms)
    > vs
    > http://www.imaging-resource.com/PRODS/D70/D70A.HTM
    > D70/300D/10D 100..150ms shutter release lag.


    Remember there are two components of lag - time to focus, and time for
    the picture to be taken after focus is complete.I know the poster was
    referring to shutter lag, which is only the latter, but in many
    applications (like shooting athletics or active young children), you
    really must consider the total lag as you will not be able to pre-focus
    in these situations.

    Which is why I kept my film SLR's until now (just bought a D70) - I
    needed them to shoot my kid's soccer and volleyball games, and for
    situations where I wanted to capture spontaneous reactions on faces. My
    digital P&S would take a second or two to fire in these dynamic
    situations, making it useless for them.

    With the D70, both focus and shutter lag are very short, and I don't
    miss the moment when shooting.

    While the A70's shutter lag is commendably short, check out the time it
    takes to focus on the same page referred to above (1 to 2 seconds). It
    too would be useless for capturing events that are over in a hundred or
    two milliseconds, where the subjects are moving.

    You have to pay more (at least today - maybe not tomorrow) for the
    mechanicals in the body and in the lens to get rapid focus in a digital
    camera - it's a non-trivial exercise engineering-wise, and not just an
    artificially induced marketing differentiator.

    B.A.S.
    B.A.S., May 3, 2004
    #4
  5. "B.A.S." <> writes:

    >You have to pay more (at least today - maybe not tomorrow) for the
    >mechanicals in the body and in the lens to get rapid focus in a digital
    >camera - it's a non-trivial exercise engineering-wise, and not just an
    >artificially induced marketing differentiator.


    It's not just this. DSLRs do autofocus with separate optical systems
    independent of the main CCD. This requires beamsplitters, lenses, and
    often several additional small CCD sensors, but it ultimately gives the
    camera an output that tells the camera how far and in which direction
    the lens needs to move to focus.

    In comparison, the P&S digicams use the main CCD for focus, and just
    move the main lens through a series of positions looking for best focus.
    It's cheaper but slower.

    Dave
    Dave Martindale, May 3, 2004
    #5
  6. Jonathan Wilson

    B.A.S. Guest

    Dave Martindale wrote:

    > It's not just this. DSLRs do autofocus with separate optical systems
    > independent of the main CCD. This requires beamsplitters, lenses, and
    > often several additional small CCD sensors, but it ultimately gives the
    > camera an output that tells the camera how far and in which direction
    > the lens needs to move to focus.
    >
    > In comparison, the P&S digicams use the main CCD for focus, and just
    > move the main lens through a series of positions looking for best focus.
    > It's cheaper but slower.
    >
    > Dave


    Is that really how it works? (Not challenging you - just curious.)

    The DSLR works out how far to move the lens elements to obtain focus,
    and then just moves them precisely that amount (as opposed to moving the
    lens elements until the image appears to be infocus)? Interesting.

    Of course you still need a speedy motor mechanism in the lens to move
    the elements after the calculation has been made, which you would expect
    to cost more to make than the slower one in the P&S.

    B.A.S.
    B.A.S., May 3, 2004
    #6
  7. "B.A.S." <> writes:

    >Is that really how it works? (Not challenging you - just curious.)


    That's how a phase-detecting autofocus system works, more or less. The
    IR systems in film P&S cameras are different, but they also give an
    estimated distance to subject in a single reading.

    >The DSLR works out how far to move the lens elements to obtain focus,
    >and then just moves them precisely that amount (as opposed to moving the
    >lens elements until the image appears to be infocus)? Interesting.


    Well, in more detail: there are two optical systems that sample light
    coming from the lens, at positions somewhat left of centre and right of
    centre. Each optical system focuses an image on a tiny CCD. It may be
    just a line of sensors, so it's very fast to read. Now, the optics are
    set up so that when an edge is in focus, it falls on the *same* pixel in
    both sensors. When the lens is moved towards infinity from the correct
    focus point, the image on one of the little sensors becomes less sharp
    but it also moves left (because its light comes from one side of the
    lens). At the same time, the other autofocus sensor's image moves
    right. So the camera reads both sensors and performs a "correlation"
    between them, which is a standard image processing operation.

    The output of the correlation is a pattern with a bunch of peaks. For
    each position in the correlation, the height (amplitude) of the
    correlation function tells you how much the two patterns are alike when
    they are shifted a particular amount relative to each other. If you
    select the largest of these peaks, the position of the peak tells you
    the amount of shift needed to make the two images most like each
    other. And that gives an estimate of which direction to turn the motor,
    and how hard you should drive it, to head towards correct focus. As the
    lens approaches focus, the camera will be taking more readings, and
    using them as feedback for the motor controller. This makes for fast
    response.

    >Of course you still need a speedy motor mechanism in the lens to move
    >the elements after the calculation has been made, which you would expect
    >to cost more to make than the slower one in the P&S.


