Site Sponsors
Results 1 to 14 of 14

Thread: focus distance accuracy

  1. #1
    Workshop Member
    Join Date
    Nov 2007
    Location
    Brooklyn
    Posts
    4,043
    Post Thanks / Like
    Images
    1253

    focus distance accuracy

    i had a bit of a diversion, so:
    regarding the alpa lens shimming, most of the concept is a red herring, since all you are doing is ensuring that when the lens helicoid is set to the infinity stop, the camera is actually focused at infinity. very important, to be sure, but after that, focus at any intermediate distance is entirely up to how accurately you can estimate that distance (maybe to a few inches, if you are lucky) and then, how accurately you can set the distance scale (and ultimately the image distance) on the helicoid to that number.

    Here the arca leads the way with the accurate and fine pitch helicoid, set by working from a chart, I believe. Alpa offers add-on scales with intermediate markings as useful aid.

    for example, using a 28mm lens with a subject distance of 10,000mm (10meters or about 33ft)and comparing that to a subject distance of 10.2 meters ( 33ft-8inches)

    to give you and idea of the dimensions, the lens to image plane distances are:

    inf: 28.000mm
    10M (33ft): 28.079mm
    10.2M (33ft-8inches): 28.077mm

    so at about 33ft, 8" subject distance change means about .002mm distance to sensor change

    the Arca has five turns for it's helicoid and my estimate is it moves about 7mm (6mm is what my 120mm lens has to move to get from inf to 2.6M (8ft). that works out to about .004mm movement per degree of rotation and for a 3" dia lens adjustment ring, one degree is about 1/2mm on the barell, or less than 1/32"

    it would take 1/2 of one degree for the Arca to focus between 10M and 10.2M with a 28mm lens

    for a 43mm lens, about .004mm difference
    for a 70mm lens, about .010mm difference between 10 M and 10.2M
    for a 120mm lens, about .029mm difference

    using 1/f=1/I + 1/O


    any discussion?
    Last edited by jlm; 12th May 2011 at 14:43.

  2. #2
    Garcia
    Guest

    Re: focus distance accuracy

    With a 28mm @ f/5.6 focused at 10 meters, DOF is so large that you can afford a good bit of focus error.

  3. #3
    Senior Member
    Join Date
    May 2008
    Location
    Hong Kong / Asia
    Posts
    524
    Post Thanks / Like

    Re: focus distance accuracy

    Quote Originally Posted by Garcia View Post
    With a 28mm @ f/5.6 focused at 10 meters, DOF is so large that you can afford a good bit of focus error.
    Yes, but afford minor error since in particular for wides the critical is the tolerance on depth of focus (not depth of field).

    Alpa and Arca addresses this by two different means; shimming and fudge factors.

    The rest is about tolerances of individual components of the systems and how they add up (and adjusted/shimmed in factory). Under assumption that Alpa's tolerance of their system is suffice small, then there should be no issue to accurate focus with an Alpa helical compared to Arca. A weakness of the Arca in regards to tolerances is the tilt that is built into the body since for sharpest optimum focus without tilts, and in particular for wides, the lens need to be perfect parallel to sensor (within 0.02mm). This is possibly a reason to why Alpa is providing tilt adapter for 80mm lenses and longer. An Alpa also avoids table look ups for each focus, as is Arca's method...

    For the Schneider 24mm Digitar as example the total focus travel from 1.5m to infinity is only 0.3mm and if instead an error causes defocus by 0.1mm one will focus on 5m instead of infinity. Thus the large depth of field would not be suffice helpful to achieve optimum sharpnes of image... Now compare that with if the tilt neutral position is out of similar tolerance... or not perfect adjusted for where the plane of focus is intended at tilt.

    Regards
    Anders

  4. #4
    Garcia
    Guest

    Re: focus distance accuracy

    Good points, Anders.

    Now I realize that Arca helicoid moves equal amount no matter which focal lens you are shooting with, but Alpa or any other traditional helical mount displaces the lens as a function of its focal length so you get more accurate focus setup thanks to fine pitch thread for wider lenses.

    So, in wide angles you get more precise lens placement per degree of focus ring turn with a classic helical mount than with Rm3d five turn mount albeit Arca system is more precise for focusing teles.

  5. #5
    Workshop Member
    Join Date
    Nov 2007
    Location
    Brooklyn
    Posts
    4,043
    Post Thanks / Like
    Images
    1253

    Re: focus distance accuracy

    measured travel of my lenses:
    43mm: 2.5mm travel, about 180 degree rotation, close focus limit 0.4M, pitch 5mm/rotation

    70mm: 8.3mm travel, about 270 degrees rotation, close focus 0.8M, pitch about 11mm/revolution

    120mm: 6.1mm travel, about 270 degrees rotation, close focus 2.6M, pitch about 8mm per revolution

    I don't have an arca to compare, but if it travels enough to cover the longest travel range of these lenses, say 8.3mm in five revolutions, the pitch would be 1.6mm per revolution, about 3 times finer pitch than my widest, the 43mm lens

  6. #6
    Member
    Join Date
    Feb 2011
    Posts
    102
    Post Thanks / Like

    Re: focus distance accuracy

    Quote Originally Posted by Anders_HK View Post
    Alpa and Arca addresses this by two different means; shimming and fudge factors.
    I'm not an Arca user, but can we all please stop referring to the Arca focusing system as using a "fudge" factor. It's an incorrect description and does a disservice to the R camera system. It implies that it's applied randomly and/or by guesstimate differently in every situation, whereas the Arca focusing offsets are actually very precisely measured for each individual lens and applied consistently.

