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autofocus modes on the Canon 1D Mark III

Introduction:

  • Canon is generally regarded as being the leader in AF technology and this is the main reason it's film cameras became more popular than Nikon ever since the early 1990's, and remains so in the digital SLR world.
  • Canon EOS dSLR cameras use TTL-SIR (through-the-lens Secondary Image Registration) auto-focus mechanism
  • the Canon 1D Mark III's auto-focus mechanism consists of:
    • light passes THROUGH the middle of the main mirror and is deflected DOWNWARDS via a curved secondary mirror attached to the rear of the main mirror.
    • light then passes forwards and down to a flat mirror at the base which reflects the light backwards through IR blocking filter then a secondary image-forming lens then onto the AF sensor chip.
    • the secondary image-forming lens is optimised for f/2.8 and f/5.6 lenses.
    • the AF sensor chip has:
      • central crosshair sensor works as crosshair aperture f/4 or brighter or horizontal only if aperture f/8-f/4
      • 18 crosshair sensors
        • work in crosshair mode if aperture f/2.8 or brighter and in horizontal mode if aperture f/5.6 - f/2.8
        • these cross-type sensors are no longer clustered centrally as in Mark IIN and earlier, but are in both inner and outer rings.
      •  26 AF assist sensors that work in horizontal mode only if apertures f/5.6 or brighter.
        • unlike with the 5D, the assist points also work in One-Shot mode when camera set to auto AF point selection or if focus point expansion is on and a cross-hair point is selected.
      • AF sensitivity -1EV to 18EV (twice as sensitive as the Mark IIN)
    • AF thus not available for Live Preview as the mirror is in the up position.

Focusing with the 1D Mark III:

using AF EOS lenses:

  • 19 user-selectable AF points plus additional 26 AF assist points with centre AF point usable down to f/8
  • when adjusting MF yourself, hold shutter half down & the camera will beep and centre AF point will light up if the centre point subject is in focus, this works with all lenses that have AF-confirm (ie. all EOS lenses and MF lenses such as Olympus OM when used with a AF-confirm OM-EOS adapter).
  • you can change AF modes:
    • Single shot vs AI-Servo for continuous action shots
    • all AF points mode vs a selected AF point mode
  • all AF points mode:
    • camera uses an algorithm to determine which subject lying on one of the AF points it will select to be put into focus
    • this appears to choose the closest subject with most brightness and contrast - unfortunately, this will probably not be a person's face, but as this mode is designed for action work then maybe this is OK.
    • in One-Shot Mode with auto AF point selection:
      • as with the 5D & 30D, the lower AF points are eliminated, but different to previous models, the AF point is then chosen to be the closest subject.
    • in AI-Servo mode with auto AF point selection:
      •  the subject is initially acquired using the centre AF point and subsequently tracked by all 45 AF points.
    • Canon recommend using centre AF only when using the 70-200mm f/2.8 with a tele-extender as other points may cause errors (see their manual).
    • predictive AF in AI-Servo mode should be able to track a subject moving towards the camera at 31mph until it gets to 26 feet away using a EF 300mm f/2.8L lens at 10fps. (similar performance to the Mark IIN but this tracked at 8.5fps).
    • note: it cannot predict what you are thinking and it cannot yet recognise faces to prioritise.
    • AF bugs:
  • selected AF point mode:
    • this is what I prefer for relatively stationary subjects as the all AF points mode is way to frustrating for general use
    • if I am hand holding the camera then I choose the centre AF point and move the camera so it falls on the subject then I lock the focus and re-compose.
    • if I am using it on a tripod and moving the camera around is too onerous, then you have the option of selecting a different AF point to use - just hope your subject lies in the central half or so of the viewfinder as there are no AF points outside this region.
    • choosing your AF point to use:
      • in shooting mode, press top right button (zoom in or cross button) to put SEL [] mode in top LCD panel
      • then select AF point by using either the half dial (moves left/right or ALL AF mode) or rear dial (adjacent AF points to current)
      • clicking on the rear toggle button allows toggling between centre AF point and ALL AF mode
    • locking AF:
      • must be in ONE-SHOT AF mode
      • either:
        • keep finger half-pressed on shutter, or,
        • keep finger on AF-ON button (this allows shutter half-press to meter at different spot to AF if recompose)
        • disable shutter button setting AF and use AF-ON button only to set AF (my preference)
          • set C. Fn IV.1 to setting 3
          • NB. you can also lock AE by pressing the adjacent star button (zoom in button) - keep it pressed if you want AE locked for more than one exposure. In spot metering mode, press FEL button for each spot metering (but setting only kept for 16secs and will be cancelled if you take finger off the shutter button after an exposure).
  • micro-adjustments to AF calibration:
    • one rather unique feature is the ability to adjust the back-focus or front-focus for each EOS lens you use in case that lens always focuses a little out (now this is an important option) or if you want to do it for a specific purpose such as focus on bridge of nose and have the setting actually focus a little back so the eyes will be in focus - who would really bother??
      • strangely, it seems Canon EF lenses that need micro-adjustments with the Canon 1D Mark III apparently work OK on older models such as the 1D Mark II, 5D, 30D without such need for micro-adjustments - did the Canon team feel that just because they provide this capability they can now skimp on quality control of calibrating accurate focus?
    • see below for more details.

