how close is infinite focus?

Discussion in 'Digital Photography' started by Lazarus Long, Aug 9, 2004.

  1. Lazarus Long

    Lazarus Long Guest

    I'm a relative newbie to most of the features of a "modern" camera.

    My question is about focusing - how close, is infinite focus. My
    camera is a Coolpix 5400. 20 feet? 30 feet?

    I was at an air show this past weekend. With my film camera I would
    have simply twisted the lens to the infinite focus and left it there.
    However, with this new digital autofocus, I let the camera pick the
    focus. Well, for a lot of the shots of the planes overhead, they're
    not that well focused. I wrongly thought the camera would
    automatically go to infinite focus. So after looking around a bit
    (including the manual) I note that AF systems are confused by
    relatively featureless backgrounds (sky). So if I had manually set
    infinite focus, how close to the camera would something appear to be
    in focus?
    Lazarus Long, Aug 9, 2004
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  2. Depends on the lens. Bear in mind that the greater depth of field because of
    the smaller than film sensor makes it almost moot if you are out of doors.
    Gene Palmiter, Aug 9, 2004
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  3. You are right that the autofocus in a digicam doesn't work on a featureless
    field. It would certainly work to set the focus at infinity at an air show
    or a fireworks display.

    You had a good learning experience. The quick feedback with a digicam is
    going to help you to learn more.
    Marvin Margoshes, Aug 9, 2004
  4. Lazarus Long

    skymuffins Guest

    Use the "sports" scene mode. With most cameras that allows the autofocus to
    lock on to a moving subject.

    (sorry if this has already been suggested... I haven't caught up on this
    group in a while.)
    skymuffins, Aug 9, 2004
  5. Lazarus Long

    Lazarus Long Guest

    Good suggestions - sports mode or simply infinite focus. I've also
    thought about the cameras "landscape mode" too.

    But my basic question still stands - how close is "infinite focus"? I
    know aperture plays a role in apparent sharpness, but focus is focus.
    Lazarus Long, Aug 9, 2004
  6. The cute answer is that "infinity focus" is focussed on things
    infinitely far away.

    The helpful answer - though it may not seem like it - is this. When your
    camera is focussed at infinity, objects considerably closer than this
    will appear to be sharp. (This is because the amount of "fuzziness" -
    the diameter of the circle of confusion - is too small to be seen). The
    bad news is that the limit (i.e. how close they get and still appear
    sharp) depends on the aperture of your camera lens, and its focal
    length; it also varies according to what you propose to do with the
    picture (small web picture to huge enlargement - they have different
    demands on sharpness). This zone for which objects appear sharp is known
    as depth of field (DoF).

    With 35mm film cameras, it is easy to find tables (in books, on the web
    etc) which will tell you DoF details for any common focal length. For
    example, my 50mm f/1.4 lens shows that at f/22 I might expect everything
    from 3m to infinity to be in focus at the infinity setting.

    With lenses of longer focal length, the DoF for any lens aperture gets
    very much smaller; conversely, for wide angle lenses, it is larger. Thus
    if you take a portrait with a long lens at wide aperture, DoF is so
    small it is often impossible to get both eyes in sharp focus unless the
    subject is dead square to the camera. OTOH, if you use a very wide lens
    for a landscape at f/16, you can often get everything from a rock 1m
    away to the distant mountains in sharp focus.

    There is a slightly better way than setting the camera focus at
    infinity, and that is to set it (in my above example with the 50mm f/1.4
    lens) to focus at 3m, then everything from about 1.5 m to infinity
    should be sharp. This setting is called the "hyperfocal distance". For
    any lens, at any particular aperture, there is such a setting which
    gives you maximum DoF, from about half the hyperfocal distance to

    Unfortunately, because these numbers are different for every focal
    length of lens and every aperture, I can't tell you what they are. You
    will have to do some research yourself, either to work it out (the maths
    is not that difficult) or to find a suitable table.

