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Thread: What is Aperture or f-stops/f number & Depth of Field

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    Default What is Aperture or f-stops/f number & Depth of Field

    This is one of the fundemental parts of photography when you advance past a point and shoot, the biggest controls you have how your picture appears is Aperture and Shutter speed. Shutter speed is more obvious in that a slower shutter gives a longer exposure and more movement or motion blur, where as a faster shutter speed freezes action.

    Some people seemed a little confused about this, so I published an article about it with some examples and tips.

    It's a bit long to publish in full here, so you can find it at:

    What is Aperture or f-stops/f number & Depth of Field

  2. #2

    Default Re: What is Aperture or f-stops/f number & Depth of Field

    Thumbs up!
    Well illustrated and very informative for beginners!

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    Default Re: What is Aperture or f-stops/f number & Depth of Field

    Yup agree, thanks for the contribution to the beginners

    Since we are at this topic on aperture and DOF, can I seek you guys' comments on this:

    At a fixed aperture, does DOF change as you move closer or further away from your subject? I believe it is a yes. If so, how does it change? Any calculations for this?

    I find this esp challenging to master when you tend to shoot head portraits at f1.4-f1.8. Sometimes you get both eyes in focus sometimes it is just one eye in focus. How do you guys judge what f-stop and how far to stay away fr your subject to achieve the desired DOF?

    Thanks!

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    Senior Member zac08's Avatar
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    Default Re: What is Aperture or f-stops/f number & Depth of Field

    Quote Originally Posted by yehosaphat View Post
    Yup agree, thanks for the contribution to the beginners

    Since we are at this topic on aperture and DOF, can I seek you guys' comments on this:

    At a fixed aperture, does DOF change as you move closer or further away from your subject? I believe it is a yes. If so, how does it change? Any calculations for this?

    I find this esp challenging to master when you tend to shoot head portraits at f1.4-f1.8. Sometimes you get both eyes in focus sometimes it is just one eye in focus. How do you guys judge what f-stop and how far to stay away fr your subject to achieve the desired DOF?

    Thanks!

    Well... When you are closer to the subject and you have a shallow DOF, the focus will be limited more on the subject and can be pretty thin. But when you are focusing further (i.e. when the subject is further) , the DOF does not thin out so much


    blur blur blur blur blur blur blur blur blur blur blur |||||| blur blur photographer


    blur blur blur not so blur |||||| not so blur blur blur blur blur blur photographer


    |||||| - being your subject

    I hope that this can illustrate a simple idea of a shallow DOF at different distances.
    Michael Lim
    My Flickr Site

  5. #5

    Default Re: What is Aperture or f-stops/f number & Depth of Field

    hahhaha cute illustration

    yes, closer u are to ur subject, the shallower the DOF.
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    Default Re: What is Aperture or f-stops/f number & Depth of Field

    So the closer you are to the subject the smaller the DOF right?

    But the qn is how do you judge... at this distance what will be in focus and what will be out...


    Quote Originally Posted by zac08 View Post
    Well... When you are closer to the subject and you have a shallow DOF, the focus will be limited more on the subject and can be pretty thin. But when you are focusing further (i.e. when the subject is further) , the DOF does not thin out so much


    blur blur blur blur blur blur blur blur blur blur blur |||||| blur blur photographer


    blur blur blur not so blur |||||| not so blur blur blur blur blur blur photographer


    |||||| - being your subject

    I hope that this can illustrate a simple idea of a shallow DOF at different distances.

  7. #7

    Default Re: What is Aperture or f-stops/f number & Depth of Field

    Quote Originally Posted by yehosaphat View Post
    So the closer you are to the subject the smaller the DOF right?

    But the qn is how do you judge... at this distance what will be in focus and what will be out...
    hmm, is it not possible to see through the VF?
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    Senior Member zac08's Avatar
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    Default Re: What is Aperture or f-stops/f number & Depth of Field

    If there is a DOF preview button on the camera, use it. It can help loads when you need to have a better picture on the DOF you are trying to get out of the pic. The other is to snap and check thru the screen on the captured shot.



    Cheers,
    Michael Lim
    My Flickr Site

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    Default Re: What is Aperture or f-stops/f number & Depth of Field

    Quote Originally Posted by zac08 View Post
    If there is a DOF preview button on the camera, use it. It can help loads when you need to have a better picture on the DOF you are trying to get out of the pic. The other is to snap and check thru the screen on the captured shot.



    Cheers,
    My experience w the DOF button on the 350D is that it just darken the overall view over the viewfinder... cant really tell the DOF thru it lei...

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    Default Re: What is Aperture or f-stops/f number & Depth of Field

    wow nice article...

  11. #11

    Default Re: What is Aperture or f-stops/f number & Depth of Field

    Quote Originally Posted by yehosaphat View Post
    So the closer you are to the subject the smaller the DOF right?

    But the qn is how do you judge... at this distance what will be in focus and what will be out...
    Other than the DOF button on some DSLRs, the main thing is to be familiar not only with all the factors which affect DOF but also the camera and lens you're using by studying the DOF calculation table and through practical experience. The formula below will tell you which factors determine DOF (where DOF = Df - Dn) and which are directly proportional or inversely proportional to the DOF :

    http://www.dofmaster.com/equations.html

    http://photoinf.com/Tools/Don_Flemin...alculator.html
    Last edited by Clockunder; 18th October 2006 at 12:22 AM.

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    Default Re: What is Aperture or f-stops/f number & Depth of Field

    Quote Originally Posted by yehosaphat View Post
    My experience w the DOF button on the 350D is that it just darken the overall view over the viewfinder... cant really tell the DOF thru it lei...
    But Canon have DOF priority mode which is helpful to maintain good DOF on your shoot .