using MF lenses with AF-confirm adapters:

  • camera will beep and centre AF point will light up if the centre point subject is in focus if you are holding shutter half down.
  • eg. Olympus OM or Nikon F lenses with AF-confirm EOS adapters (buy them on Ebay).

using lenses in MF without AF-confirm or when AF is unreliable:

  • accurate MF is practically impossible through the viewfinder with the supplied focusing screen, thus you have 2 options:
    • use Live Preview:
      • this is brilliant and should be on ALL cameras as a backup for accurate MF
      • unfortunately there is no Live Preview BOOST mode as with Olympus cameras so focusing through R72 IR filters becomes very difficult, and I have yet to test it on stars.
    • alternatively one could buy a split image focusing screen to assist manual focus

Custom functions that relate to auto-focus:

C. Fn III Autofocus/ Drive functions:

  • 1. USM lens electronic MF:
    • 0 = enable after One-Shot AF
    • 1 = disable after One-Shot AF
    • 2 = disable in AF mode
  • 2. AI servo tracking sensitivity  (NEW) slow - normal - fast
    • this determines how quickly AF moves from your subject to another subject (eg. foreground obstacle)
    • slow setting for subjects rapidly moving in and out of AF points eg. slalom skiing, breaststroke swimming
    • for birding with a clear sky background, many use fast setting.
    • avoid fast setting with small subjects with busy backgrounds
  • 3. AI Servo 1st/2nd image priority
    • It deals with two issues: 
      • how quickly the camera will fire if you suddenly press the shutter button fully, and,
      • if the shutter button is held down for continuous shooting, whether AI servo AF will always take time to ensure correct focus for each shot (which may result in slowing down the drive speed), or whether the camera will always fire at top fps speed (even if proper focus cannot be ensured for each shot in the sequence)
    • 0 = AF priority / Tracking priority
      • focus priority for 1st and subsequent shots
      • useful for car races:
        • Since race cars move very quickly, if the lens cannot track the car quickly, it will not achieve correct focus.
        • From the 2nd shot on, priority is given to subject tracking instead of continuous shooting speed to give the camera more lens-driving time to achieve AF.
      • seems to be best mode for bird photography
    • 1 = AF priority / Drive speed prior
      • focus priority for 1st shot then continuous shutter speed priority after that
      • best suited to sports like football and sports at night - in low light, the lens requires more time to achieve focus so:
        • for the first shot, priority is given to gaining focus. 
        • for the second and subsequent shots, priority is given to shooting speed to capture the moment.
    • 2 = Release / Drive speed priority
      • shutter has priority irrespective if focus achieved
      • often used by press photographers who need to capture the instant even if focus not achieved.
      • seems to give poor in focus rate for bird photography though
  • 4. AI Servo AF tracking method
    • “Main AF point” = centre AF point unless manually selected AF point
    • This custom function requires two other settings:
      • Automatic AF point selection
      • AF expansion with C.Fn III-8
    • 0 = Main focus point priority
      • camera will start focusing on the closer subject that entered the picture. When the center AF point detects that closer subject, the focusing priority will then shift to the main AF point.
    • 1 = Continuous AF track priority
      • the camera will ignore any closer subject entering the picture. The focus-tracking immediately before the obstacle entered the picture will continue, and the AF point can shift to any other point while tracking the original subject instead of being fixed at the main AF point. 
      • useful for football, slalom skiing when other players or obstacles may enter the foreground and need to be ignored.
  • 5. Lens drive when AF impossible
    • 0 = focus search on
    • 1 = focus search off
      • useful for super telephotos to prevent them becoming extremely out of focus
      • when using this mode, try pre-focusing manually then hit the shutter when needed to activate AF
  • 6. Lens AF stop button function
    • 0 = AF stop
    • 1 = AF start
    • 2 = AE lock
    • 3 = AF point: M ⇒ Auto / Auto ⇒ctr
    • 4 = ONE SHOT / AI servo mode toggle
  • 7. AF microadjustment (NEW)
    • 0 = disable
    • 1 = adjust all lenses by the same amount
    • 2 = adjust by individual lens - see below for more details
  • 8. AF expansion with selected AF point (NEW)
    • expansion enabled makes AF on a fast moving small subject easier BUT risks focusing on an adjacent object such as grass in front of a ball - thus choose this depending on speed/size of subject and presence of obstacles.
    • 0 = disable
    • 1 = enable left/right assist points
    • 2 = enable surrounding assist pts
      • especially good for low contrast subjects or in low light
  • 9. Selectable AF point (NEW)
    • choosing 9 points allows manual selection of an AF point quicker.
    • 0 = 19 points
    • 1 = inner 9 points plus centre point
    • 2 = outer 9 points plus centre point
  • 10. Switch to registered AF point
    • 0 = disable
    • 1 = enable
      • you can register a AF point you use frequently by selecting it then pressing zoom out + ISO button simultaneously.
      • this AF point then becomes the point the rear toggle button defaults to when pressed.
  • 11. AF point auto selection
    • determines if rear dial &/or half dial can be used to select auto AF point selection
    • 0 = rear dial direct: disable / half dial: enable
    • 1 = rear dial direct: disable / half dial: disable
    • 2 = rear dial direct: enable / half dial: enable
  • 12. AF point display during focus
    • 0 = on
    • 1 = off
    • 2 = on when focus achieved
  • 13. AF point brightness
    • 0 = normal
    • 1 = brighter
  • 14. AF-assist beam firing
    • 0 = enable
    • 1 = disable

