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Thread: Nikon 50mm F1.8 Front focus on D80

  1. #21
    Senior Member zoossh's Avatar
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    Default Re: Nikon 50mm F1.8 Front focus on D80

    am i right in say so far that all what you guys are discussing in terms of how much focusing moves front and back are determined by how much a wheel moves inside moves in terms of the circumferential distance (which moves in intervals of the teeth on the wheel)?

    getting a bit too technical for me. but still want to clarify.

  2. #22
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    Default Re: Nikon 50mm F1.8 Front focus on D80

    Hi zoossh, if I did not misread your post, you are right. Motor is a rotary actuator while focusing needs linear motion. So the motor will have a set of gearing mechanism to translate its motion from rotary to linear action. But exact type of gear mechanism I am no sure.

    Quote Originally Posted by zoossh View Post
    am i right in say so far that all what you guys are discussing in terms of how much focusing moves front and back are determined by how much a wheel moves inside moves in terms of the circumferential distance (which moves in intervals of the teeth on the wheel)?

    getting a bit too technical for me. but still want to clarify.

  3. #23

    Default Re: Nikon 50mm F1.8 Front focus on D80

    Quote Originally Posted by lastboltnut View Post
    Hi zoossh, if I did not misread your post, you are right. Motor is a rotary actuator while focusing needs linear motion. So the motor will have a set of gearing mechanism to translate its motion from rotary to linear action. But exact type of gear mechanism I am no sure.
    I once dismantled the LX-2 lens module, and noticed that it translate rotary motion to linear motion via groove sliding action, i.e. while rotating the elements, because guided by leading grooves, elements hence move forward and backward based on the direction of rotation, this is same concept as rig drill. Not sure if this is a common methodology used?

  4. #24
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    Default Re: Nikon 50mm F1.8 Front focus on D80

    Worm gear set?

    Quote Originally Posted by scaredcloud View Post
    I once dismantled the LX-2 lens module, and noticed that it translate rotary motion to linear motion via groove sliding action, i.e. while rotating the elements, because guided by leading grooves, elements hence move forward and backward based on the direction of rotation, this is same concept as rig drill. Not sure if this is a common methodology used?

  5. #25

    Default Re: Nikon 50mm F1.8 Front focus on D80

    Quote Originally Posted by lastboltnut View Post
    Worm gear set?
    Nope, sun gear set.

  6. #26
    Senior Member zoossh's Avatar
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    Default Re: Nikon 50mm F1.8 Front focus on D80



    think i'm losing it again. technical talks.... Worm gear? sun gear?

  7. #27

    Default Re: Nikon 50mm F1.8 Front focus on D80

    Hi,
    managed to borrow the D80 from my friend.
    the new lens still have slight front focus. The problem is obvious at F1.8. But when I step down further to F2.2 or F2.8 and beyond, the front focus is less obvious due to the increase in DOF.

    pictures taken at F1.8 is soft, I guess partly due to the shallow DOF, aggravated by the front focus.

    pictures taken at F2.8 and beyond is sharp. For that, I am going to accept the fact that this lens has a slight inherited front focus problem. And I will continue to enjoy using the lens at smaller aperture size of say F2.2 and above.

    I still do not accept NSC's explanation about this lens is not designed for digital camera , and therefore is expected to have front focus.

    Thanks for the valuable info provided.

  8. #28

    Default Re: Nikon 50mm F1.8 Front focus on D80

    Quote Originally Posted by scaredcloud View Post
    I once dismantled the LX-2 lens module, and noticed that it translate rotary motion to linear motion via groove sliding action, i.e. while rotating the elements, because guided by leading grooves, elements hence move forward and backward based on the direction of rotation, this is same concept as rig drill. Not sure if this is a common methodology used?
    Zoom rings in SLR and other lenses have been working the same way for ages.
    Last edited by lsisaxon; 29th January 2008 at 04:33 PM.

  9. #29

    Default Re: Nikon 50mm F1.8 Front focus on D80

    Quote Originally Posted by lastboltnut View Post
    Hi Isisaxon,

    I am not sure about the cam AF mechanism....but base on mechanical defition, I think scaredcloud is right. By mechanical definition, backlash means the error when the gear move in one directionthen reverse. For example at a location "0" the motor moves the gear 1deg Clockwise (CW), and then the motor moves it back 1 deg, but actually the gear in not in location "0" as there is a backlash error.....thus a backlash error only comes in if the focus hunts.....if the focus only move in one direction, there won't be any backlash problem....

    The overshooting of the focus point, like cam ask motor to stop, but motor did not stop on time is a control problem.... like the gain of the control is not correct...

    I am just trying to discuss in mechanical point of view...Hope someone can shed more light to this issue regarding the Nikon AF mechanism.
    Hmm... I don't know the exact terms because I'm neither a mechanical engineer nor a control engineer.. hahaha.. but it probably has something to this effect because there is a backlash correction factor in the motor drive of my telescope.

    I believe the body should be using a servo motor. For AF-S lenses, the SWM may be similar to a stepper though..

  10. #30
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    Default Re: Nikon 50mm F1.8 Front focus on D80

    hmm.. i guess we have to give small allowances for variance when using such shallow depth of field. nikon also has pretty accurate AF. if you recall those days when camera companies used to advertise how many steps their AF system takes to achieve focus, you will note that nikon system has the most number of steps. (something like 200+ steps versus another top brand's 144+ steps)

  11. #31

    Default Re: Nikon 50mm F1.8 Front focus on D80

    I do agree that we need to give small allowance for variance, but the point is this lens is consistently front focus, so that mean this should be a design issue/limitation that Nikon should be able to overcome.

