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Thread: DOF

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    Member superbee15's Avatar
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    DOF

    I recently got back from my 1st NZ trip (stunning country). I was just taking normal holiday snaps but wished to get background as well as subject (person) in focus. We were on top of a hill and background was the beautiful harbour and then city then mountains in background. Just a beautiful vista and it would have been great to get at least the harbour in better focus. The photo covered subject but just nothing much else. So ultimately it was a fail. I used f9 on a 55mm focal length and was about 7 metres from subject.

    Since getting back and investing how I may have done better it seems I may have been better with shorter FL and higher f number maybe to around f18.

    When I looked at a DOF calculator it seems that there is a point at which the DOF combination causes a jump for near distance to infinity and you cannot get the in between distances - if that makes sense. For instance, in my shot it would have been good to get good focus to other side of harbour and maybe into city and accept rest as background. This would mean that I needed about 6 metres to maybe 3 klm.

    Does this then translate to infinity to get the harbour in more focus?
    In the DOF calculator I set my camera with say f18 and subject distance to 6 meters. When I set FL to 45 it gives DOF out to 150m while reducing FL to 45 means it goes to infinity. Is this the case?
    Am I understanding this correctly?

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    Administrator ricktas's Avatar
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    Yes, infinity is everything from the closest focusing distance. S of your DOF calculations state 6metres to infinity. Everything from 6 metres and further away should be in focus. However, remember that lenses are not perfect so this may not be 100% correct and things like weather conditions can adversely affect how clear distant objects appear. Stuff like mirages on hot days, UV light levels etc.
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    Arch-Σigmoid Ausphotography Regular ameerat42's Avatar
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    Quote Originally Posted by superbee15 View Post
    ...So ultimately it was a fail. I used f9 on a 55mm focal length and was about 7 metres from subject.

    Since getting back and investing how I may have done better it seems I may have been better with shorter FL and higher f number maybe to around f18.


    When I looked at a DOF calculator it seems that there is a point at which the DOF combination causes a jump for near distance to infinity and you cannot get the in between distances - if that makes sense. For instance, in my shot it would have been good to get good focus to other side of harbour and maybe into city and accept rest as background. This would mean that I needed about 6 metres to maybe 3 klm.

    Does this then translate to infinity to get the harbour in more focus?
    In the DOF calculator I set my camera with say f18 and subject distance to 6 meters. When I set FL to 45 it gives DOF out to 150m while reducing FL to 45 means it goes to infinity. Is this the case?
    Am I understanding this correctly?
    Superbee. I'll try to give a color-coded reply

    Firstly, pick a useful and clear-to-understand DOF calculator. I have used this PhotoPills one because it allows
    incremental measurements to be put in. I had set it up for your camera and lens, but I don't know if
    you'll get that screen.

    Now, for the replies:
    Still, seeing the picture and with EXIF intact would be good.
    f/18 sounds rather "harsh" to me, as you could get IQ loss through diffraction.
    However, setting your focus (you call this "subject distance") PAST where your
    subject is will help increase the DOF for the situation you described. Try it for about
    10m on the calculator.


    I don't quite understand your "causes a jump" paragraph, but I suspect the last line above
    still covers the situation. "Infinity" for focus purposes can mean anything that the lens can focus when set to
    that mark. I would think that "yes" is the answer for that question.


    You use "set FL to 45" and "reducing FL to 45". I don;t know what you mean, as it's 45 in both cases.

    In summary, use perhaps a FL of 45mm, set your focus to about 10 m, place the people you want in the frame at about 7m, and
    set about f/9 to f/11 on your camera.

    A BIG NOTE: The DOF will appear to decrease at ANY GIVEN F-STOP (such as f/9) when you increase the focal length.

    A slightly technical point: "acceptable DOF" depends on the "circle of confusion" (CoC) that the lens produces for the chosen
    Focal Length/f-stop combination. A smaller CoC gives a higher DOF.

    OK, enough said... Go and try a few things
    CC, Image editing OK.

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    Quote Originally Posted by ameerat42 View Post
    I don't quite understand your "causes a jump" paragraph, but I suspect the last line above
    still covers the situation. "Infinity" for focus purposes can mean anything that the lens can focus when set to
    that mark. I would think that "yes" is the answer for that question.


