dof tables not accurate why?

Discussion in 'Sony Alpha E-Mount Lenses' started by adwb, Apr 28, 2016.

  1. adwb

    adwb TalkEmount Regular

    128
    Sep 30, 2015
    Bristol UK
    Alistair
    according to dof tables from several sources,
    if
    sony A7 full frame.
    lens focal length is 35mm
    aperture is f8
    focus distance to subject is 10 feet
    then:-
    closest focus point is 6.3 -6.2 feet
    furthest point in focus is 18.1 ft

    So images taken at 10 foot are pin sharp.
    With out altering any thing on the camera or its position I then move the object to 6 foot and the image is soft,
    move it to 7 ft its still soft
    move it to 18 foot its sharp.

    why?

    I am wanting to pick a focus at f8 and be able to let the subject move around not need to refocus every shot but with this discrepancy it does not work
    what am I missing??
     
  2. addieleman

    addieleman Passionate amateur

    Nov 13, 2012
    Netherlands
    Ad Dieleman
    Obviously the circle of confusion at these figures must have been 30 µm, which was a fairly common value in the film days. Apparently the depth of field is not centered around the focus point which leads me to think that you didn't focus exactly at 10 feet but maybe more like 12 or 13 feet.

    If you're using manual focus, use magnified view of the A7 to focus as accurately as you can, also focus at the working aperture of f/8. Also, the values here would be too generous for me, I'd use a circle of confusion of 15-20 µm depending on the circumstances. If you're using autofocus, set the A7's "Live View settings" to ON to force focussing at working aperture and be aware that the A7 might make small focussing errors, my A7 does that anyway.
     
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  3. adwb

    adwb TalkEmount Regular

    128
    Sep 30, 2015
    Bristol UK
    Alistair
    Ad, thanks,
    perhaps I should also have said i had the camera on a tripod , camera in manual focusing; the mode in manual,the iso fixed ,focus zoom to set the focus, the ais off and 10 second timer to trigger the shutter.
    the distances were measures with a tape measure
    the data came from dof master and a couple of other sites and the calculators were set to either full frame or in a couple of cases the actutal camera make and model.

    That's why I do not understand the soft front of the origin focus images
     
  4. addieleman

    addieleman Passionate amateur

    Nov 13, 2012
    Netherlands
    Ad Dieleman
    What lens did you use? Did you focus with the aperture set at f/8?
     
  5. MAubrey

    MAubrey TalkEmount Top Veteran

    The circle of confusion of 30µm for FF/35mm film is standardized on the basis of a 8x10 print. The assumption of "tact sharp" is predicated on that print.

    So when you open up the file on your monitor and zoom into 1:1 pixel size on a 1080p monitor and a 24MP sensor you're viewing 1/4 of the 6000x4000 pixels at on a screen that's probably far larger than 8x10. There's simply no CoC table that can account for that. The best way to go about it would be to adjust your viewing size and not use 1:1.
     
    Last edited: Apr 28, 2016
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  6. adwb

    adwb TalkEmount Regular

    128
    Sep 30, 2015
    Bristol UK
    Alistair
    Thanks every one for your replies to answer a couple of questions,

    Lens is a 28-70 and meta data shows 35 mm on every image
    Focusing was done at. At f8 camera used A7r2
    Images not viewed at 1:1 but adjusted to similar viewing size.

    Mike from what you say it sound like dof tables are a waste of time in that cas
     
  7. robbie36

    robbie36 TalkEmount Veteran

    488
    Nov 21, 2014
    Well DOF calculations stem from the 1950s, and our concept of what is acceptably sharp has clearly changed over the last 20 years.

    I was told that a good rule of thumb was to calculate the DOF from a table 2 stops wider than the f stop you are using. So for the DOF calculation at f8 you use f4. That would give you for 35mm @ f8 @ 10ft DOF from 7.72ft to 14.1ft.
     
  8. MAubrey

    MAubrey TalkEmount Top Veteran

    Well, that depends on a couple things. Hyperfocusing is still useful and that's dependent on DOF tables.

    It also depends on what your output is going to be. If the end goal is a print of a particular size, you can use them to extrapolate to that print size. If the end goal is particular size image for web viewing (e.g. a 1080p image, perhaps), then the final downsampled result will likely look perfectly sharp for that purpose.

    They still have their place. You just need to know your output from the start--something that was easier back in the days of film and has gotten harder and harder with digital.

    With that said, Robbie's recommendation is a solid, practical one.
     
  9. adwb

    adwb TalkEmount Regular

    128
    Sep 30, 2015
    Bristol UK
    Alistair
    thanks every one , wiser now which is is always good:2thumbs:
     
  10. mstphoto

    mstphoto TalkEmount Regular

    140
    Feb 26, 2016
    Aberdeen, NE Scotland
    Mike Stephen
    The chart that I use uses a CoC of 0.02 which apparently is more accurate these days.
    For 35mm at f8, Hyperfocal distance is 25' 1" (7.65m) and Near Focus is 12' 7" (3.84m)

    Mike
     
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