C. Fn IV Operation functions:

  • 1. Shutter button / AF-ON button functionality
    • 0 = metering + AF start
    • 1 = metering + AF start /AF stop
      • during AF, you can press AF-ON to stop autofocusing
    • 2 = metering start/Meter + AF start
      • useful for subjects which keep start/stop moving
      • in AI-Servo mode, you can press AF-ON to repeatedly start or stop the AI-Servo AF operation and exposure set when photo taken
    • 3 = AE lock/Metering + AF start
      • this is similar to C. Fn IV-1 in previous cameras and allows back button focusing which is what I prefer but confuses people who expect the shutter button will activate AF.
    • 4 = Metering + AF Start / disable
  • 2. AF-ON / AE lock button switch
    • the functions of the AF-ON button and AE lock button (star button) are switched.
  • 3. Quick control dial (rear dial) in meter function
    • 1 = Exposure comp (aperture if Manual metering mode) 
    • 2 = AF point selection
    • 3 = ISO speed
  • 4. SET button function when shooting
    •  
  • 11. installed focusing screen type

Calibrating your lens AF using micro-adjustment function:

  • surely you don't need to do this, you just AF and it all works - wrong!
  • check your lenses and you may well find as I did that your brand new, expensive 135mm f/2.0L EF lens is soft wide open because it has a back focus problem with this camera body.
  • my lens was so soft, I felt like throwing it in the bin until I did some tests and found I could manually focus it MUCH sharper, then the penny dropped - I need to micro-adjust it - so here's how you do it:
  • set your camera and lens on a sturdy tripod and aim at a textured object say 3m away
  • ensure maximum sharpness by using self timer and if you want to be pedantic, set mirror lock up.
  • set AF to only use the centre AF point and AF on the subject and take the photo
  • replay the image and go to maximum zoom in to view the textured or target object to assess it for sharpness
  • now incrementally repeat the following steps to get the sharpest image
    • NB. on playback, this camera has a way cool feature that by rotating the circle dial, it will display previous images at the same zoom making it so much easier to compare each image.
  • go to Menu and select C.FnIII : 7 and select 2. Adjust by lens and press SET to select it
  • adjust the microadjustment by 1 increment (mine needed setting 3 in the backwards direction) and press INFO to set it.
  • retake the image after setting AF again and view the image to compare
  • continue doing this until you get the sharpest image.
  • important points:
    • the adjustment is stored for a model of lens so if you have more than one lens of that model, you are out of luck as camera does not recognise serial numbers of lenses to differentiate them.
    • a different setting is used if you use a tele-extender either 1.4x or 2x with the lens - guess you have to set a setting for each combo.
    • can only set one adjustment for a zoom lens, so usually best to do the tele end only.
    • you can register up to 20 lenses

Canon 1D Mark III AF for sports/wildlife photography:

  • this is the raison d'etre for the existence of the Canon 1D Mark III - its ability to take pro quality sports/action photos.
  • Canon made some changes in the way AF works in this camera, including:
    • using area AF mode, it now chooses the closest AF point instead of the greatest number reporting sharp focus
  • many seem to recommend the following for most team sports:
    • center point only:
      • ALL AF pts area auto focus point selection useful only for birds in flight or perhaps track sports where no closer subject intervenes
      • unfortunately, choosing centre point means either:
        • you need to keep your subject in the centre - boring composition, or,
        • you need to lock the focus by half-press shutter + star button +/- AF button but not live AF anymore.
    • AI servo mode
    • CFIV-3 to activate AF with the * button
      • That takes a little getting used to, but once you do it makes it a lot easier to lead the AF before hitting the shutter, and to keep tracking the subject with AF on while hitting the shutter periodically.
      • allows you to effectively use AI servo as one-shot to lock focus and recompose, just by focusing with * and then releasing it to lock focus, e.g., for reaction shots of the bench.
      • CFIV-3 gives you a new AE reading for each frame in a burst. This is useful when you're tracking players in and out of sun and shadow.
      • CFIV-1 locks AE at the half shutter press, so every frame in a burst gets the same exposure.
  • some recommend following for birding:
    • Custom Function III:
      • 2 fast
      • 3-0 AF priority/tracking priority
      • 4-0 Main focus point priority (I know its not what you'd expect)
      • 5-0
      • 8-2
    • If the background is not busy then set 8 to disable and then enable all 45 AF points so the camera can use all AF points to track once you lock on with center.

 

 

photo/canon1diii_af.txt · Last modified: 2011/10/03 12:12 by gary