    Yes, but film P&S cameras do have fast focusing lenses. They use an
    infrared beam and sensor to measure subject distance, then focus the
    lens based on that information. Many of them are very fast in focusing.
    Sometimes I wonder why digital P&S cameras don't use IR focus.

    So I think the slowness of digital P&S cameras is not the focus time of
    the lens, but the time required for the microcontroller to make a
    sequence of measurements using the main CCD to estimate where the best
    focus point is.

    Dave
    Dave Martindale, May 3, 2004
    #7
  8. Jonathan Wilson

    Lionel Guest

    Kibo informs me that (Dave Martindale) stated that:

    >Yes, but film P&S cameras do have fast focusing lenses. They use an
    >infrared beam and sensor to measure subject distance, then focus the
    >lens based on that information. Many of them are very fast in focusing.
    >Sometimes I wonder why digital P&S cameras don't use IR focus.


    Perhaps the possibility of the image sensor picking up some of the IR
    from the focusing beam? But I think it's even more likely it's just to
    reduce costs.

    >So I think the slowness of digital P&S cameras is not the focus time of
    >the lens, but the time required for the microcontroller to make a
    >sequence of measurements using the main CCD to estimate where the best
    >focus point is.


    Yes, so do I.

    --
    W
    . | ,. w , "Some people are alive only because
    \|/ \|/ it is illegal to kill them." Perna condita delenda est
    ---^----^---------------------------------------------------------------
    Lionel, May 4, 2004
    #8
  9. (Dave Martindale) wrote in message news:<c76gvf$gcu$>...

    > Yes, but film P&S cameras do have fast focusing lenses. They use an
    > infrared beam and sensor to measure subject distance, then focus the
    > lens based on that information. Many of them are very fast in focusing.
    > Sometimes I wonder why digital P&S cameras don't use IR focus.


    P&Ss use a wide variety of methods depnding on the camera, as do
    DSLRs. As a minimum, you can buy an active IR flash for all DSLRs for
    no-light focusing, and for many P&Ss too.

    > So I think the slowness of digital P&S cameras is not the focus time of
    > the lens, but the time required for the microcontroller to make a
    > sequence of measurements using the main CCD to estimate where the best
    > focus point is.


    As processing power gets better, P&Ss are getting a bit faster. They
    still aren't up to pro DSLR standards, but some do exceed the
    operating speed of the low end prosumer DSLRs like the Canon 10D,
    which lags much longer in review modes than almost all current P&Ss.
    The 10D does have average shutter lag for a DSLR, though, which all
    hover around 0.1 secs.

    Another big, processing method issue is shoot priorty, which only
    Sigma and Canon DSLRs provide (though the Canon is only fully shoot
    pri in JPEG modes). Nikon-based DSLR bodies like Fuji and Kodak use
    (and obviosly Nikon) can't do it. Pentax also failed to provide SP,
    for some reason. One would have to tag inferior processing methods,
    in addition to overall power, as the culprit. I suppose there are
    probably some mechanical issues as well, since Nikon bodies are a
    definitive show stopper.
    George Preddy, May 4, 2004
    #9
  10. Jonathan Wilson

    George Kerby Guest

    On 5/4/04 10:33 AM, in article
    , "George Preddy"
    <> wrote:

    > (Dave Martindale) wrote in message
    > news:<c76gvf$gcu$>...
    >
    >> Yes, but film P&S cameras do have fast focusing lenses. They use an
    >> infrared beam and sensor to measure subject distance, then focus the
    >> lens based on that information. Many of them are very fast in focusing.
    >> Sometimes I wonder why digital P&S cameras don't use IR focus.

    >
    > P&Ss use a wide variety of methods depnding on the camera, as do
    > DSLRs. As a minimum, you can buy an active IR flash for all DSLRs for
    > no-light focusing, and for many P&Ss too.
    >
    >> So I think the slowness of digital P&S cameras is not the focus time of
    >> the lens, but the time required for the microcontroller to make a
    >> sequence of measurements using the main CCD to estimate where the best
    >> focus point is.

    >
    > As processing power gets better, P&Ss are getting a bit faster. They
    > still aren't up to pro DSLR standards, but some do exceed the
    > operating speed of the low end prosumer DSLRs like the Canon 10D,
    > which lags much longer in review modes than almost all current P&Ss.
    > The 10D does have average shutter lag for a DSLR, though, which all
    > hover around 0.1 secs.
    >
    > Another big, processing method issue is shoot priorty, which only
    > Sigma and Canon DSLRs provide (though the Canon is only fully shoot
    > pri in JPEG modes). Nikon-based DSLR bodies like Fuji and Kodak use
    > (and obviosly Nikon) can't do it. Pentax also failed to provide SP,
    > for some reason. One would have to tag inferior processing methods,
    > in addition to overall power, as the culprit. I suppose there are
    > probably some mechanical issues as well, since Nikon bodies are a
    > definitive show stopper.

    Moron


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    George Kerby, May 4, 2004
    #10
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