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Help understanding the distance scale?

Discussion in 'Adapted Lenses' started by DDBazooka, Oct 24, 2011.

  1. DDBazooka

    DDBazooka Mu-43 Veteran

    211
    Sep 3, 2011
    On my lens I see:

    [​IMG]

    Using a calculator I get:

    [​IMG]

    The lens show that DOF would be around 6 ft to 30+ ft but the calculator outputs 7.77 to 14 ft.

    Am I reading the distance scale wrong? Or is it because the numbers on the focus scale bunch up at the end? I'm guessing because of the ways the numbers are spread out, 100% accuracy is impossible...

    Forgive me if I'm a couple decades too late, I just started using manual focusing lens! :smile:
     
  2. RnR

    RnR Mu-43 All-Pro

    Sep 25, 2011
    Brisbane, Australia
    Hasse
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  3. DDBazooka

    DDBazooka Mu-43 Veteran

    211
    Sep 3, 2011
    Thanks! Looks like I got some reading to do.

    It seems that on the bigger f-stops the distance scale is very close to being accurate, but apparently, not exact, which is why I thought it was weird how only f/16 was off.
     
  4. shnitz

    shnitz Mu-43 Top Veteran

    989
    Aug 25, 2011
    Austin, TX
    It's because of two things:
    -Your calculator is different than the lens maker's
    -You're using a tiny sensor

    If you take that calculator, and you put in 135 film (35mm) instead of your camera, you'll notice that despite the same lens settings, the depth of field jumps in size, from 6.35 feet away to 23.6 feet away (instead of your 7.7 to 14 feet). This compares apples to apples, since your lens scale was obviously made for 35mm film. So, let's notice first, that the online depth of field calculator is more conservative than the scale on the lens. "Acceptably sharp" varies, depending on your definition of acceptable. The lens maker assumed a photographic print made from film and enlarging paper, which isn't as harsh as trying to eke it out of a hundreds of dpi printer, made from a digital sensor.

    Second, notice that changing the sensor, from a larger sensor to a smaller sensor decreases the depth of field, given that you keep the same focal length, aperture, and subject distance. This happens even if you stay digital, and go from the Olympus PEN to a Leica M9, which funny enough is the same reading as 35mm film. The link above explains it pretty well. Here's another, that is tailored a bit more towards 4/3 cameras
    wrotniak.net: Depth of field and your digital camera
    Those two links, along with some creative google searches, should keep you busy to your heart's content.

    One thing that this does show, however, is how much harder it is to make lenses for micro 4/3 cameras. Being smaller sensors, you need better resolution lenses. My medium format Mamiya, for example, has lenses that are relatively cheap, for their size and heft, since the resolution is made from size, not accuracy. That's part of why micro 4/3, despite being significantly smaller than DSLRs, costs so much to get a solid kit going.
     
  5. RobWatson

    RobWatson Mu-43 Hall of Famer

    This just makes my bells go off. I take the same setup and put a half size piece of film and magically the DOF changes? No way.
     
  6. shnitz

    shnitz Mu-43 Top Veteran

    989
    Aug 25, 2011
    Austin, TX
    The actual depth of field doesn't change, it's independent of the camera, as you suspect. However, notice that the above calculator has an entry for camera. If you're wondering why they did that, they were nice enough to mark the heading "Camera, film format, or circle of confusion." Nothing mathematically changes with regards to what the lens is sending back to the camera, but you have to factor in that a smaller sensor means that you're trying to extract more resolution from the same scene. While a larger format may capture an image of a head, torso, etc., a smaller format will use the same lens, at the same settings, to only capture the head. It's the same thing as if you were to take a medium format digital camera and then crop 90% of the image.

    I'll give you the pleasure of learning about it on your own. Again, click the above two links, plus google it till the cows come home.
     
  7. With a standard optical lens there can only ever be one distance from the camera where objects are perfectly in focus. Depth of field relates to what we consider to be in acceptable focus. It's more about perception than a definitive measurement, and is also dependent on the size of the viewing medium and the distance you are viewing it from. You know how it is; the best way to hide an out-of-focus image is to view it small :wink:

    If you take two images taken with the same lens but on one larger sensor (A) and one smaller sensor (B) and print them to the same size, the image taken with the smaller sensor has effectively been magnified. The small background object that may appear in-focus in the image taken with sensor A, will appear larger and fuzzier in the image taken with sensor B to the point where we would now consider it to be out-of-focus. Note that it was always out-of-focus (see the first sentence), but previously we were viewing it at a size small enough not to notice.
     
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  8. Hikari

    Hikari Mu-43 All-Pro

    Nov 26, 2010
    DoF DOES change with format size simply because you have different permissible circles of confusion. DoF is a subjective quality based on perceived sharpness.

    If you are using a 35mm lens, use the scales for the aperture two stops below the set aperture--if the aperture is set to f/16, use the f/8 scales. The two largest apertures will naturally have no scales to use. (The crop factor is directly related to to the shift in DoF. On an APS-C size sensor, you compensate for about one stop--1.4 is the square root of 2. m4/3 has a crop factor of 2, or the square root of 4.)
     