    I think others have been referring to it as using a "lens offset" factor.

    (I'm not aiming this specifically at Anders, it's just something that irks me.)

  7. #7
    Workshop Member
    Join Date
    Nov 2007
    Location
    Brooklyn
    Posts
    4,043
    Post Thanks / Like
    Images
    1253

    Re: focus distance accuracy

    I'm guessing here, but i assume the arca helicoid is calibrated in arbitrary units, probably quite finely, and then for each lens, they produce a chart that relates the helicoid scale to the actual distance for that lens?

    so i look up say, 10M distance, for a 120mm lens on the chart, then set the helicoid scale to that number?

  8. #8
    Super Duper
    Senior Member
    Shashin's Avatar
    Join Date
    May 2011
    Location
    Florida, USA
    Posts
    4,501
    Post Thanks / Like
    Images
    141

    Re: focus distance accuracy

    Hi. I have been visiting Getdpi for a while, but have never joined. I work in an imaging center dealing with scientific, technical, and applied imaging.

    Just to throw my 2-cents i here about image plane tolerances. All you need is to calculate the depth of focus. You only need two things to work it out: permissible circle of confusion and the f-number--focal length is not a factor.

    Depth of focus = 2 x circle of confusion x f-number

    The constant of 2 is that the depth of focus is in front and behind the plane.

    If you take the pixel pitch for the circle of confusion, that will give you a number to work with. With a sensor with a 5 micron pitch (0.005mm) with a lens at f/5.6 would result in a depth of focus of 0.056mm. That would be the tolerance at the focal plane.

  9. #9
    Workshop Member
    Join Date
    Nov 2007
    Location
    Brooklyn
    Posts
    4,043
    Post Thanks / Like
    Images
    1253

    Re: focus distance accuracy

    interesting, working the equation the other way:

    using .056mm as the image distance spread for an object distance or 10M (32ft), for a 28mm lens at f5.6, the object distance range is 10M to 5.8M

    using a 70mm lens at f5.6, same .056mm, a target 10m away would have a 1M depth of field


    these seem overly optimistic

    feel free to check my math, i used 1/f=1/I +1/O,

    or O=1/((1/f)-(1/I))

  10. #10
    Super Duper
    Senior Member
    Shashin's Avatar
    Join Date
    May 2011
    Location
    Florida, USA
    Posts
    4,501
    Post Thanks / Like
    Images
    141

    Re: focus distance accuracy

    Depth of focus is the total tolerance of the area--0.056mm. To put it another way, +/–0.028mm from the focal plane is the tolerance for the sensor. So depth of field should be on both sides of the 10m mark.

    Sorry, I am out the door and so I may not be spending the time to do justice to your comment.

  11. #11
    Workshop Member
    Join Date
    Nov 2007
    Location
    Brooklyn
    Posts
    4,043
    Post Thanks / Like
    Images
    1253

    Re: focus distance accuracy

    Comment:

    jack stated in his IQ review that the use of focus mask might neuter the need for shimming your back. I think what he means is that you don't need to be limited by the ability to first, accurately measure the distance to the subject and then second, set the lens to that number.

    you just guess the distance, keep taking shots and chimp using focus mask until the LCD satisfies you. Sounds good to me.

    you will still have to shim to make sure the lens can mechanically get to inf however, but this will only pertain to shots taken at...infinity

  12. #12
    Senior Member
    Join Date
    May 2008
    Location
    Hong Kong / Asia
    Posts
    524
    Post Thanks / Like

    Re: focus distance accuracy

    Quote Originally Posted by cng View Post
    I'm not an Arca user, but can we all please stop referring to the Arca focusing system as using a "fudge" factor. It's an incorrect description and does a disservice to the R camera system.

    I think others have been referring to it as using a "lens offset" factor.
    Fudge factor was in no ill meaning

    "Lens offset" is not correct term since the factors applies to "fudging" /adding factors to focusing in order to compensate fabrication tolerances for camera, lens and back.

    Perhaps we can hereby instead name them "adjustment factors"??

  13. #13
    Member
    Join Date
    Feb 2011
    Posts
    102
    Post Thanks / Like

    Re: focus distance accuracy

    Quote Originally Posted by Anders_HK View Post
    Fudge factor was in no ill meaning
    I know, I know. It just sounds so random and haphazard. "Adjustment factor" is lots better.

    LOL, I don't even own an Arca. Yeah, I'm a grumpy old man.

  14. #14
    Subscriber & Workshop Member GrahamWelland's Avatar
    Join Date
    Nov 2007
    Location
    Vancouver, WA
    Posts
    5,802
    Post Thanks / Like
    Images
    564

    Re: focus distance accuracy

    Quote Originally Posted by jlm View Post
    Comment:

    jack stated in his IQ review that the use of focus mask might neuter the need for shimming your back. I think what he means is that you don't need to be limited by the ability to first, accurately measure the distance to the subject and then second, set the lens to that number.

    you just guess the distance, keep taking shots and chimp using focus mask until the LCD satisfies you. Sounds good to me.

    you will still have to shim to make sure the lens can mechanically get to inf however, but this will only pertain to shots taken at...infinity
    Exactly.

    So long as you can actually focus to infinity it becomes an iterative process where you can basically treat the lens barrel distances as merely a guide and then fine tune using focus masking.
    Remember: adventure before dementia!

    As Oscar Wilde said, "my tastes are simple, I only like the best"

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •