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Thread: Focus Lock

  1. #1
    bisso7's Avatar
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    Focus Lock

    I don't quite understand something about focus lock on my Canon T3. The manual talks about locking the focus, and then "recomposing the shot." What I don't understand is, if you lock the focus in on your original composition, and then recompose the shot, would not recomposing alter your focus from the original that is "locked" into the camera??? I don't get it! Please help. LOL

  2. #2

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    Re: Focus Lock

    Yes it does. It's usually not noticeable, but is most noticeable when using 35mm or 50mm, f/1.8 lenses at f/2.8 or so because the lens is sharp enough and DOF shallow enough to exhibit the problem.

    The solution is to shift the focus point instead.

  3. #3
    dje's Avatar
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    Re: Focus Lock

    Hi Jeff

    In AF mode, the T3 has 9 focus points. If you have focus point selection set to manual, then you can select any one of these as the focus point. If you are set up on a tripod, this is easy to do although 9 focus points is a bit limited (compared to the higher end cameras which have a lot more focus points). If you are shooting handheld, you can just use the centre focus point and shift the camera alignment so the centre focus point is on the subject you want to focus on, press the shutter halfway to lock in the focus, and shift the camera back to the original alignmnent and press the shutter fully to take the shot.

    Here is an example. The first shot was taken without shifting the camera for focusing and it has focused on the distant section of the image. The second shot was taken by first changing the alignment of the camera down so the centre focus point was on the fence pillar, half pressing the shutter to lock the focus on the fence pillar, then angling the camera up to it's original position to take the shot.

    Hope this clarifies things.
    Dave

    PS Click on image to open up Lightbox and get a larger view.

    Focus Lock

    Focus Lock

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    William W's Avatar
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    Re: Focus Lock

    Another example.

    Focus and Recompose is often used for Portraiture.
    In the shot below the Centre Point AF was used to Focus on the Subject, Focus was then Locked and the shot Recomposed and then the Shutter was released.
    Focus Lock

    If the Centre AF Point had been used for this shot (i.e. using the centre AF Point as it is aligned in the final composition), the Subject would be Out of Focus and the Background would be In Focus . . . and that was not required.

    There are sometimes reasons not to use the outer AF points and although the LH outer AF points could have been used to achieve Focus in this example, the Photographer chose to use the Centre AF Point and Focus and Recompose, because he was shooting in Available Light and the Scene was a low EV (Low Light) and the Outer AF points on most DSLRs are less accurate, especially in Low Light.
    Even though the Aperture was fairly large; the Framing quite tight and the Shooting Distance quite short, there was never any doubt that the Recomposing would move the Plane of Sharp Focus enough, to hinder the sharpness of the Subject.

    If one uses the Focus and Recompose technique enough, one becomes accustomed to when there is danger of moving the Plane of Sharp Focus enough, such that the Subject’s Sharpness will be affected.

    Key considerations for those danger signals are usually a combination of two or more of any of these factors:

    > Using very large apertures. (i.e. small F-numbers, like F/1.4~F/2.8)
    > Using very short Shooting Distances. (i.e. tight framing of the Subject)
    > Using Larger Format Cameras. (i.e. like a 5D, compared to a Point and Shoot)
    > Using a great distance across the scene, for the Recompose.
    > Using a Pan or Tilt of the Camera Body to Recompose (i.e. ‘swivelling’); rather than Dollying (moving the camera parallel to the Subject).

    WW

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    Re: Focus Lock

    I have yet to learn how focus lock works on any of my cameras [ I have a rough idea obviously ] However I have been using it from very early days of digital by pointing the camera at the object I want sharp and taking half trigger. Then holding half trigger I re-frame for the shot I want and fully press the trigger. The danger in this practice, which I guess is why FL was invented, is that if you are also using AE the exposure for the object may not suit the shot you are taking.

    The companion tool is Exposure Lock and I also frequently use this to avoid burning out highlights with of course the danger that AF may pick an unsuitable focus when I tilt the camera up to include more of the sky etc.

