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Thread: Focus trick?

  1. #1

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    Focus trick?

    I was wondering, because sometime I have problem to have a correct focus of my subject when I don't really want it to be center of the shot.

    So what I tried to do it, in "One shot" mode I point the center AF point on my main subject, wait till the focus is made, and without releasing the button, I point the shot how I want it and take it.

    Is that a correct way to focus on things I have problem to have a good focus?

  2. #2
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    Re: Focus trick?

    Hi Mathieu,

    Just googled your camera. It has a nine point autofocus system which means you'll be able to move the point around within your viewfinder. On Nikon cameras there is multi selector on the back of the body. I wouldn't imagine Canon to be that different. Hope this helps.

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    Re: Focus trick?

    This approach is called "focus then compose (or focus then frame)." It is a common approach, and a good one. The alternative, as indicated by the other poster, is to change the focal spot to one of the allowed auto-focus points that is nearest to your subject and focus using that. If your composition is really critical, you might combine the two methods and focus using the nearest point and recompose like you do now.

    So what are the issues with either approach? First, most cameras do not use as sophisticated a focusing method on some or all of the alternative focusing points as they do on the central one. Most cameras will focus using both horizontal and vertical data on the center point, but only horizontal or only vertical data (depending on position) with some of the outlying focal points. In very recent cameras, the number of focal points that use both horizontal and vertical data has increased significantly, but AFAIK, they don't use as thorough a data set for focusing on every point that they do on the center point. Your manual will be able to identify how many and which type of focusing points your camera has. My D5000 has 11 focusing points, "including one cross-type sensor." What that means is that my camera only uses both horizontal and vertical data for focusing with the center point. It is generally recognized that cross-type sensors are more accurate than the linear type, so you might well decide to use your focus and recompose method with the center point on a camera like mine. I should also mention that lenses are generally sharper in the center than away from the center, so you can get a better focus even with all cross-type sensors in the center than at the edges with many lenses.

    OTOH, when you focus then compose, you move the focus plane as you rotate to the actual place you want in focus (think of the focal plane as rotating around you as the central spoke.) As it happens, most focal planes aren't flat, so this focus then frame approach comes close to staying on-focus if you don't rotate too much. However, macro lenses normally DO have a flat focal plane, because macro photography has such a critical DOF. So, if you are using a macro lens for portrait work, you may find that the focus then frame approach gives you a noticeable focusing error.

    So, the upshot is that nothing is going to work right, so you might as well give up photography and take up a more determinate hobby like golf.

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    Re: Focus trick?

    ... or manual focus. I really don't care for golf.

    - Bill

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    Re: Focus trick?

    Personally, I don't like mutliple focusing points. I find them confusing and too often the wrong area gets selected.

    The simple focus, hold focus, and recompose (while just using the centre point) works fine for me.

    But one point to remember is that depending on your metering mode, when you recompose the exposure can change unless you lock it.

    Or do it manually as Bill suggests. Providing of course that the scene stays constant. I'm too slow at manual focusing for fast action shots like sports or flying birds, etc.

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    Re: Focus trick?

    Quote Originally Posted by Geoff F View Post
    Or do it manually as Bill suggests. Providing of course that the scene stays constant. I'm too slow at manual focusing for fast action shots like sports or flying birds, etc.
    I don't think this "trick" works on fast action stuff anyway... by the time you would lock focus and recompose, you'd miss the shot.

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    Re: Focus trick?

    That is where AI Servo works, Bill. Or whatever the option is callled in other camera brands.

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    Re: Focus trick?

    Quote Originally Posted by Geoff F View Post
    That is where AI Servo works, Bill. Or whatever the option is callled in other camera brands.
    Right - but AI Servo works only if your subject remains on that focus point. If you move the focus point away from the subject, the camera will begin hunting again.

    - Bill

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    Re: Focus trick?

    Thank for the answers guys.

    I'm happy to know that this trick work because I took many photo today using this

    Yes my camera has 9 AF point, 1 Cross Type. I could lock only one of these point or many, but using this method is sometime a bit too slow, depending on the subject so that's why I tried to focus-lock and take the shot

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    Re: Focus trick?

    Quote Originally Posted by Geoff F View Post
    The simple focus, hold focus, and recompose (while just using the centre point) works fine for me.
    It get's a bit tedious during a 800+ frame studio shoot though

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    Re: Focus trick?

    Was wondering, can I do the same thing for Spot metering to adjust a correct exposure based on what I want the most to be correctly exposed?