    One cautionary word: whilst you may be used to seeing the focal lengths
    of the lenses of digital compacts being quoted as "35mm equivalent"
    numbers - e.g. 35 - 135 or 28 - 80mm - these are a complete trap for
    this purpose. You will have to use the actual focal length of the lens,
    and the real sensor size, to do the maths.

    In the end, it may be easier to find the answer empirically. Even if you
    calculate or find a figure, you still have to test it - it will vary, as
    I said above, with the use to which you will put the pictures. So, set
    the camera to manual, find some regularly spaced objects (lamp posts or
    telegraph posts spring to mind) and get to it. You will find a setting
    which will work well for your purpose. I'd guess 20m at f/8 will be fine
    - but please come back and tell us yea or nay.

    At least (being digital) it will be quick and cost nothing except your

    BTW - I don't want to rain on your parade, but I would imagine that
    digital compacts are about the worst possible type of camera for air
    displays. The shutter lag would make it impossible to catch the critical
    moment and the focal length at the longest end inadequate for any but a
    large formation. If you can't afford a DSLR, a film SLR (with a zoom up
    to at least 300mm) would give much better results at reasonable price.

    David Littlewood, Aug 9, 2004
  7. David Littlewood wrote:
    For much action phtotography, the secret is to half press the shutter to
    lock focus and exposure - keep your finger on the button and press at the
    critical moment. Zoom up to 420 mm at f/2.8 with image stabilisation is
    available in cameras such as the FZ10:

    at a considerable saving on the price of the DSLR equivalent. So you pays
    your money and takes your choice.....

    David J Taylor, Aug 9, 2004
  8. Infinite focus means just that. If you set the camera to infinity, then
    the objects in best focus is something so far away that the rays of
    light reaching the camera are parallel - a distant landscape for

    Closer objects will be less sharp, though *how much* less sharp depends
    on focal length and aperture. This is where "depth of field"
    computations come into play.
    No, it probably picked some random value that seemed best to it (without
    a real target to focus on). With my cameras, at least (Canon), they
    light a LED and draw a different-coloured box on the LCD to tell you
    that they failed to focus, though you can force the camera to take a
    picture anyway in that state.
    Do a web search for "depth of field" for the answer to that question.
    But with the very short focal length lenses on most digicams, probably
    anything that was still in the air would be far enough away for good

    For this type of photography, switching to manual focus and setting
    infinity is the right approach.

    Dave Martindale, Aug 9, 2004
  9. Lazarus Long

    Ron Hunter Guest

    For practical purposes, anything over 50 away from an ordinary sized
    lens will be 'infinite' focus.
    Ron Hunter, Aug 9, 2004
  10. David L has the most complete and totally accurate answer to your

    I will try to have the shortest accurate answer:

    Infinity is infinity, but it is safe to say that any aircraft flying
    that you are photographing at an air show will be as sharp as they can get
    with the camera focused on infinity. You best bet is to lock the focus on

    Motion (yours and the aircraft's) is a common cause of less than ideally
    sharp images at such events.
    Joseph Meehan, Aug 9, 2004
  11. Yepp - I have also been irritated with the strange behaviour
    of digital cameras to lock on any focus when they cannot focus.
    It is more logical to move to infinity.

    Now - how near is anything sharp if you focus on infinity? Very
    near with your camera in bright sunlight. The depth of focus (DOF)
    of small sensor digital cameras is very large. So - it is totally
    safe to put the camera on infinity and shot at an air show.

    Roland Karlsson, Aug 9, 2004
  12. Lazarus Long

    Frank ess Guest

    What is the longest focal length of the CP5400? How much of the frame
    did your aeroplanes take up? It seems to me they would not be very
    large, and the camera may have focused on them but spread them over too
    few pixels to give an impression of sharpness.

    Are you sure the blur was focus-related? You might have good focus but a
    very slight motion of the camera during exposure could have a blurring
    effect, not easily distinguishable from focus blur.

    Truth be told, a digital point-and-shoot, even such advanced ones as CPs
    5400, 5700, and 8700 are not the best choice for action photography. It
    can be done, but not as easily as with a good film camera or a dSLR.

