Why "Focus and Recompose" causes back-focus issues

Discussion in 'Digital Photography' started by Carl Miller, Mar 7, 2005.

  1. Carl Miller

    Carl Miller Guest

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  2. Carl Miller

    Bill Guest

    Bill, Mar 8, 2005
    #2
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  3. Carl Miller

    paul Guest

    paul, Mar 8, 2005
    #3
  4. Carl Miller

    Owamanga Guest

    On Tue, 08 Mar 2005 09:18:59 -0800, paul <> wrote:

    >Carl Miller wrote:
    >
    >> http://visual-vacations.com/Photography/focus-recompose_sucks.htm

    >
    >
    >Hmm OK so it's better to manually select a side AF sensor than recompose
    >with the middle one because panning the camera back to position changes
    >the distance causing back focus. That makes sense.


    Just learn to focus on something slightly closer - the subject's ear
    for example and it'll all be corrected.

    Canon has an advantage over Nikon on this front - their focus sensors
    are on the thirds intersections vs Nikon's positioning of 'anywhere
    the subject shouldn't be'.

    --
    Owamanga!
     
    Owamanga, Mar 8, 2005
    #4
  5. Carl Miller

    Steve Guest

    Owamanga wrote:

    > Just learn to focus on something slightly closer - the subject's ear
    > for example and it'll all be corrected.


    No, you want to focus on something at the same distance. The whole point of the
    article, explaimed in a long, roundabout fashion, was that pointing a camera at a
    point that's on the plane of focus >but not perpendicular to the axis of the lens
    when it's in the position in which the shot will be composed< results in an increased
    lens to subject distance. The article used a lot of words to let us know that the
    hypotenuse of a right triangle is longer than the base.

    The article is flat out wrong that focus and recompose will always result in
    incorrect focus. The author's very own example of focusing on the subject's eyes
    could also have been achieved by raising the camera to the height of the subject's
    eyes, thus moving the camera parallel to the plane of focus, and maintaining the same
    lens to subject distance.

    If focusing on the ear results in a sharp picture it will be because the ear is the
    same distance from the lens as the plane of focus. I'm pretty sure that every camera
    manaul that explains focus and recompose says "focus on a different subject at the
    *same* distance". Apparently some people have a hard time with the "same distance"
    concept.

    --
    Steve

    The above can be construed as personal opinion in the absence of a reasonable
    belief that it was intended as a statement of fact.

    If you want a reply to reach me, remove the SPAMTRAP from the address.
     
    Steve, Mar 9, 2005
    #5
  6. Carl Miller

    Owamanga Guest

    On Wed, 09 Mar 2005 06:22:33 GMT, Steve <>
    wrote:

    >
    >
    >Owamanga wrote:
    >
    >> Just learn to focus on something slightly closer - the subject's ear
    >> for example and it'll all be corrected.

    >
    >No, you want to focus on something at the same distance. The whole point of the
    >article, explaimed in a long, roundabout fashion, was that pointing a camera at a
    >point that's on the plane of focus >but not perpendicular to the axis of the lens
    >when it's in the position in which the shot will be composed< results in an increased
    >lens to subject distance. The article used a lot of words to let us know that the
    >hypotenuse of a right triangle is longer than the base.


    Dude, just stop and think for a second. If you estimate the back-focus
    error in this situation to be around 2" then choose something 2"
    closer to focus on (ie his ear rather than his nose) and you'll be
    focussed correctly when you re-frame.

    It's *very* simple.

    >The article is flat out wrong that focus and recompose will always result in
    >incorrect focus. The author's very own example of focusing on the subject's eyes
    >could also have been achieved by raising the camera to the height of the subject's
    >eyes, thus moving the camera parallel to the plane of focus, and maintaining the same
    >lens to subject distance.


    That would help a lot, but is the plane of focus truly a plane?

    I haven't done this test, and am sure it depends on the lens quality,
    but if you placed a camera onto a piece of graph paper and set a wide
    open aperture, focusing it at say 12", would the focused horizontal
    line remain in sharp focus as it exits the frame to the far right and
    left. Even though the distance to those ends of the line is
    significantly further away than the center is?

    Turn the camera 90deg and you have the same problem with the portrait
    (if one exists).

    >If focusing on the ear results in a sharp picture it will be because the ear is the
    >same distance from the lens as the plane of focus.


    That *was* my point, and is why you would choose the ear.

    > I'm pretty sure that every camera
    >manaul that explains focus and recompose says "focus on a different subject at the
    >*same* distance". Apparently some people have a hard time with the "same distance"
    >concept.


    --
    Owamanga!
     
    Owamanga, Mar 9, 2005
    #6
  7. Carl Miller

    Steve Guest

    Owamanga wrote:
    > If you estimate the back-focus
    > error in this situation to be around 2" then choose something 2"
    > closer to focus on (ie his ear rather than his nose) and you'll be
    > focussed correctly when you re-frame.
    >
    > It's *very* simple.


