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Thread: Another DOF question

  1. #1
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    Default Another DOF question

    I've read through several articles on DOF but i'm still confused...

    1) what is hyperfocal length? When they say focus at infinity, what does that mean?
    2) i tried using the DOF calculator to try figure things out and i came to a conclusion that what you see on the viewfinder (the DOF?) is what you get? is this true? OR the outcome of the calculation of the distance, etc is only seen after you take the snapshot?
    3) lastly, if you take a landscape pic, you won't need to worry abt DOF right?

    thanks!!

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    Default Re: Another DOF question

    Hi mfbatzap,

    1) Hyperfocal distance is the point of focus where everything from half of that distance to infinity falls within the DOF. E.g. when you focus at the hyperfocal distance of 5m, everything from 2.5m to infinity will be sharp. It is a function of focal length, f-number, and circle of confusion. Old lenses have DOF markings on them and are easy to find the hyperfocal distance. With newer lenses you can use DOFMaster to make the DOF wheel for your lens. Let us know if you need help on how to use the wheel to find the hyperfocal distance.

    2) No, what you see in the view finder of an SLR/DSLR is with the aperture wide open. This is done so that you can see the brightest image the lens allow. Simply look into your lens and vary the f-stop and you'll notice that the aperture does not change. To see the actual DOF under a certain f-stop, you need to use the DOF preview button, which will release the aperture to the set f-stop, resulting in a darker view finder image.

    3) Yes you do. To get max DOF you will need to find and focus at the hyperfocusing distance, given the lens focal length and desire aperture. But of course if you are not too concern about the foreground then you can just use infinity.

    Cheers.
    Last edited by ziploc; 1st April 2007 at 01:16 PM.

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    Default Re: Another DOF question

    wow thanks man!!

    so that is what the DOF preview is for.. LOL..
    Anyway how do you pros gauage the DOF sia?? I keep getting the settings wrong.. hahaha..
    everytime the object in focus quite blur... and I cant seem to find any sharp areas in the pic!?

    the DOFmaster really look very cheem! wahaha..

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    Moderator ziploc's Avatar
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    Default Re: Another DOF question

    With older lens you can gauge the DOF by reading the DOF markings on the lens. With newer ones without the markings you'll need to use other tools like DOF calculator or the DOF wheel as mentioned above.

    Anyway, when you focus on your subject it should not be blur, unless it is due to improper focusing, camera shake (shutter speed too slow) or motion blur. If you focus correctly your subject should be at the sharpest point (the focal point), unless you're talking about insufficient DOF when taking macro pics.

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    Default Re: Another DOF question

    And btw, using the DOFMaster wheel is really quite easy. This is how you use it:

    Using this wheel on the DOFMaster page as an example (which is for a lens of 50mm focal length). Let's assume you focus your lens at 12 feet which is where your subject at. Now let's say you want to find out what the DOF would be at f/4. Turn the DOF wheel until the arrow points to 12 feet (outer ring). Now look at the inner ring for the number "4" which is the f-stop, and the corresponding outer ring marking (distance) it is pointing to. The left "4" points to the near end of the DOF and the right one the far end. So in this case we have DOF of about 10.5 to 14 feet.

    Now to find the hyperfocal distance. Let's say you want to use f/11. Turn the wheel so that the right "11" is pointing to infinity. The arrow will now be pointing to the hyperfocal distance, while the left "11" is at the near DOF distance, which should be half of the hyperfocal distance.
    Last edited by ziploc; 1st April 2007 at 10:02 PM.

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    Default Re: Another DOF question

    The DOF is also affected by the zoom of the picture, right? If so, what is the dimension of the picture that this wheel is applicable to?

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    Default Re: Another DOF question

    Quote Originally Posted by cantaresg View Post
    The DOF is also affected by the zoom of the picture, right? If so, what is the dimension of the picture that this wheel is applicable to?
    zoom?

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    Default Re: Another DOF question

    yah. If you zoom out the picture on the screen, some areas that seems in focus may be thrown out of focus right?

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    Default Re: Another DOF question

    Quote Originally Posted by cantaresg View Post
    yah. If you zoom out the picture on the screen, some areas that seems in focus may be thrown out of focus right?
    no.. it doesn't matter.. the DOF calculator uses focal length, not zoom...