    The reason I have not bothered with EL/FL is becuase having learnt the half-trigger practice without these modern gadgets I have yet to strike an situation where I need them.... obviously there will be a frantic scratching of my head when I do strike that situation

    My current most used camera doesn't have just nine points but anywhere on the touch screen Even the T4i cannot do this.

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    bisso7's Avatar
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    Re: Focus Lock

    Thanks to all for taking the time to explain how FL works. It's been VERY helpful to me, especially including the photos with the explanation. Also good to know that FL is used most often in portraiture, which I never knew. I'll be traveling to Florida the first week in July to visit family, so this knew "revelation" on FL just may be useful.

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    Moderator Dave Humphries's Avatar
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    Re: Focus Lock

    Quote Originally Posted by William W View Post
    > Using a Pan or Tilt of the Camera Body to Recompose (i.e. ‘swivelling’); rather than Dollying (moving the camera parallel to the Subject).
    Hi Bill,

    Blindingly obvious, but of course, where you can; a 'sidestep' to recompose has to be better than standing in place and panning (what I usually do - but usually only at distances and DoFs where it doesn't matter)

    I must remember this one for shots like your example.


    Quote Originally Posted by jcuknz
    The companion tool is Exposure Lock and I also frequently use this to avoid burning out highlights with of course the danger that AF may pick an unsuitable focus when I tilt the camera up to include more of the sky etc.
    I guess an alternative to worrying about exposure lock or messing with EC is simply to shoot "Manual" - having set the exposure before taking the shot, it won't change when you half press or re-compose. (the meter will, but who cares).

    Cheers,

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    Re: Focus Lock

    For me, recomposing on a shot is usually due to speed and ease of use. Although it only takes a second or two to move the focus to another sensor, it's just easier and rarely does it make much of a difference. Unless you are photographing close subjects and/or use large apertures you wont notice. Be safe and check the DOF information for each of your own lenses. A bit of an error here above. Moving the camera laterally to the subject is going to alter the focus more than rotating the camera. Stick to proper panning techniques.

  9. #9
    Moderator Dave Humphries's Avatar
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    Re: Focus Lock

    On the DSLR, I will move the focus point as long as things that Bill mentioned aren't going to cause problems, but I now also shoot with two other cameras that only have a central point, making re-composing necessary.

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    William W's Avatar
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    Re: Focus Lock

    Quote Originally Posted by Dave Humphries View Post
    Originally Posted by William W

    > Using a Pan or Tilt of the Camera Body to recompose (i.e. ‘swivelling’); rather than Dollying (moving the camera parallel to the Subject).


    Blindingly obvious, but of course , where you can; a 'sidestep' to recompose has to be better than standing in place and panning (what I usually do - but usually only at distances and DoFs where it doesn't matter)
    Thank you.

    Yes.

    Confirming that the Dolly Technique for Recomposing is only necessary when shooting at short SD and when there is a narrow DoF.

    Typical examples of indicative range for the usefulness and need to employ the technique:

    Tight Head Shot to Head Shot using 24mm to 85mm Focal Length Lens (on a 135 Format Camera) and shooting between F/1.0 ~ F/4.

    We practice the exercises in my workshops: I'd suggest practicing, remember that you might have to recompose vertically.

    ***

    Quote Originally Posted by Dave Humphries View Post
    Originally Posted by jcuknz

    [I have yet to learn how focus lock works on any of my cameras] . . . The companion tool is Exposure Lock and I also frequently use this to avoid burning out highlights with of course the danger that AF may pick an unsuitable focus when I tilt the camera up to include more of the sky etc.

    I guess an alternative to worrying about exposure lock or messing with EC is simply to shoot "Manual" - having set the exposure before taking the shot, it won't change when you half press or re-compose. (the meter will, but who cares).
    Most DSLRs allow the SEPARATION of the Exposure Lock Function in an Automatic Acmera Mode and the Focus Lock from BOTH Functions being on the SAME (half depress) Shutter Release.

    Most commonly the Auto Focus is moved to a secondary control button, other than the Shutter Release.

    Learning how Focus Lock works, would benefit: the separation of these functionalities is usually explained quite fully in the Camera's User Manual.

    WW

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