    It is very simple, so why complicate it? You've now said twice that the ear is
    closer, but if that gives you good focus it's because it's at the *same* distance.
    Why estimate an error of 2" when you can just choose a spot to focus on that's at the
    same distance, and if the DoF is small enough that a 3.5% difference in focus
    distance is critical (as in the article) why would you be estimating the distance to
    a different point? The scenario in the article requires either a tape measure or a
    piece of string. (The author apparenty uses a tape measure graduated in 64ths.)


    > if you placed a camera onto a piece of graph paper and set a wide
    > open aperture, focusing it at say 12", would the focused horizontal
    > line remain in sharp focus as it exits the frame to the far right and
    > left.


    It depends on the lens and what you mean by "sharp".

    > Even though the distance to those ends of the line is
    > significantly further away than the center is?


    I expect that's one of the things that makes the design of lens optics challenging. I
    don't really know anything about optical design, but it seems intuitive that it would
    be easier to design a lens with less error if it were designed to focus on a curve,
    since the light would hit the front element at less of an angle. Of course there's a
    lot about physics that is counter-intuitive. There's no doubt that it's the edges of
    the field of view that are the most problematic.


    >>If focusing on the ear results in a sharp picture it will be because the ear is the
    >>same distance from the lens as the plane of focus.

    >
    >
    > That *was* my point, and is why you would choose the ear.


    Then why do you insist on saying it's closer?

    --
    Steve

    The above can be construed as personal opinion in the absence of a reasonable
    belief that it was intended as a statement of fact.

    If you want a reply to reach me, remove the SPAMTRAP from the address.
     
    Steve, Mar 10, 2005
    #7
  8. Carl Miller

    Owamanga Guest

    On Thu, 10 Mar 2005 08:04:27 GMT, Steve <>
    wrote:

    >Owamanga wrote:
    >> If you estimate the back-focus
    >> error in this situation to be around 2" then choose something 2"
    >> closer to focus on (ie his ear rather than his nose) and you'll be
    >> focussed correctly when you re-frame.
    >>
    >> It's *very* simple.

    >
    >It is very simple, so why complicate it? You've now said twice that the ear is
    >closer, but if that gives you good focus it's because it's at the *same* distance.


    Yes. I qualified the statement 'in this situation'.

    >Why estimate an error of 2" when you can just choose a spot to focus on that's at the
    >same distance,


    Because you can't. There are typically 5 AF focus points, and on a
    Nikon, none of them are where you'd need them. On a canon they are
    better placed, but still, if you look at the photo in question, none
    of either camera exactly lie on the right part of the subject without
    some form of re-framing.

    I checked in photoshop. Using Nikon's points, the center focus point
    would lie on his furthest collar, left would be background, right
    would be his closest shoulder, top would be his cheek, bottom would be
    his close arm. All useless. Canon in this situation (where the
    photographer is ignoring rule of thirds) isn't any better.

    http://visual-vacations.com/ProfessionalServices/Portraits/187U1318.jpg

    > and if the DoF is small enough that a 3.5% difference in focus
    >distance is critical (as in the article) why would you be estimating the distance to
    >a different point?


    Because an estimation is better than no estimation by focusing on a
    point that becomes irrelevant once you reframe.

    >The scenario in the article requires either a tape measure or a
    >piece of string. (The author apparenty uses a tape measure graduated in 64ths.)


    Which isn't going to fly in the real world. Yes, we did it 80 years
    ago, today we lean on technology. Even if I had a tape measure that
    showed 64ths, my lens markings are in feet!

    >> if you placed a camera onto a piece of graph paper and set a wide
    >> open aperture, focusing it at say 12", would the focused horizontal
    >> line remain in sharp focus as it exits the frame to the far right and
    >> left.

    >
    >It depends on the lens and what you mean by "sharp".


    Not blurry :), what does sharp mean to you then?

    >> Even though the distance to those ends of the line is
    >> significantly further away than the center is?

    >
    >I expect that's one of the things that makes the design of lens optics challenging. I
    >don't really know anything about optical design, but it seems intuitive that it would
    >be easier to design a lens with less error if it were designed to focus on a curve,
    >since the light would hit the front element at less of an angle. Of course there's a
    >lot about physics that is counter-intuitive. There's no doubt that it's the edges of
    >the field of view that are the most problematic.
    >
    >
    >>>If focusing on the ear results in a sharp picture it will be because the ear is the
    >>>same distance from the lens as the plane of focus.

    >>
    >>
    >> That *was* my point, and is why you would choose the ear.

    >
    >Then why do you insist on saying it's closer?


    Look at the photo in question and tell me again that the ear isn't
    closer:

    http://visual-vacations.com/ProfessionalServices/Portraits/187U1318.jpg

    --
    Owamanga!
     
    Owamanga, Mar 10, 2005
    #8
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