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Thread: question abt autofocus.

  1. #1

    Default question abt autofocus.

    Assuming a cam is using contrast detection af, i focus on a subject 3meter away using f1.8. If the hyperfocal dist is 1m for eg, the focused area will be between 2.5m to 3.5m or 3m to 4m from the camera?

    Thanks!
    [Canon EOS 5D II + 40D | EF 16-35mm f/2.8L II + 70-200mm f/2.8L IS II] [Fuji X-Pro1 + XF35mm f/1.4]

  2. #2
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    Default

    Without focal length, this question is not complete right?
    Equipment: D800|D700|11-16|28-75|105 Micro VR|50 F1.4G|85 F1.8G
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    Moderator catchlights's Avatar
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    Default Re: question abt autofocus.

    nope,
    the depth of field cover from 1/3 at the front, to 2/3 at the back of focusing point.
    Shoot to Live, Live to Shoot
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    Moderator catchlights's Avatar
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    Default Re: question abt autofocus.

    btw, auto focusing or manual focusing, is irrelevance.
    Shoot to Live, Live to Shoot
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  5. #5

    Default Re: question abt autofocus.

    Online DOF Calculator

    Assuming a 1.6x Canon xxxD camera, to get 1m hyperfocal distance at your requirements, you need a focal length of 5.8mm! No such lens exist, AFAIK.
    Assuming using FF 5DMk2, to get 1m hyperfocal distance at your requirements, you need a focal length of 7.3mm! Again no such lens exist, AFAIK.
    Plug in the figures and see the results.

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    Moderator Octarine's Avatar
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    Default Re: question abt autofocus.

    Quote Originally Posted by zzyzx View Post
    Assuming a cam is using contrast detection af, i focus on a subject 3meter away using f1.8. If the hyperfocal dist is 1m for eg, the focused area will be between 2.5m to 3.5m or 3m to 4m from the camera?
    It does not matter which AF system is in use (contrast of phase detection) - what matters is: selected focal length and aperture. Btw: f/1.8 is the maximum opening of the lens. For hyperfocal distance it's irrelevant. Once you got all details feed them into the DOF calculator.
    Hyperfocal distance is used when one wants to have everything from the front object to infinity to be sufficiently sharp. At such short distances as you describe here you will need to stop down a lot to get objects further behind in focus as well. Or you will need a wide angel lens .. back to focal length as mentioned ..
    EOS

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    Senior Member ZerocoolAstra's Avatar
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    Default Re: question abt autofocus.

    Quote Originally Posted by zzyzx View Post
    Assuming a cam is using contrast detection af, i focus on a subject 3meter away using f1.8. If the hyperfocal dist is 1m for eg, the focused area will be between 2.5m to 3.5m or 3m to 4m from the camera?

    Thanks!
    using DOF calculator, you will get a value for near limit and far limit, which extends 1/3 to the front and 2/3 to the rear of the focus distance.

    hyperfocal distance is something different. Since you are inquiring about using f/1.8 and also about near and far limit, I will presume you are mistaken here
    Exploring! :)

  8. #8

    Default Re: question abt autofocus.

    Sorry guys for the confusion. The values i posted are only assumption. Those are example to illustrate a senerio only. I was more interested in the 1/3 front, 2/3 back part.

    Since more focused part is towards the rear, how would we know the af system is not suffering from back focus problem? And why would designer it in this ratio more towards the rear?

    Thanks again..
    [Canon EOS 5D II + 40D | EF 16-35mm f/2.8L II + 70-200mm f/2.8L IS II] [Fuji X-Pro1 + XF35mm f/1.4]

  9. #9

    Default Re: question abt autofocus.

    The 1/3, 2/3 is a result of the physics. In fact, its only a very general guideline, like the 1/f guideline for handheld shutter speeds. If you play around with the various inputs, you can get 1/2, 1/2 as well.

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    Senior Member ZerocoolAstra's Avatar
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    Default Re: question abt autofocus.

    Quote Originally Posted by zzyzx View Post
    Sorry guys for the confusion. The values i posted are only assumption. Those are example to illustrate a senerio only. I was more interested in the 1/3 front, 2/3 back part.

    Since more focused part is towards the rear, how would we know the af system is not suffering from back focus problem? And why would designer it in this ratio more towards the rear?

    Thanks again..
    To test for focusing errors, you can use a focus chart and set up your test accordingly. That should give you the clearest indication of whether your autofocus is out or not.
    Exploring! :)

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    Moderator Octarine's Avatar
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    Default Re: question abt autofocus.

    Quote Originally Posted by zzyzx View Post
    Since more focused part is towards the rear, how would we know the af system is not suffering from back focus problem? And why would designer it in this ratio more towards the rear?
    It's the laws of Physics and the perception of eyes and brain. If you have doubts about your AF system please use a focus test chart of any other lens calibration tools. If you camera has AF fine tuning then use this method. Otherwise feel free to drop by the service center of your camera maker. But bear in mind the variance of (all) systems. AF systems of cameras vary in their accuracy within a certain (manufacturer defined) margin. The same goes for the accuracy of lenses to focus correctly after getting the focus command from the body. Both together can give acceptable results or may result in front / back focusing. Acceptable sharpness varies between people and pixel peeper can spend times and money on the search for Holy Sharpness Grail.
    On the other end: downsizing image for web display usually will result in increased sharpness and can make up for a slight focus error.
    EOS

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