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    Re: Focus trick?

    I'm not familiar with the T3 controls, but my Nikon has a button for AE/AF lock. You can configure this buttton in various ways. If you use the focus-then-frame approach to focus, a natural thing to do is configure this button for exposure lock (at least on my camera this can be done as a toggle, where pressing once locks and pressing again unlocks, or as a press-and-hold lock. I use press-and-hold because I assume I'd get the toggle state wrong as often as not...) When using spot metering, you can press this button to lock the exposure you want, then focus where you want, then frame and shoot. It sounds more awkward than it is. Consult your manual to see if you have a button for AE/AF lock and how it can be configured.
    Last edited by tclune; 23rd June 2011 at 03:11 PM. Reason: typos

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    Re: Focus trick?

    After a little bit of test the "*" button on the upper right of my camera seems to be an "Exposure Lock". So I can separate the focus subject and exposure target.

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    Re: Focus trick?

    Quote Originally Posted by kontrol View Post
    Was wondering, can I do the same thing for Spot metering to adjust a correct exposure based on what I want the most to be correctly exposed?
    Spot metering DOESN'T ensure that the spot will be correctly exposed - it only ensures that the spot will be exposed as a medium grey. You have to add or subtract exposure compensation for the amount the spot varies from a medium gray to ensure it exposes correctly.

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    Re: Focus trick?

    Quote Originally Posted by kontrol View Post
    I was wondering, because sometime I have problem to have a correct focus of my subject when I don't really want it to be center of the shot.

    So what I tried to do it, in "One shot" mode I point the center AF point on my main subject, wait till the focus is made, and without releasing the button, I point the shot how I want it and take it.

    Is that a correct way to focus on things I have problem to have a good focus?
    The technique is absolutely fine to my knowledge . I would suggest you to check whether the sharpness in the focus that you are suspecting is actually the blurriness due to camera shake or not . You can find so by checking whether the entire picture is blur or only part of it looks blur.

    I hope the information helps

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    Re: Focus trick?

    Quote Originally Posted by Colin Southern View Post
    Spot metering DOESN'T ensure that the spot will be correctly exposed - it only ensures that the spot will be exposed as a medium grey. You have to add or subtract exposure compensation for the amount the spot varies from a medium gray to ensure it exposes correctly.
    Oh? Sounds like I misunderstood something then. Any tutorial or explanations on that?

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    Re: Focus trick?

    Quote Originally Posted by kontrol View Post
    Oh? Sounds like I misunderstood something then. Any tutorial or explanations on that?
    Hi Mathieu,

    Yes - spot-metering is often misunderstood (as is all metering to be honest).

    I wrote a bit about it some time ago - have a read of my posts in this thread here, and see if this helps?

    In summary though, I'm sure you can appreciate that a black object reflects less light than a white object - but when you spot-meter a black cat or a white bride's dress, the camera has no way of knowing that what you're metering is dark or light - so all it can do is expose it "in the middle". So if you spot meter a white dress you'll get a grey dress unless you tell the camera to increase the exposure by around 2 stops. Conversely, if you spot meter a "black cat", you'll need to tell the camera to decrease the exposure by 2 stops or you'll have a gray cat.

    Larger area metering modes still work the same way, but get it right most of the time (or close enough) because most scenes average out to a medium gray anyway, but they can still be easily fooled.
    Last edited by Colin Southern; 24th June 2011 at 07:34 AM.

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    Re: Focus trick?

    Thanks guys, very apreciated.

    Colin, Thank, I'm going though all the posts and re-reading the tutorial on the website also because I clearly misunderstood something. .

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    Re: Focus trick?

    Quote Originally Posted by kontrol View Post
    Thanks guys, very apreciated.

    Colin, Thank, I'm going though all the posts and re-reading the tutorial on the website also because I clearly misunderstood something. .
    No worries

    Please, if you need help, just ask - it's a topic of confusion for many, and one that I understand well - so I'm more than happy to educate people about it - because then they educate others about it - and between us, we start to dispel some of the (many) myths

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    Re: Focus trick?

    Quote Originally Posted by kontrol View Post
    Thanks guys, very apreciated.

    Colin, Thank, I'm going though all the posts and re-reading the tutorial on the website also because I clearly misunderstood something. .
    No worries

    Please, if you need help, just ask - it's a topic of confusion for many, and one that I understand well - so I'm more than happy to educate people about it - because then they educate others about it - and between us, we start to dispel some of the (many) myths

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