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Thread: Depth of field on a prime lens

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    Member Lesley Bray's Avatar
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    Depth of field on a prime lens

    Hopefully I'm posting this in the correct thread - if not please feel free to move it.
    I would love it if someone could explain to me simply and so I can understand it - how to use the depth of field markings on a prime lens.
    Yes I have read the instructions, and yes I have searched the tutorials here.
    I can see the distance markings on my lens and the depth of field markings under that. I cannot get my head around any more than that at this moment.
    Thanks
    Lesley

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    A royal pain in the bum! arthurking83's Avatar
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    it depends on the style of DOF markings, but basically there are coloured lines for various aperture values. eg, you may see blue for f/16, red for f/11 and green for f/8... etc, etc(or whatever).

    You line up each corresponding line with a distance marking on the distance scale.

    So, eg if you are at f/16(and hence using the blue lines marks, you may line up the near mark at the 0.5m range and the far distance mark at the 20m range. This then tells you that at this focal length and at f/16 your apparent DOF may be from 0.5 to 20m(close enough to infinity if you're using a wide angle lens).

    etc etc.

    notice how the smaller the f number the wider the spacings between corresponding lines(colours).

    EDIT: I forgot to say too. the other way to use them is to get infinity itself in focus(but in general you don't always want to do that. If you did tho, you would set the far end mark at infinity, and the near end falls where it does, and you live with that closest focus distance(that's why you generally don't want to do that, if you have interesting subject matter closer than the closest focus distance. Infinity is so far out that having it in focus at the expense of closer subject matter is wasted DOF. You can't see much detail at infinity anyhow when compared to subject matter at closer distances.
    Last edited by arthurking83; 10-02-2011 at 9:43pm.
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    Lesley, a lens is only ever sharpest at the point where it is actually focused. Using DOF by stopping down extends the area of 'acceptable sharpness' to both sides of the point where you have focused but that area is still less sharp than the point where you have actually focused until sharpness becomes unacceptabe (ultimately a subjective decision). The markings on the lens are used to help to determine where sharpness becomes 'unacceptable'. If you view the image at a fairly small magnification (ie a small print) then you won't be able to tell just how unsharp that area is and in fact it will extend even further either side of the focus point. However if you where to blow up the same image into a fairly large print then the lack of sharpness will become evident and objectionable.

    The point is that DOF is a bit of a moving target and you are probably better off experimenting and seeing what works for you and the typical print sizes that you will have as an end product. Use the markings as a guide.

    JJ

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    A royal pain in the bum! arthurking83's Avatar
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    a pictorial version. Note the colour coding. Not all lenses have colour coding. I haven't seen this on any modern Nikon lenses. Older MF primes have them this way. And also note some zoom lenses do also(from that same era, where zooms used to be of the push pull variety. The DOF markings were on the extendable part of the barrel, and fluted in that they converged as they drew closer to the camera body(ie. as the lens was zoomed out to the longer focal lengths.
    FWIW, the Nikon 105mm VR macro lens only has markings for f/32(who on Earth uses f/32?? ) and they are so close together due to the 105mm focal length and relatively short throw of the focus ring, that they're useless. The distance between the two dots marking each end, is close to equal of the size of the small dot mark itself. In the case of this lens, the markings make accuracy difficult and hence usefulness doubtful.

    anyhow....(the easier lens to use)


    DOF set from x to infinity at f/y

    1. from ~2.75m to infinity at f/16


    2. from 4m to infinity at f/11


    using infinity as out goal. The near end falls where it falls according to the focal length and aperture value.

    if you use it in the normal manner, where you preset near focus(closest focus point) you get:

    3: focus from 1m to 1.5m at f/16


    4: focus from 1m to about.. (whaddya reckon?.. make that 1.3m? at f/11


    They're only rough guides, and higher quality lenses with proper focusing rings of higher quality may have clearer markings and more accurate setting, but DOF is a bit of an illusion anyhow and you're trying to give the appearance that something at the distances you're trying to get in focus, is in focus(it's not!.. it's within an acceptable focus range).

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    Lesley Bray's Avatar
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    Thank you Arthur and JJ - the light is gradually getting brighter in my brain. Thank you JJ, your explanation helped a lot.
    Arthur many thanks to you - the photos helped so much - I was able to see what you were describing. The lens I am using is a Canon (135mm) and only has the f32 markings on it and as you said - who uses f32 !!!! Now I am wishing I had some old fashioned lenses.
    Lesley

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    Can't add anything here other than to keep looking at the DOF guide on the lens, keep thinking about it and finally your understanding will become clear(er).

    I have needed to do this with many photographic principals, like lighting setups in the studio. It's not rocket science, but it does require many reviews before you finally see the light, as as Kym says "it's all about the light" lol
    .
    Cheers, Mal

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