Depth of Field Calculators.. Are they all wrong??

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  • MsmotoMsmoto Posts: 5,396Moderator
    @spraynpray

    Thanks, but it is the equipment which does all the work… Nikon! And surrounded by the gray lens crowd…LOL
    Msmoto, mod
  • imhumanimhuman Posts: 1Member
    I am going to share my findings with indications of DOF effects on sharpness rather than what we usually see with depth of field

    The dof calculators are both correct and incorrect. If you are focusing on something that dominates the frame dof calculators are quite accurate. Think about what it is you want in focus. If you were to shoot a portrait that fills the frame you may want most of the face in focus, or you may want to focus on a small section like an eye. A depth of field calculator will get this right, since the general rule of photography is to fill the frame.

    I first experienced problems with dof when I attempted to shoot a wide angle shot of a bride and groom occupying only the center of the image. Even though in accordance with depth of field would put the couple in focus with plenty of room to play when zooming in to 100% the bride and groom were very soft requiring a good amount of sharpening.

    This has been bugging me and I continued to practice at home focusing on small distant objects. On occasions the subject was sharp but most of the time very soft. I used a tripod, and a cable release but this made no difference, only a change in focus would impact image sharpness.

    It became much more clear when using live view. I focused on a cushion across the room, zoomed into 100% in live view, and manually focused on stitching on the cushion. With just a micro turn on the lens the stitching went from focus to out of focus. I thought maybe it could be the live view, but in taking pictures I could see at 100% zoom the stitching being soft and sharp just as seen in live view.

    What I find is that when focus on a small section of the frame and then consider what lens I would have used to capture something so small and distant in focus (a guestimate of focal length) when I input the f-number and imaginary lens into dof calculators the shallow depth of field relates. In this case I felt the crop resembled using an imaginary 1200mm f1:2.8 lens. This gave me no depth of field and room to play with focus. It was either sharp or not. When zooming out the overall image had plenty of depth of field and overall sharpness, but a given point in the image had no sharpness.

    For me my findings are still preliminary and require more studies. I should add that I print larger than the guidelines. I have always fount that when the subject fills the frame my images look fantastic. This is the case even when I used to create 30 X 40 inch prints from my old 10 megapixel Nikon D200. I always struggled when shooting wide with plenty negative space surrounding the bride and groom. Now with using Live View even these images look fantastic. With the ability to focus more precisely, my images are very sharp where I want the sharpness to be.

    Try for yourself, live view changes everything
  • PB_PMPB_PM Posts: 4,312Member
    I think part of the issue is that the focus scales on most modern AF lenses are not terribly accurate, which would make getting accurate hyper-focal distance difficult. On the other hand, I have used some older Nikkor lenses which have accurate focus scales, and depth of field markings on the lens barrel. When used with those lenses hyper-focal distance scales work very well.
    If I take a good photo it's not my camera's fault.
  • shoot123shoot123 Posts: 2Member
    Smaller sensor cameras provide a greater depth of field for a given "relative" focal length at the same aperture. So a Nikon 50mm focal length on an "crop frame" DX camera has more depth of field than a 75mm focal length on a "full frame" FX camera.

    Fotosharp makes a nifty "slide chart" calculator for "full frame" sensor cameras (or 35mm cameras). I've used one since film camera days, and have found it to be pretty useful.

    http://fotosharp.com/depth_of_field_calculator.html
  • PapermanPaperman Posts: 469Member
    edited June 2014
    @Inhuman,

    I wrote it in another topic before; too much is expected from/AF systems; their capabilities are highly exaggerated.

    As you have indicated, your sharpness/AF expectation from a 1:1 magnification of a small object is similar to what one can achieve from an imaginary 1200mm/f2.8 lens from a few meters ( if that were possible ). We are then talking about +/- 0.5mm AF accuracy which I don't believe any AF system in any lens can detect ( unless we're shooting macros or close-ups which has no comparasion ). Even if the AF system were accurate enough up to catch/detect the fraction of a milimeter difference in 3-5 meters, there may not be such a small "increment" in the mechanics of a lens to adjust it that accurately.

    Just also take into consideration that the AF system is trying to "catch" focus not the way you see through your eyes but trying to read the phase/contrast change created by the less than a mlimeter difference a few meters away ON A 24x36mm PLATE ! And then try to calculate how many microns that 1mm in real life would relate to on the focusing screen.

    And one wonders how many of us have the perfect eyesight to notice the focus issues on prints that can only be seen at the pixel level.

    We are expecting too much, we are worrying too much .....


    Post edited by Paperman on
  • PB_PMPB_PM Posts: 4,312Member
    edited June 2014
    @Inhuman,

    Just also take into consideration that the AF system is trying to "catch" focus not the way you see through your eyes but trying to read the phase/contrast change created by the less than a mlimeter difference a few meters away ON A 24x36mm PLATE ! And then try to calculate how many microns that 1mm in real life would relate to on the focusing screen.
    In fact it is much worse than that with a traditional DSLR, which uses a sub-mirror at the bottom of the prism box. Even with a full frame camera the focus sensor (the entire module itself) is smaller than a DX sensor. Now take one focus point which is either a "+" or "|" shape which is fraction of that size, and you can see how easy it is for focus to be off. Also, if the main mirror and the sub-mirror are not aligned properly that can cause even more problems.
    Post edited by PB_PM on
    If I take a good photo it's not my camera's fault.
  • AdeAde Posts: 1,071Member
    In this case, it's not the size or shape of the AF points that's causing issues.

    The fundamental problem is that the phase-detect AF sensor is at a different location from the actual imaging sensor, so light takes a different optical path to reach the AF sensor vs. the real sensor.

    This difference creates AF accuracy errors regardless of the size or shape of the AF sensor. These errors can be "minimized" through calibration; however, in practice there is always a slight error.

    On new mirrorless designs, the AF points can be placed on the imaging sensor itself, eliminating this error.

    Contrast detect systems (used in Live View) are also immune to this particular issue.

    Another fundamental problem is the precision and accuracy of the AF drive (the type of motors used, gearing, etc.) Most AF lenses today are designed for speed, not accuracy. E.g., the typical Nikon SWM motor is quite suitable for shooting fast action sports at f/8, but is less suitable for situations calling for very shallow DoF.

    @imhuman

    From your description it sounds like your AF lens calibration is significantly off. You may want to use LensAlign or similar systems to AF Fine Tune your lenses.
  • spraynprayspraynpray Posts: 6,387Moderator
    You beat me to the draw @Ade. In this case it is nothing at all to do with DoF, just AF calibration (from how it was described by @Imhuman).
    Always learning.
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