800mm f6.7 rear filter

Discussion in 'Medium Format' started by steve_phillipps, Dec 2, 2010.

  1. Managed to get hold of an 800mm f6.7 - hell of a lens!
    Unfortunately the image through the camera is incredibly soft. I've got this on approval so no probs, but with it came a repair note stating that it had had the second from front element damaged and repaired so I'm assuming this is probably the reason. BUT, one thing that did occur to me was that it doesn't have a filter in its rear slot and just wondering if this is part of the optical path and not having it in place could be the reason for softeness?
    Steve
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  2. Using the pan arm to support the body
    00Xmi4-307789784.jpg
     
  3. Hopefully my O'Connor video head should be a match for even this monster!
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  4. WJT

    WJT Moderator

    Hi Steve, nice lens. I do not know about the requirement (if any) for a rear filter on the 800, but there is a discussion regarding the 400 here: http://www.photo.net/medium-format-photography-forum/00FU53
    Apparently a rear filter on that lens is not needed to complete the optical path, might even hurt it. By the way...I'm envious! Regards.
     
  5. Thanks Walter, well if it performs as badly as it looks through the viewfinder you're welcome to it!
    Seriously though, it is a nice lens, fairly well built, I always liked side focus wheels (although the one on this is not super smooth but OK).
    I'll try to post some test shots if I can get them scanned.
    Steve
     
  6. I'm pretty sure the element that you described that was repaired is one of the ED elements for the 800. The fact that this element was surrounded by two negative elements leads me to believe that this was one of those highly weatherable exotic glasses. You might try shining a flashlite into the front of this beast to see if there are any hazy elements, especially the one that was repaired. I do not recommend the use of rear filters for the long Pentax lenses. Too many reflections are created, even when using multi-coated filters. The 800, like the other long lenses is susceptible to shutter vib in the 1/2 to 1/30 second range.
     
  7. These test shots are only poor printer scans from a 5x7" print so the quality's not good, but it does give an idea of how they look. The first one is wide open, the second at f11.
    Steve
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  8. This is at f11, which is much better but still very poor, and especially so towards the edges.
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  9. Thanks Steve the Sage, good to have your input! I shot at 1/125th to 1/500th as I'm aware of the vibration issues (although have you seen my tripod? 14kg there!)
    "Hazing" sounds pretty much spot on to me.
    Steve
     
  10. The repair doc says "second element from front had severe chipping due to knock or drop, holding ring was bent, re-grind and re-coat element".
    Steve
     
  11. Yes, that is a giant tripod and head but try shooting it at an 1/8 second to see if it has any shutter problems.
    The fact that the image improves with stopping down leads me to think that it is not haze but aberrations, since most aberrations are reduced by the diaphragm. Faulty repair? Probably.
     
  12. Most ED elements are brittle, so a drop could chip it, even though it is not the front element. Your lens uses a quadruplet front group. I'm hoping the repair people put the first element back in place!
     
  13. I must say "re-grind" element sounds quite severe.
    Steve
     
  14. Regrinding an element changes the whole lens. Send it back.
     
  15. My opinion (like others) is that the lens elements were specified, probably made, ground, polished, coated, etc. then assembled by Pentax.
    Any alteration to that optical path, like the stated "re-grind" would remove material from an element designed by Pentax.
    I would not keep the lens, if you have right of return. It may take awhile, but there are other 800s out there.
    Best of luck.
     
  16. I agree. Send it back. What you've shown does NOT look at all like shutter induced vibration. I does look an awful lot like a poor lens.
     
  17. Thanks guys. Trouble is once you see it you kind of want it then! But no way I'd keep anything if was even 10% below par let alone like this. I have found another one but it's £5000, plus it doesn't have the 1.4x converter with it. Are they hard to come by?
    Steve
     
  18. That looks a lot like bad spherical aberration. My guess is that one of the front lenses was reassembled backwards, or placed in the wrong order, or spaced incorrectly. As for regrinding, they can hardly mean what it comes across as: whittling down the glass thickness just to get rid of a defect. That would be plain idiotic. So I'm sure they mean that a replacement lens element was ground to match the one which was damaged.
    You could give them one more chance: send it back and ask for the elements to be checked, reassembled and optical-bench tested before returning it to you again. Specifically ask for an interferometric test or at least a Ronchi grating test, and to refund your money if they cannot deliver it back with Rayleigh diffraction limited performance (max 1/4 wavelength error). Pentax produces outstanding long focus ED optics (highly prized by astro-imagers); they would never stand over an optic as poor as the one you got; and they must instantly admit there's a problem if these tests reveal it.
     
  19. Thanks Ray, it's going back on Monday. Not sure I'd ever trust or be happy with a repair that appears that drastic. Thanks for your expertise.
    Steve
     
  20. "Are they hard to come by?"
    Yes it is a rare lens. Few are seen on the used market. I did see a new one for sale at Tri-State Camera in NY.
     

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