    You use "set FL to 45" and "reducing FL to 45". I don;t know what you mean, as it's 45 in both cases.
    Sorry all

    I meant to say "set FL to 46" and "reducing FL to 45".

    I put 2 screen shots into file by way of demonstrating. Hope these load.

    - - - Updated - - -

    now to look at CoC
    Attached Images Attached Images

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    Once upon a time, Canon DSLRs had A-DEP mode. It seems that has now gone. Not sure if Nikon had anything similar.

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    Ausphotography irregular Mark L's Avatar
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    What if you simply missed the focus??
    Can you post a photo?
    Otherwise we are sort of guessing at the problem you are trying to calculate the problem you have.

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    Here is link to shot. I did not post originally as wife probably not impressed. I can delete later.

    IMG_8424 by Henry Lennon, on Flickr

    I would have been nice to have more of harbour into better focus. Still struggle a bit with DOF.

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    Arch-Σigmoid Ausphotography Regular ameerat42's Avatar
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    Really hard [or read "impossible"] to do with that FL, f-stop, and subject distance.

    FL is mainly to blame. A shorter FL lens and a fuller length pic of her would have to be
    the only practical way to get ALL the scene in focus.

    SO, consolation: nothing wrong with your gear

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    Alternatively: Learn to use hyperfocal in situations like these.

    Hyperfocal can allow you to refocus to a 'more appropriate' plane which allows the main subject(your wife in this instance) to still be in acceptable focus whilst capturing the background a little more sharply(again more acceptable focus).

    Hyperfocal work on the premise that there is an otherwise wasted zone of focus in front of the subject(wife here) that could have been better used if needed.
    Note that it's not all beer and skittles tho, and none of that lunch is free!
    Had you tried hyperfocal, your wife may not have been (as)sharply rendered .. so it's a matter of how much are you willing to forgo to achieve your desired effect.

    With hyperfocal, to get use of that wasted zone of focus in front of your wife, you would have refocused slightly behind her.
    So if she was at a distance of 7m, then using the hyperfocal principle here, you would have re focused at say 8 or 9m which would have kept your wife in a zone of acceptable focus, but also set a deeper focus level behind here too.

    I generally use hyperfocal in some landscape shots, but only if or where the foreground detail is important too.
    On a tripod and with plenty of time to fiddle about, it's easy to do. AND I usually use liveview now, rather than just guesstimate as I used too prior to cameras without liveview ability.

    The trick to doing this quickly and seamlessly in a faster paced situation such as yours, is to practise it with the specific lenses you think you may use this method.

    You focus using AF, quickly guesstimate a hyperfocal distance and before you actually make the exposure manually reset the focus on the lens. It usually only takes a half a mil of focus ring tweaking, but it has to be in the correct direction, otherwise you will get worse results!
    And remember, this only works OK if you don't need the main subject to be in pin sharp focus.
    If you really want to get pedantic and serious about it all, then you may want to learn to prefocus and have your main subject positioned perfectly for the shot.
    That is, pre focus the lens to say 9m mark. Get wife to stand at the 7m point(or you'd move to a point 7m back from wife) .. and shoot manually focused.
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    Ausphotography irregular Mark L's Avatar
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    Quote Originally Posted by superbee15 View Post
    So ultimately it was a fail. I used f9 on a 55mm focal length and was about 7 metres from subject.

    Since getting back and investing how I may have done better it seems I may have been better with shorter FL and higher f number maybe to around f18.
    For what you wanted it was a fail. Still a decent photo.
    The shorter the FL the less DOF. So don't get closer, get further away for the purposes you want with this photo. Further away, increased DOF and if you have to crop the photo later so be it.
    Also don't be afraid to try ISO400 here to get the shutter speed up. Strap over shoulder looks in focus but Mrs superbee15's face looks not so sharp. She smile and thus introduced a fraction of movement at the very instant you pressed the shutter button? Increased ISO also helps increase the f/#.

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    Arch-Σigmoid Ausphotography Regular ameerat42's Avatar
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    Quote Originally Posted by Mark L View Post
    ...The shorter the FL the less DOF...

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    Appreciate your inputs. Yep after reading more and your inputs, I think I was expecting a little too much. However, all part of learning. Hindsight says to use shorter focal length and crop to some extent maybe. They never give you enough time on organised tours to muck about - unfortunately.
    Cheers

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