    Regards,
    Arto.

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    Default Re: What is Aperture or f-stops/f number & Depth of Field

    with the old lenses with apertuer rings, you could tell the amount of defocus infront and back of the subject for any given aperture settings. Now with the new lenses, not too sure anymore. I guess with instant preview of your pictures, such information have been rendered unecessary.

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    Default Re: What is Aperture or f-stops/f number & Depth of Field

    Quote Originally Posted by Clockunder View Post
    Other than the DOF button on some DSLRs, the main thing is to be familiar not only with all the factors which affect DOF but also the camera and lens you're using by studying the DOF calculation table and through practical experience. The formula below will tell you which factors determine DOF (where DOF = Df - Dn) and which are directly proportional or inversely proportional to the DOF :

    http://www.dofmaster.com/equations.html

    http://photoinf.com/Tools/Don_Flemin...alculator.html
    Thanks for the useful resource... but surely one cannot do this calculation or measurement when shooting right? So I guess it is still practise and reviewing, and trial and errors ba...

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    Senior Member zac08's Avatar
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    Default Re: What is Aperture or f-stops/f number & Depth of Field

    Quote Originally Posted by yehosaphat View Post
    Thanks for the useful resource... but surely one cannot do this calculation or measurement when shooting right? So I guess it is still practise and reviewing, and trial and errors ba...
    Yup... Keep trying at the constant parameters for easier learning.

    That's why most ppl advise that you start off with a prime lens. Focal length being fixed will allow for easier learning on the other factors.


    ,
    Michael Lim
    My Flickr Site

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    Default Re: What is Aperture or f-stops/f number & Depth of Field

    Quote Originally Posted by zac08 View Post
    Yup... Keep trying at the constant parameters for easier learning.

    That's why most ppl advise that you start off with a prime lens. Focal length being fixed will allow for easier learning on the other factors.


    ,
    ya... once you start on jumbo-aperture-primes, you start to loose interest in the rest of the f4 or higher f-stop zoom lens ... and no $$ to think abt the 2.8L

  17. #17

    Default Re: What is Aperture or f-stops/f number & Depth of Field

    Thanks for the feedback guys, hope someone finds it useful

    yehosaphat yes there are calculations for everything, it gets quite complicated though!

    Most cameras do have a DoF preview button and yes it does darken the screen, but actually as we are shooting digital, it doesn't matter, just shoot the pic, then check the LCD with medium zoom for correct focus on the points you wish.

    Also bare in mind the focal plane will effect it a lot, so if you want both eyes in focus you just have to make sure your camera back is parallel to the plane of the eyes.

    Practise is the best thing, you'll get to learn what aperture is required in each situation.

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    Senior Member creampuff's Avatar
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    Default Re: What is Aperture or f-stops/f number & Depth of Field

    Quote Originally Posted by yehosaphat View Post
    My experience w the DOF button on the 350D is that it just darken the overall view over the viewfinder... cant really tell the DOF thru it lei...
    That is precisely how the DOF preview. Your viewfinder darkens because unless you are using the lens' largest aperture, you are previewing how focus will be like stopped down. If you carefully see in the viewfinder, you will see the DOF for the aperture you have set. Believe me it is darn accurate.

  19. #19

    Default Re: What is Aperture or f-stops/f number & Depth of Field

    The DOF equations may look daunting and complicated but if you rearrange it slightly, then you could deduce whether a factor is directly or inversely proportional to DOF.

    DOF(near) = s(H - f)/(H + s - 2f)

    Rearrange the denominator, becomes s(H - f)/[H - s + (2s - 2f)]

    DOF(near) = s(H - f)/[H - s + 2(s - f)]

    DOF(far) = s(H - f)/(H - s)

    DOF = DOF(far) - DOF (near)

    So if s=f, then DOF = zero because DOF(far)=DOF(near) although it's not practical.

    The closer s is to f, the smaller is s-f, meaning that DOF(far) will be nearer to DOF(near) ==> shallower DOF. Therefore for a given focal length, the closer you are to the subject (i.e. smaller s makes s-f smaller), the shallower the DOF is. For a given subject distance, the longer the focal length you use (i.e. bigger f makes s-f smaller), the shallower the DOF.

    Other relationships between DOF and aperture, focal length etc. can be deduced in similar fashion.

    http://www.dofmaster.com/equations.html
    Last edited by Clockunder; 19th October 2006 at 01:42 AM.

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    Default Re: What is Aperture or f-stops/f number & Depth of Field

    Quote Originally Posted by Clockunder View Post
    The DOF equations may look daunting and complicated but if you rearrange it slightly, then you could deduce whether a factor is directly or inversely proportional to DOF.

    DOF(near) = s(H - f)/(H + s - 2f)

    Rearrange the denominator, becomes s(H - f)/[H - s + (2s - 2f)]

    DOF(near) = s(H - f)/[H - s + 2(s - f)]

    DOF(far) = s(H - f)/(H - s)

    DOF = DOF(far) - DOF (near)

    So if s=f, then DOF = zero because DOF(far)=DOF(near) although it's not practical.

    The closer s is to f, the smaller is s-f, meaning that DOF(far) will be nearer to DOF(near) ==> shallower DOF. Therefore for a given focal length, the closer you are to the subject (i.e. smaller s makes s-f smaller), the shallower the DOF is. For a given subject distance, the longer the focal length you use (i.e. bigger f makes s-f smaller), the shallower the DOF.

    Other relationships between DOF and aperture, focal length etc. can be deduced in similar fashion.

    http://www.dofmaster.com/equations.html
    THanks!

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