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DOF & Backfocus question.

Discussion in 'General Technical Discussion' started by D. L. Miller, May 25, 2005.

  1. I know about DOF, but my question is about Back-focus & Front-focus.

    I realize the total DOF is the result of many factors (i.e. lens length, f stop, focal distant etc), and DOF changes as the variables change, but I get a little confused with BF & FF problems.

    Is it possible to have no BF or FF problems at one focal point but as the subject you are focusing on, gets closer (or farther), BF or FF can show up?? Or is it safe to say if you test your lens at a short distance (let say 3 feet) and it shows no problem of BF or FF, then it is safe to say you will not have any problems at any range of focus with that lens?

    I might not be saying it correctly, but can BF or FF come and go as you change the distance of your subject?

    Can you hide BF & FF by stopping down?
  2. Sorry,

    forgot to sign my thread.

  3. Flew


    Jan 25, 2005

    This is not an exact answer to your question, but these are the thoughts that I had when I read your question. As you change the DOF based on all of the parameters that you listed, the effects of FF or BF will certainly be either emphasized or de-emphasized. Also, what many think of as FF (or BF) is really the AF system locking onto an item other than what you expected.

    This can be especially confusing when you consider the the actual AF sensors don't always align with the grids in your viewfinder.

    Hope this helps a little.
  4. By DOF do you mean Depth of Field or Depth of Focus? Field is at the subject plane and Focus is at the sensor/film plane. If you are talking about the D2X front/back focus problem then, as this seems to be an alignment problem, it is Depth of Focus that matters. Depth of focus extends as far in front of the image plane as behid it. It is given by
    d = depth of focus in either direction
    c= Limiting circle of confusion (one or two pixle spacings say?)
    v= Distance between lens and image (as you focus closer this increases)
    f= f number of lens
    F= Focal length of lens.
  5. I realise I should have given the depth of focus formula in terms of subject distance (note this formula strictly falls apart if you try to put infinity in as the subject distance but if you pretend infinity/infinity is 1 and not indeterminate it works.)
    d=depth of focus in either direction
    c=limiting circle of confusion
    D=distance to subject
    f=f-number of lens aperture
    F=focal length of lens.
  6. I have had futher thoughts on this and with a subject at infinity the formula reduces to d=cf, i.e. the depth of focus is proportional to the circle of confusion, this leads to some interesting conclusions.
    The same autofocus mechanism is used on the F6, D2Hs and D2X but the circle of confusion requirements are different.
    From experience over many years with film you are pushing things if you go beyond 12.5x enlargement from 35mm, taking the accepted 0.25mm as the point at which normal sighted person cannot distinguish between a point and a disc then the circle of confusion you need for the F6 is .02mm. (0.25/12.5)
    For the D2Hs and D2X let us assume that there is no point, due to aliasing concerns etc., in having a circle of confusion less than 2 pixles in diameter. For the D2H this gives a circle of confusion of 23.3/1232 (23.3mm wide sensor 2464 pixles accross) which equals .019mm while for the D2X the circle of confusion is 23.7/2144 which equals .011mm.
    From this it can be seen that the tolerence in setting up the focusing mechanism in the D2X must be approximately half that used in the F6 and D2H if similar results in focusing accuracy are to be achieved.
    With the number of complaints about back focus problems on D2X I wonder if Nikon have fully taken this decreased set up tolerance into account on their production line.
  7. Ken-L

    Ken-L Guest

    (The answer to your question)
  8. Given the same lens and camera, the same f stop, if you don't have BF or FF at 2 feet, then you won't have it at 50 feet. Am I correct in making this statement?

    If you do have BF/FF it will vary as does the distance of the subject. Is this a correct statement?

    What I am tryinbg to eliminate is the following concern. If I test my camera and lens at 2 or 3 feet and it shows no BF/FF then any focus problem I have at 100 feet is me, or some other problem....
  9. If the problem is one of accuracy of setting up camera internals and we are talking about depth of focus then the answer to your question is that you are incorrect, depth of focus is least when you are focused on infinity, it increases as you focus closer (the opposite to depth of field). You should check on a subject 20 or 30 ft away rather than 2 ft.
  10. Dave,

    I hate to keep beating my question to death, but I am still confused on BB/FF and its effect on close and/or distance focusing problems.
    If I check a given lens on a camera and I use all the different tests people use to check BB/FF (i.e. lining up CD covers, rulers, Tim Jackson chart etc.) and there is no BF/FF showing at 3 feet. Is it possible for FF/BF to show up at (let say) 100 feet.

    Since BF/FF can be a lens or camera problem (& combo) I am trying to find out if you don't have the problem at a given distance, can one come to the conclusion you will not have it at any distance.

    Example: I used a 50mm f/1.4 & a D2x and checked out BF/FF at 3 feet with my lens wide open (f/1.4)
    It was almost perfect. The point I was focusing on was "sharp" and the D.O.Field ratio (almost 1/3 front and 2/3 back) was more than acceptable. Can I concluded, I will not have BF/FF at any distance with this combo of lens & camera.

    I have seen results of other "postings" where the focus started 2mm to 3mm back of the point of focus at 3 feet. Let say they did everthing correct. I call that a BF problem. I assume there will be a BF problem at every distance with that example.
    It might even get worse as you increase distance. (?).

    So is it possible that a camera or lens might not have a BF/FF problem at 3 feet but might have one at 100 feet (or any distance you might want to give as an axample).
  11. Ken-L,

    You said the answer to my question is NO.

    I had two major questions>

    1. Can BF/FF come and go as you change distance?

    2. Can you hide BB/FF by stopping down?

    So are you saying NO to both questions?


    This might require a 3rd answer:

    3. I realize you can make the DOField larger by stopping down, but if you have a major BF/FF problem, the picture could still be out of focus and the object you thought were going to be in focus won't be. Is this correct?

    Thanks for all the replies.
  12. Ken-L

    Ken-L Guest

    I am not certain that my answer is correct, it's only my opinion based on how I interpreted your questions. :shock:



    If there is a percieved problem, acting the way it seems you are describing, the camera would not be at fault. The lens could have a problem. Trial and error will show you what the limitations are and any "work-around" you discover. :lol:

    Caveat: With "digital" and all the software/firmware/cpu issues almost anything is possible! If I ran into what you describe I would probably have my lens and camera checked by an authorized repair shop, so I didn't have to worry and got a definitive answer. :cry: 
  13. Can I just sum up what I said earlier. The accuracy which you have to set up the mechanical parts of the autofocus system is dependent on the minimum depth of focus, this is a minimum when the lens is focused at infinity and at this point depth of focus equals whatever you choose as your limiting circle of confusion size times the lens aperture, focal length disappears from the equation. In my earlier posting I suggested that for the D1X a circle of confusion of roughly .01 mm would be suitable. With an f1.4 lens focused at infinity this gives a depth of field of + or - .014mm (.0006 inches) this implies that Nikon technicians have not much more than than plus minus half a thousandth of an inch to play with in setting up the autofocus system. For the D2H they would have twice that.
    From the above argument you will see that back/front focus problems are going to show up most on wide aperture lenses focused at 100 times their focal length or more.
  14. Thanks to Ken-L & Dave Rosser for your replies.

    I had another question but I think I have the answer know with your reponse to my general question on BF/FF.

    I own a 17-55mm f/2.8

    There is a small BF at 20mm to 24mm. None at any other length. Plus my other two lens check OK at a 3 foot test. All wide open. Sounds like a lens problem with the 17-55mm.

    The back focus is so small I can overcome it by stopping down to f/8. I think it causes the DOField to over take the BF problem at the point of focus on the subject.. I have no front focus ratio, but the subject is in acceptable focus, but not sharp. (no front focus ratio, all focus is rear. I am use to a 1/3 - 2/3 with film. And my digital DOF ratio on most shots is pretty close to that split.)

    I think I'll send the lens in. The camera seems to be working so well, I don't want any one to touch it. I'll call Nikon and see if they need the camera with the lens to make adjustments. I hope they need just the lens....If they need the camera too, I might just stay with what I have.


    Thanks for replies.


  15. My wife tells me I get into a lecturing mode at times :(  which I guess is why my previous answers look like college notes.
    I think this whole BF/FF problem is a camera one due to the fact that the D2X demands an accuracy in setting up the mechanical parts (mirror, focus sensor position etc.) far tighter than any previous Nikon Camera - it's part of the reason the camera is so expensive. It also gets confusing because depth of field (at the subject plane) increases as depth of focus (at the film/sensor plane) decreases and there is some point where the error in depth of focus is hidden by the wide depth of field. This is why the problem shows up at a focused distance where depth of focus is not necessarily at its minimum.
    If the problem shows up with any lens then Nikon should adjust the camera because that is where I think the fault will lie.
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