focus, then change focal length

Discussion in 'Digital Photography' started by Poldie, Jul 10, 2008.

  1. Poldie

    Poldie Guest

    If I manually focus on something, then change the focal length, it seems
    that the subject is still in focus. This is handy, but can it be relied
    on? Is this something which is a given, or have I just been lucky so
    far? Perhaps it depends on the camera/lens? I'm using an EOS 400d with
    the stock lens (for now - thinking of an upgrade but I'm in no hurry).

    Cheers.
     
    Poldie, Jul 10, 2008
    #1
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  2. Poldie

    Bert Hyman Guest

    _.@._ (Poldie) wrote in news:g55r03$en3$:

    > If I manually focus on something, then change the focal length, it
    > seems that the subject is still in focus. This is handy, but can it
    > be relied on? Is this something which is a given, or have I just
    > been lucky so far? Perhaps it depends on the camera/lens? I'm
    > using an EOS 400d with the stock lens (for now - thinking of an
    > upgrade but I'm in no hurry).


    Trying to remember back to the days when I actually bought lenses for
    my SLRs, there are "true zooms" and "varifocal" lenses.

    A "true zoom" will maintain focus as you change focal length, a
    varifocal lens will not. Zoom lenses from different makers maintained
    focus in various proprietary ways, often moving the internal elements
    around with complicated cam actions. Zoom lenses were sometimes called
    "parfocal". Some zooms tried to maintain focus throughout the focal
    length and focus distance ranges, but didn't always make it with every
    combination.

    I don't know if there's any legal constraint on what kind of lens gets
    to be called a zoom lens, or if you might get a varifocal lens with
    that label.

    If your camera/lens combo is auto-focus, are you sure that's not what
    you're seeing?

    --
    Bert Hyman | St. Paul, MN |
     
    Bert Hyman, Jul 10, 2008
    #2
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  3. Poldie

    Poldie Guest

    Bert Hyman wrote:
    > _.@._ (Poldie) wrote in news:g55r03$en3$:
    >
    >> If I manually focus on something, then change the focal length, it
    >> seems that the subject is still in focus. This is handy, but can it
    >> be relied on? Is this something which is a given, or have I just
    >> been lucky so far? Perhaps it depends on the camera/lens? I'm
    >> using an EOS 400d with the stock lens (for now - thinking of an
    >> upgrade but I'm in no hurry).

    >
    > Trying to remember back to the days when I actually bought lenses for
    > my SLRs, there are "true zooms" and "varifocal" lenses.
    >
    > A "true zoom" will maintain focus as you change focal length, a
    > varifocal lens will not. Zoom lenses from different makers maintained
    > focus in various proprietary ways, often moving the internal elements
    > around with complicated cam actions. Zoom lenses were sometimes called
    > "parfocal". Some zooms tried to maintain focus throughout the focal
    > length and focus distance ranges, but didn't always make it with every
    > combination.
    >
    > I don't know if there's any legal constraint on what kind of lens gets
    > to be called a zoom lens, or if you might get a varifocal lens with
    > that label.
    >
    > If your camera/lens combo is auto-focus, are you sure that's not what
    > you're seeing?


    I'm a little new to this, so I'll have to read up on some of the stuff
    you just wrote (thanks!). But I'm talking about manual mode. In
    auto-focus then yes, it'll do it automatically when I half-press the
    shutter. It's just that the other day I was doing 15-30 second
    exposures of a candle in the dark, which required manual focus, and it
    was then that I noticed it.
     
    Poldie, Jul 10, 2008
    #3
  4. Poldie

    sarge137 Guest

    On Jul 10, 3:23 pm, Poldie <_.@._> wrote:
    > Bert Hyman wrote:
    > > _.@._ (Poldie) wrote innews:g55r03$en3$:

    >
    > >> If I manually focus on something, then change the focal length, it
    > >> seems that the subject is still in focus.  This is handy, but can it
    > >> be relied on?  Is this something which is a given, or have I just
    > >> been lucky so far?  Perhaps it depends on the camera/lens?  I'm
    > >> using an EOS 400d with the stock lens (for now - thinking of an
    > >> upgrade but I'm in no hurry).

    >
    > > Trying to remember back to the days when I actually bought lenses for
    > > my SLRs, there are "true zooms" and "varifocal" lenses.

    >
    > > A "true zoom" will maintain focus as you change focal length, a
    > > varifocal lens will not. Zoom lenses from different makers maintained
    > > focus in various proprietary ways, often moving the internal elements
    > > around with complicated cam actions. Zoom lenses were sometimes called
    > > "parfocal". Some zooms tried to maintain focus throughout the focal
    > > length and focus distance ranges, but didn't always make it with every
    > > combination.

    >
    > > I don't know if there's any legal constraint on what kind of lens gets
    > > to be called a zoom lens, or if you might get a varifocal lens with
    > > that label.

    >
    > > If your camera/lens combo is auto-focus, are you sure that's not what
    > > you're seeing?

