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Depth of field - hopefully explained

Discussion in 'Open Discussion' started by arad85, Oct 14, 2012.

  1. arad85

    arad85 Mu-43 Veteran

    477
    Aug 16, 2012
    Prompted by a recent discussion on here, and the fact I have a more than healthy interest in the maths behind all of this photo malarkey:wink: I thought it time to write down in one place my understanding of DoF.

    I set out to do this without any formulas, just to describe what is actually going on. It's here:

    Depth of Field and what affects it – explained. | Andys photography

    If anyone has any comments or suggestions for what needs doing to it - good or bad - I'd be more than happy to receive them.

    Thanks
     
    • Like Like x 6
  2. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    Linky doesn't work.
     
  3. arad85

    arad85 Mu-43 Veteran

    477
    Aug 16, 2012
    Ooops... thanks. Should be there now :smile:
     
  4. F1L1P

    F1L1P Mu-43 Veteran

    388
    Jan 2, 2010
    Europe
    Nice article.

    However I didn't get the "The second thing to learn is that DoF is not captured into the image when you take it....– the fact that it isn’t a property of the image capture, but that of the image viewing." part.

    Will have to study it a bit more :biggrin:
     
  5. arad85

    arad85 Mu-43 Veteran

    477
    Aug 16, 2012
    Thank you...

    Think of it this way. Every point that is not on the plane of focus is out of focus to some greater or lesser extent. You only see it as out of focus when you print it. Print and view it so that the out of focus area is smaller than what your vision can discren and it is in focus. Take the same photo and print it 2x or 4x or 8x the size and that out of focus blob may now be big enough to see as a blob - it will now be out of focus.

    Same image, different result depending on how you print it....
     
  6. dancogan

    dancogan Mu-43 Rookie

    15
    Aug 31, 2012
    SE Michigan
    Yes, but won't you view the smaller image from a closer distance? So you will discern the same circle of confusion whether you make a small print or a large one, if viewing distance is proportional to the size of the print. Is that correct? Enjoyed the article, except for the same confusion expressed by F1L1P. (no pun intended ... lol)
     
  7. usayit

    usayit Mu-43 Hall of Famer

    Sharpness is an illusion....

    It took me a while to grasp that concept.
     
  8. arad85

    arad85 Mu-43 Veteran

    477
    Aug 16, 2012
    Yup. a 10x8 print viewed from 12" will look the same as a 20x16 print viewed from 24". But look at the 20x16 print from 12" and the depth of field will alter (slightly).

    Most of the time, the DoF calculations hold good, because we naturally follow the rules when viewing images. I don't go around saying "I'm viewing this image 10% further away, so my perception is that DoF is 10% deeper". I really just wanted to explain why depth of field is dependant on printing and viewing. It is too easy to go somewhere like dofmaster and get a value that says at 10', anything between 9' 3.256" and 11' 2.132" will be "in focus". That doesn't match up with reality.
     
  9. Cruzan80

    Cruzan80 Mu-43 All-Pro

    Aug 23, 2012
    Denver, Co
    Sean Rastsmith
    May also want to go into usnig adapted lenses on a smaller sensor than originally designed for, and how the barrel markings change. You did go into the assumptions made at that marking, but not how putting the same lens with the same aperture on a smaller frame camera applies to how much "DoF" at various apertures, vs. original. Aka. a 50mm lens on FF at f4 has barrel markings saying from "x to y" is in focus. How does that change on a m4/3, or other sensor?
     
  10. arad85

    arad85 Mu-43 Veteran

    477
    Aug 16, 2012
    Crikey, that's a good question... I know DoF will change so that you get half the DoF on a 2x crop sensor, but noi idea on the ratios of f-number and distance scales (although I suspect you just need to use the f-number scaled appropriately...)
     
  11. zpuskas

    zpuskas Mu-43 Veteran

    459
    Feb 25, 2011
    Santa Barbara, CA
    To paraphrase..."I cannot define DOF, but I know it when I see it!".
     
  12. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    Nope, DOF doesn't change because the sensor size changes. That's a complete misnomer, although it's widely believed.

    DOF is dependent on the physical diameter of the aperture and shooting distance. A 50mm lens shot at, say, f/2.8 will produce exactly the same DOF on m43, APS-C, or FF if the photo is shot from the same distance from the subject. The only difference is that the shot taken on smaller sensors will be cropped relative to the ones on larger sensors.

    So why do so many people think the smaller sensor results in more DOF? Because to get the same angle of view as a 50mm lens on FF, you need a 25mm lens on m43 (more or less--the difference in aspect ratio has a small impact). And the aperture of a 25mm at f/2.8 is half the diameter of the 50mm lens, so you get more DOF.

    If you want to keep the 50mm lens but get the same framing as you would on FF, you can back up further from your subject. That, of course, changes perspective as well as DOF. You would end up with a photo that looks as if it were taken from the new location with a 100mm lens on FF. But that 100mm lens, at f/2.8, has an aperture twice as large as the 50mm, which means it has shallower DOF, but not because of the different sized sensor.

    Clear as mud?
     
  13. Cruzan80

    Cruzan80 Mu-43 All-Pro

    Aug 23, 2012
    Denver, Co
    Sean Rastsmith
    Yes, but I also get the arguement that if you take a photo on a sensor 1/2 as big, and blow it up to the same size, it does chage DoF, per the OP arguement (size of blob). Even though the edges show more framing (since you don't have a cropped image).

    I think part of the problem is people are going with cropped image (m43 sensor) compared to uncropped (FF sensor) at the same focal length, and others are comparing "effective focal length" pictures (aka. m43 50mm vs FF 100mm).
     
  14. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    Yes, the increased enlargement does affect DOF. I deliberately left that out for simplicity. And the effect is the opposite of what people generally think. The crop sensor image will have less DOF than the larger sensor. :eek: It's not as simple as saying "half the DOF", though.
     
  15. Kees

    Kees Mu-43 Regular

    56
    Jun 30, 2011
    The Netherlands
    Kees Dignum
    Very clear explanation. Well done!
     
  16. arad85

    arad85 Mu-43 Veteran

    477
    Aug 16, 2012
    Nope. It's circle of confusion that is dependent on lens aperture and focal length.

    Depth of field is decided when you render the image and view it. The size you print to and view from will dictate whether you can see the circles of confusion captured (and there's one for every point in the image) and therefore see that point as out of focus. A photo taken with the same lens with the same settings on different sized sensors will, when printed out to the same size and viewed from the same distance, have different depths of field. They will also be different pictures, but they will have different depths of field - all from the same lens.

    Sensor size comes into it in two places. Firstly it helps dictate focal length used for a photo. Secondly you have to magnify the image more from a smaller sensor to get to a certain print size and this magnification also magnifies the circles of confusion more which directly affects the perceived depth of field.

    Did you read the article I posted?