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    Default Re: Another DOF question

    Quote Originally Posted by cantaresg View Post
    The DOF is also affected by the zoom of the picture, right? If so, what is the dimension of the picture that this wheel is applicable to?
    Hi cantaresg,

    Yes, DOF is affected by the focal length of the lens, so you'll need to make multiple wheels for different focal length if you're using a zoom lens. Of course you can't make a wheel for every possible focal length of you lens, so the compromise is to make wheels for some selective length (e.g. 28, 50 and 80mm for a 28-80mm zoom lens).

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    Default Re: Another DOF question

    Quote Originally Posted by cantaresg View Post
    yah. If you zoom out the picture on the screen, some areas that seems in focus may be thrown out of focus right?
    Ok now I know what you mean. If you enlarge the picture, you'll of course change the percieved "sharpness" as you're effectively magnifying it. This is where the "circle of confusion" comes in, which defines the cutoff point where it is considered as "sharp". A larger picture will need a smaller circle of confusion to have the same perceived sharpness if viewed from the same distance.

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    Default Re: Another DOF question

    oh yah. the term is called circle of confusion. But concept is similar? Because amongst the books that I read, this was listed as a factor affecting the DOF as well.

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    Default Re: Another DOF question

    Quote Originally Posted by cantaresg View Post
    oh yah. the term is called circle of confusion. But concept is similar? Because amongst the books that I read, this was listed as a factor affecting the DOF as well.
    Well, this actually depends on your definition of the circle of confusion. Let's say you define 0.03mm as the acceptable CoC regardless of the size of your final output. Then in this case DOF will not change. Of couse, in this case if you're printing two pictures of the same shot with one twice as large as the other, you'll need to stand twice the distance away for the larger on in order to the have the same perceived sharpness as the smaller.

    If on the other hand you want the same percieved sharpness for the larger pic as the smaller one when viewing form the same distance, then you'll need to half the CoC for the larger print, and this will then affect the DOF for a given f-stop and focal length.

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    Default Re: Another DOF question

    DoF governed by this 4 factors:
    - Lens' focal length. The more tele, the more shallow the DoF.
    - Aperture of the lens. The bigger aperture, the more shallow the DoF.
    - Subject to camera distance. The closer subject to camera, the more shallow the DoF.
    - CoC of the camera's sensor. To determine how big is your print out/viewing is.

    Regards,
    Arto.

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    Default Re: Another DOF question

    Quote Originally Posted by Artosoft View Post
    - Lens' focal length. The more tele, the more shallow the DoF.
    Just one minor clarification though. The DOF will only be affected by focal length if the object size at the focal point changes in the image. This is due to the magnification/reduction effect and hence the CoC comes into play. If on the other hand the object size is maintained by changing the lens to object distance, then DOF will remain constant regardless of the focal length.

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    Default Re: Another DOF question

    thanks for all the info!!

    one last thing, if DOF is a factor of dist and focal length, then if i'm using a prime lens, to get the correct DOF i have to move around my focused object?? LOL

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    Default Re: Another DOF question

    Quote Originally Posted by mfbatzap View Post
    thanks for all the info!!

    one last thing, if DOF is a factor of dist and focal length, then if i'm using a prime lens, to get the correct DOF i have to move around my focused object?? LOL
    just change you aperture.

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    Default Re: Another DOF question

    basically, if I get it right, use the focal length and distance to get your composition and perspective, then use the aperture to change the dof. that should be the way.

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    Default Re: Another DOF question

    ohh okok thanks!!
    i'll play around wif a prime lens and try get the right settings hahaha..

    how about zoom lens? if I need to zoom say in a concert badly lighted hall w/o tripod, I will need a larger aperture to compensate for the faster shutter speed... then how do i adjust the DOF? or in this case die die must pump up the ISO, reduce shutter speed and step up the aperture?

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    Default Re: Another DOF question

    If you are in a concert, then chances are you are rather far from your subject. So even at a moderate aperture, you should get a reasonable DOF. Like if you are taking landscape and your subject is very far, even if you open up your aperture, the DOF is still rather shallow. Right?

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