A&P Focusing the Canon 50mm f/1.4 (part II) **pics** **56k why are you in this forum?**

Discussion in 'Lifestyle' started by JaimeZX, Jan 7, 2007.

  1. JaimeZX

    JaimeZX Formerly of :Sep 2001: fame - Also: Sprout Crew OT Supporter

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    In order to test my theory of back-focusing with this lens, I set up a little test rig:

    I used pink insulation foam as a backdrop due to its plainness to deliberately make it difficult for the camera to focus on it, thus (in theory) isolating the targets better.


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    The camera rested on teh black case at bottom and I used each AF point to try to focus on the various targets as indicated in the above image. Between each of three shots I focused elsewhere, forcing the AF to reacquire the target for each shot.

    I used a blue line to approximate the focal plane across the image and a red box of course to indicate the desired target. You'll see I get better at this as we proceed through the AF points, as I sort of figured out how this was working.

    Finally, all shots were ISO-800, f/1.4, and 1/640s. I wanted to keep shutter speed up to reduce the possibility of motion blur; lighting was 4 x 60w bulbs in the cieling and 1 x 90w bulb about 3 feet to the right of the test rig. So! Let's get to it:


    The center AF point is supposed to be the most accurate, but in all three of these shots, plus a few others I took, it consistantly back-focused by maybe a quarter of an inch? To be fair, its target was a difficult one, and perhaps I would have had better performance with something that stuck up a bit as opposed to just an opening in the T-square.

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    For the "2 O'clock" AF point, another difficult target, the back edge of the small metal ruler. Performance was simliar to the center point, back focusing a bit. Also, difficulty acquiring this target in the first place.

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    The 3 O'clock AF point I found to be the most accurate. This could, again, be attributed to the target, which stuck up a bit providing lots of edges for the camera to see. Still, I was pleasantly surprised. :)

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    The 4 O'clock point was again a tougher one for the camera. Originally it was the edge of the small ruler again, but it had so much trouble acquiring the target that I added a dime to the area to give it more edges. Some back-focusing but overall not too bad.

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    The 6 O'Clock point also showed some back-focusing initially, until when I looked at the lens itself I found that the little medallion in the label was about an inch closer than perhaps the closest focus distance. So this data may be totally invalid. :o

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    The 7 O'clock point I expected to be tough; the dog's paw in the bookmark. Surprisingly it did fairly well.

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    The 9 O'clock point had performance similar to the 3 O'clock point: pretty darn good, and this time the target was not "sticking up in the air," it was the back-edge of the bookmark. If anything, it may have front-focused that spot a smidge.

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    The 10 O'clock point also had excellent performance, the knot on the bookmark yarn.

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    Finally, the 12 O'clock point was really disappointing. The target was sticking up and had lots of edges, but it was consistantly back-focused.

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    Conclusions: clearly the camera does better with higher-contrast targets. I almost feel like re-shooting this whole series again with pop-up targets. But the PS took me a long time. :p Really the biggest surprise I guess was the 12 O'clock point. :hsd:

    Questions? Comments? Suggestions?
     
  2. not a bad way to spend 3+ hours :o
     
  3. Changed

    Changed Guest

    You can't really solidy confirm that the lens performs lower-than-par because thoses tests are far from perfect and not in a controlled environment.
     
  4. JaimeZX

    JaimeZX Formerly of :Sep 2001: fame - Also: Sprout Crew OT Supporter

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    You are absolutely correct.

    Is that an upside-down skull in your av or a boom box? :confused:
     
  5. Changed

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    It's a skull... I had it right-side-up for a while and felt like a change of late. :wiggle:
     
  6. JaimeZX

    JaimeZX Formerly of :Sep 2001: fame - Also: Sprout Crew OT Supporter

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    Looks like a jammin' boom box. :mamoru:
     
  7. Changed

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    I can't really picture that at all :rofl:
     
  8. :werd: he must have a wild imagination :rofl:
     
  9. JaimeZX

    JaimeZX Formerly of :Sep 2001: fame - Also: Sprout Crew OT Supporter

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    The boom box and handle are bouncing up and down to the beat and the speakers are pulsing with the music too, see?
     
  10. mattsb2000

    mattsb2000 OT Supporter

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    How high are you right now?
     
  11. :mamoru: :mamoru:
     
  12. JaimeZX

    JaimeZX Formerly of :Sep 2001: fame - Also: Sprout Crew OT Supporter

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    :squint: Well now I just woke up from a nap. :o
     
  13. misfits

    misfits Well-Known Member

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    :rofl:
     
  14. yonson

    yonson OT sux... OT Supporter

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    :rofl: I knew it was a downside up skull, but now that it's been mentioned, it does kinda resemble a boom box :mamoru:
     
  15. N-Word-Jim

    N-Word-Jim Cure for boredom

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    Ya know... the sensors aren't perfectly represented by those little boxes they put in your viewfinder ;). On a test like this with that shallow of an angle that could cause a fair bit of error right there.

    Also, I dispute the validity of the tests entirely due to you not framing the photos before uploading.
     

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