  12. #32
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    Default Re: Nikon 50mm F1.8 Front focus on D80

    Did you read what I wrote before ? Your test chart is invalid - download a proper test chart.

    Quote Originally Posted by gooseberry View Post
    As I mentioned in another thread, the test chart you've used - the lines are so close together, you can't rule out the fact that the camera just focused on the line(s) further in front of the X (the focusing sensor area is larger than you think it is).

    Google for D70 focus test chart to get a pdf of a much better test chart.

  13. #33

    Default Re: Nikon 50mm F1.8 Front focus on D80

    Thanks Gooseberry,

    I will try with the test chart when I could get the cam again.

    But basing on test chart printed by NSC, the previous lens did show some 20micron of front focus, this is agreed by the NSC staff. And since this new lens has the same behavior, I would think the slight front focus at larger aperture(F1.8) is inherited with the design of this lens.

  14. #34
    Senior Member zoossh's Avatar
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    Default Re: Nikon 50mm F1.8 Front focus on D80

    Quote Originally Posted by CT 3833 View Post
    I do agree that we need to give small allowance for variance, but the point is this lens is consistently front focus, so that mean this should be a design issue/limitation that Nikon should be able to overcome.
    it seems like an uniform design issue/limitation. but whether nikon can overcome now, not sure abt it.

    soomeone also had similar problems...

    http://www.clubsnap.org/forums/showthread.php?t=346803

  15. #35
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    Default Re: Nikon 50mm F1.8 Front focus on D80

    Hey scaredcloud and Isisaxon, do you have a pic/illustration on these set up? I tried searching the net and couldn't really get a look at how it is....or scaredcloud, can sketch if you don't mind? Really keen to know how its design.

    Thanks!

    Quote Originally Posted by scaredcloud View Post
    I once dismantled the LX-2 lens module, and noticed that it translate rotary motion to linear motion via groove sliding action, i.e. while rotating the elements, because guided by leading grooves, elements hence move forward and backward based on the direction of rotation, this is same concept as rig drill. Not sure if this is a common methodology used?
    Quote Originally Posted by lsisaxon View Post
    Zoom rings in SLR and other lenses have been working the same way for ages.

  16. #36
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    Default Re: Nikon 50mm F1.8 Front focus on D80

    Hi, so you sent in the 2nd lens to NSC as well?

    Quote Originally Posted by CT 3833 View Post
    Thanks Gooseberry,

    I will try with the test chart when I could get the cam again.

    But basing on test chart printed by NSC, the previous lens did show some 20micron of front focus, this is agreed by the NSC staff. And since this new lens has the same behavior, I would think the slight front focus at larger aperture(F1.8) is inherited with the design of this lens.

  17. #37

    Default Re: Nikon 50mm F1.8 Front focus on D80

    No I did not send the second copy to NSC but I mean the second copy has the same behavior.

    Manage to find a test shot tat day, I aim at button number 5, it turn out sharper at button 8. I have to iterate, this will happen to F1.8. When I step down to F2.2 or higher, the sharpness at button 5 improved because of the higher DOF.

    I have to say that this is a worst case, some shots are better but still OOF. By the way I used a tripod to eliminate handshake, focus point error.





    And as mentioned, someone else(post in Nikonians) has the same problem, but with the higher end F1.4 version. And NSC there told him the same that this lens is designed for film, therefore the front focus.

    http://www.nikonians.org/forums/dcbo...d=121003&page=
    Last edited by CT 3833; 30th January 2008 at 09:25 AM.

  18. #38

    Default Re: Nikon 50mm F1.8 Front focus on D80

    Quote Originally Posted by CT 3833 View Post
    No I did not send the second copy to NSC but I mean the second copy has the same behavior.

    Manage to find a test shot tat day, I aim at button number 5, it turn out sharper at button 8. I have to iterate, this will happen to F1.8. When I step down to F2.2 or higher, the sharpness at button 5 improved because of the higher DOF.

    I have to say that this is a worst case, some shots are better but still OOF. By the way I used a tripod to eliminate handshake, focus point error.





    And as mentioned, someone else(post in Nikonians) has the same problem, but with the higher end F1.4 version. And NSC there told him the same that this lens is designed for film, therefore the front focus.

    http://www.nikonians.org/forums/dcbo...d=121003&page=

    Looks to me even the 0 is sharper than the 8. That's quite a fair distance from the 5 button hor? I also got a 50mm f1.8 but too bad i using D40, so can't test for AF gear problem or not since I manual AF.

  19. #39

    Default Re: Nikon 50mm F1.8 Front focus on D80

    Lukesky,
    as I said, this is the worst case, there are some not too bad but net is it is still front focus.

    Did you try using manual focus at F1.8 and see if you get the same outcome?
    Last edited by CT 3833; 30th January 2008 at 03:43 PM.

  20. #40

    Default Re: Nikon 50mm F1.8 Front focus on D80

    Quote Originally Posted by CT 3833 View Post
    Lukesky,
    as I said, this is the worst case, there are some not too bad but net is it is still front focus.

    Did you try using manual focus at F1.8 and see if you get the same outcome?
    You might want to read this article.
    http://www.adorama.com/catalog.tpl?a...op=academy_new

    Nikon addressed this issue by allowing the user to fine tune the AF in D300 and D3.
    Last edited by lsisaxon; 30th January 2008 at 03:49 PM.

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