    I really enjoyed the reactions after explaining to some reputedly
    sophisticated photo-makers that cranking the lens to Infinity was
    wasting a considerable zone of useful in-focus out there "beyond

    Just wanted to form up with the other Daves and Davids in this string...

    David (Dave) (Frank) ess
    Frank ess, Aug 9, 2004
  13. Lazarus Long

    Eddy Vortex Guest

    Be careful when setting focus on infinity. I use an Olympus C2100 UZ and to
    set the focus on infinity manually. I have to push the toggle button until
    the readout says infinity (and it will go no further) and then bring the
    focus back 3 clicks. So infinity is 3 clicks before the end of range. This
    is because your camera needs room to go past infinity when the autofocus is
    'hunting. Hope this helps. Eddy
    Eddy Vortex, Aug 9, 2004
  14. Lazarus Long

    Lazarus Long Guest

    Not very much, most often. There were ground displays from which I
    got some very sharp images, I doubt they're great photos, but they are
    No obvious motion blur, just an overall fuzzy quality. The beauty of
    shooting a lot of pics was that some did look o.k., though most
    Yeah, and things like this are why I still have my film SLR. Not
    "advanced" at all in terms of things like auto this or that. But it
    gets the job done for me.

    But I REALLY need to get familiar with this Coolpix 5400 before taking
    it somewhere where the results matter or will be difficult to
    impossible to return to.
    I understand hyperfocal distance. Well, probably enough for me.
    Lazarus Long, Aug 9, 2004
  15. This feature is sometimes found on long lenses to allow for thermal
    expansion of the barrel of the lens. If this expands, the helical focus
    mechanism would otherwise be unable to get the lens elements close
    enough to the sensor to give infinity focus.

    I would agree this is unlikely to be the reason here though. It is more
    likely to be that manufacturing tolerances are to wide and some
    allowance has to be made to ensure a safety margin.
    David Littlewood, Aug 9, 2004
  16. 'ow's about this then?

    1 - find a relatively nice plain - a sports field will do, any sports field
    or maybe even a long flattish road.

    2 - focus your camera on something in the distance - a couple a hunner yards
    will do (use the half press option on your camera if it help)

    3 - take the picture

    4 - do the above a couple of times

    5 - get the prints developed and pace oyt the distance from where you shot
    the pic (distance to first bit of sharp focus)

    6 - BONGO! - oops, that should be BINGO! you have solved the problem for
    your camera irrespective of all the theorising :)

    the Arty one
    Arty Phacting, Aug 9, 2004
  17. That's true, and it could be the answer; I assumed the poster (since he
    said he had experience of using a film SLR) had some knowledge of camera
    shake, but maybe not.

    In taking pictures of the Red Arrows last year I tried to catch the
    instant in which the opposition pair just passed each other. I got a
    shot with their noses ^just^ appearing to touch, but of course I had
    panned with one plane. It was sharp, but the other one was rather
    blurred. Quite effective in its way, but I would have liked another one
    with both planes sharp.

    David Littlewood, Aug 9, 2004
  18. ah - but are there infinitely many infinities?

    the Arty one
    Arty Phacting, Aug 9, 2004
  19. Lazarus Long

    scott Guest

    You said in another post that some photos were sharp and others weren't.
    Could this be due to using too long a shutter speed, not panning with the
    plane or camera shake? What shutter speed did you use? Don't forget a
    plane can move a few metres in 1/100 s which can make it tricky to get sharp
    images. Hey, I have problems with cars sometimes, and they are much slower!
    scott, Aug 9, 2004
  20. Lazarus Long

    Lazarus Long Guest

    Most these were prop jobs - aerobatic stuff including a biplane with a
    wing walker. Faster than cars I'm sure, but also not as fast as jets.
    The ground displays were a couple of helicopters and some guy had a
    jet powered surfboard (!) there.

    Some of the sharper images were of an F-15.

    One of the fuzziest was of a slow moving (for a plane) prop transport.

    I think the camera simply couldn't focus automatically. The sky was
    completely overcast - a smooth white field.
    Lazarus Long, Aug 9, 2004
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