    >
    > I'm a little new to this, so I'll have to read up on some of the stuff
    > you just wrote (thanks!).  But I'm talking about manual mode.  In
    > auto-focus then yes, it'll do it automatically when I half-press the
    > shutter.  It's just that the other day I was doing 15-30 second
    > exposures of a candle in the dark, which required manual focus, and it
    > was then that I noticed it.


    Back in the day when I used film cameras without auto-focus I found
    that my Canon zoom lenses did a pretty good job of maintaining focus
    over their range, and still do. But, because I'm a belt and
    suspenders kind of guy, when I had time I'd touch up the focus after
    zooming. These days I rely almost exclusively on the auto-focus (my
    eye sight and reflexes aren't as good as they were 35 years ago). In
    a situation where I had to use manual focus with a zoom lens I'd
    probably revert to my old habit.

    If you're using the kit lens the zoom range isn't that great, and even
    wide open the depth of field isn't that shallow. Your manually
    focused images are probably within acceptable, if not precise, focus
    over the zoom range of the lens. Having said that, I'd personally
    refocus after zooming.

    Regards,
    Sarge
     
    sarge137, Jul 10, 2008
    #4
  5. Poldie

    sarge137 Guest

    On Jul 10, 3:23 pm, Poldie <_.@._> wrote:
    > Bert Hyman wrote:
    > > _.@._ (Poldie) wrote innews:g55r03$en3$:

    >
    > >> If I manually focus on something, then change the focal length, it
    > >> seems that the subject is still in focus.  This is handy, but can it
    > >> be relied on?  Is this something which is a given, or have I just
    > >> been lucky so far?  Perhaps it depends on the camera/lens?  I'm
    > >> using an EOS 400d with the stock lens (for now - thinking of an
    > >> upgrade but I'm in no hurry).

    >
    > > Trying to remember back to the days when I actually bought lenses for
    > > my SLRs, there are "true zooms" and "varifocal" lenses.

    >
    > > A "true zoom" will maintain focus as you change focal length, a
    > > varifocal lens will not. Zoom lenses from different makers maintained
    > > focus in various proprietary ways, often moving the internal elements
    > > around with complicated cam actions. Zoom lenses were sometimes called
    > > "parfocal". Some zooms tried to maintain focus throughout the focal
    > > length and focus distance ranges, but didn't always make it with every
    > > combination.

    >
    > > I don't know if there's any legal constraint on what kind of lens gets
    > > to be called a zoom lens, or if you might get a varifocal lens with
    > > that label.

    >
    > > If your camera/lens combo is auto-focus, are you sure that's not what
    > > you're seeing?

    >
    > I'm a little new to this, so I'll have to read up on some of the stuff
    > you just wrote (thanks!).  But I'm talking about manual mode.  In
    > auto-focus then yes, it'll do it automatically when I half-press the
    > shutter.  It's just that the other day I was doing 15-30 second
    > exposures of a candle in the dark, which required manual focus, and it
    > was then that I noticed it.


    Back in the day when I used film cameras without auto-focus I found
    that my Canon zoom lenses did a pretty good job of maintaining focus
    over their range, and still do. But, because I'm a belt and
    suspenders kind of guy, when I had time I'd touch up the focus after
    zooming. These days I rely almost exclusively on the auto-focus (my
    eye sight and reflexes aren't as good as they were 35 years ago). In
    a situation where I had to use manual focus with a zoom lens I'd
    probably revert to my old habit.

    If you're using the kit lens the zoom range isn't that great, and even
    wide open the depth of field isn't that shallow. Your manually
    focused images are probably within acceptable, if not precise, focus
    over the zoom range of the lens. Having said that, I'd personally
    refocus after zooming.

    Regards,
    Sarge
     
    sarge137, Jul 10, 2008
    #5
  6. Poldie <_.@._> wrote:
    >If I manually focus on something, then change the focal length, it seems
    >that the subject is still in focus. This is handy, but can it be relied
    >on?


    No.

    >Is this something which is a given, or have I just been lucky so
    >far?


    No and yes.

    >Perhaps it depends on the camera/lens?


    No and yes.

    >jue
     
    Jürgen Exner, Jul 11, 2008
    #6
  7. On Jul 10, 3:21 pm, Poldie <_.@._> wrote:
    > If I manually focus on something, then change the focal length, it seems
    > that the subject is still in focus.  This is handy, but can it be relied
    > on?  Is this something which is a given, or have I just been lucky so
    > far?  Perhaps it depends on the camera/lens?  I'm using an EOS 400d with
    > the stock lens (for now - thinking of an upgrade but I'm in no hurry).
    >
    > Cheers.


    There are a lot of lenses around, so it is not possible to make an
    unqualified statement but most these days do a pretty good job of
    holding focus.

    Keep in mind that if you focus at extreme longest focal length and
    then back off, you are unlikely to notice any slight shift in focus
    because fine details are lost in lower angular resolution, so the old
    rule of thumb is focus fully zoomed in, then back off to desired focal
    length. I have not had a problem in recent years doing this with
    several new lenses.
     
    Don Stauffer in Minnesota, Jul 11, 2008
    #7
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