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  9. Hikari

    Hikari Mu-43 All-Pro

    Nov 26, 2010
    It would even happen if you cropped your image too.
     
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  10. RobWatson

    RobWatson Mu-43 Hall of Famer

    DOF Not

    Circle of confusion? Confounding factors more like it. How about the guy (color blind with strong astigmatism and grimy glases) standing in front of the fancy print just made that has crappy glaring glass over it with supremely poor lighting? He may even have a neurological disorder that impairs his perception ... you want to lump all that into your DOF definition? Why not? Just stepping forward or back a few paces changes the way we percieve the printed image.

    There is a real DOF in an image that is set primarily by the optics and the pixel (or aggregate film grain) size. How well this has picked up the detail and shading from the object is what sells cameras and lenses. Pretty much every type pf reproduction (like printing a digital image) results in loss due to MTF < 1.

    Crappy monitors everywhere so viewing even at 100% on different monitors causes loss. Gonna buy a better monitor or get better camera and lenses?

    Gotta draw the line somewhere or the terms become meaningless.

    Circle of confusion. Sure, using Airy disks is arbitrary, but it certainly not subjective. When things transition from arbitrary to subjective then the meaning is lost (or better, left behind).

    Anyway, I need coffee or I'll really start to whine.
     
  11. shnitz

    shnitz Mu-43 Top Veteran

    989
    Aug 25, 2011
    Austin, TX
    It's a very real measurement, and it's as objective as depth of field is. The line is very clearly drawn, because circle of confusion is still related to the image capture. Your astigmatic viewer and sub-par monitor examples both take place after the image is already taken.

    Take a lens, and set it up with a given focal distance and aperture, without a camera behind it. Now, set up a full-frame camera. From the complete image circle of the lens, you crop out the center 864 mm^2 from it to get an image. With a micro 4/3 camera (set up to the same aspect as a full frame camera, to simplify things), you are only cropping about 25% of that, at 225 mm^2. So, now, for the same viewing medium, with the same viewer, for the same size image, you have to enlarge the micro 4/3 image about 4 times to get an equivalent size. That is how circle of confusion comes into play. You're effectively having to blow the image up more with a smaller sensor, which any way you slice it, means you're putting more strain on the lens' resolution capability. Obviously, the two photos will be different; the 4/3 camera will look like it's zoomed in twice as much.

    You are also right that after the image capture, the enlarging size, medium, and viewing distance comes into play. That is still dependent on the circle of confusion. From the Wrotniak link, because he explains it perfectly well:
    "For example, a frame from a 35 mm camera has the size of 24×36 mm. To be printed as a typical 30×40 cm (12×16 inches) enlargement, or viewed on a 20" (diagonal) screen, it has to be magnified 12.5 times (note that because of a different aspect ratio, the usable part of the frame is only 24×32 mm).

    For the same print size, an image created on the 2/3" sensor of a typical non-SLR digital camera (6.6×8.8 mm) will require a magnification of about 50×. The circle of confusion has to be five times smaller to result in similar perceived sharpness of the viewed image."

    Again, read those links. You'll find that we're very objective 'round these parts :smile: Just look at my previous posts. Math and facts: that's me!
     
  12. RobWatson

    RobWatson Mu-43 Hall of Famer

     
  13. Hikari

    Hikari Mu-43 All-Pro

    Nov 26, 2010
    Rob, it is subjective because it always refers back to the human visual system. Changes to DoF because of format, cropping, and viewing distance are very well documented and has been for a very, very long time. And this is nothing to do with the Airy disk, it is about the permissible circle of confusion--different things.
     
  14. Hikari

    Hikari Mu-43 All-Pro

    Nov 26, 2010
     
  15. RobWatson

    RobWatson Mu-43 Hall of Famer

    When the c ircle of confusion expands to the point that it is the size of the Airy disk it is called the permissible circle of confusion and sets the DOF!
     
  16. RobWatson

    RobWatson Mu-43 Hall of Famer

    Hmmm ... If the pixel/grain size is such that the image is strongly undersampled then a great many things look to be in focus when they actually are not! This is easy to see by simple resizing the image or take two images one with ful lres and the other 1024x768 pixels and look what happens to the apparent DOF.
     
  17. Djarum

    Djarum Super Moderator

    Dec 15, 2009
    Huntsville, AL, USA
    Jason
  18. Hikari

    Hikari Mu-43 All-Pro

    Nov 26, 2010
    Actually, the Airy disk is smaller until diffraction is visible--hard to get something smaller than the Airy disk. The permissible circle of confusion is the point where a circle of confusion is unresolved. As the format changes, the size of that circle also changes because of the magnification changes to reach the final print size.
     
  19. Hikari

    Hikari Mu-43 All-Pro

    Nov 26, 2010
    Actually, it does not. The DoF in my 22MP back is no different than my 40MP camera barring the slight difference in chip size. DoF never changed when I shot Tech Pan or TriX.

    Folks get confused viewing at 100% as some sort of "real world" condition, which it is not.
     
  20. RobWatson

    RobWatson Mu-43 Hall of Famer