Focus at infinity

Discussion in 'Nikon' started by raymondc, Sep 24, 2012.

  1. I was just out the other day and thought some images were a bit soft. I then ran some tests. They were shot on tripod for 100 sec each at night, manual focus at infinity. I then got some images a couple days ago and found that one of the supposedly weak lenses performed quite good as well. So I like your views ...
    Maybe the 18-70 is not that sharp at 18 or 70mm but at 35mm on the 2nd photo is not bad? Maybe that is the reason why my shots the other night was not that sharp?
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  2. Try taking the test photos with wide open aperture - I have had some zoom lenses shift focus with aperture stopped down.
     
  3. manual focus at infinity​
    Those lenses don't have hard stops at infinity - did you determine visually where infinity focus actually is? Or did you just set the lens to "infinity" and hope for the best?
     
  4. I think I found the issue. the 18-70DX at this time, will check the others, when you focus at infinity according to the marking it's not the same as AF something afar (or when the yellow dot in the viewfinder comes up).
    does this issue happen on the AF or MF prime lenses? interested to know b/c i may be getting some of those. i do a lot of low light photography so it's very handy to just focus at infinity right at the end .....
     
  5. SCL

    SCL

    Some have a hard stop and some don't. If you're shooting digital it is pretty easy to check. I usually test my lenses' infinity focus against the moon. I rarely shoot wide open, so my tests are usually around f5.6, but a wide open test, IMHO, is best because I can count on the DOF stopped down to include infinity, even if my infinity stop is a little off. Some lens manufacturers provide for a little past infinity focus in their lenses to account for thermal expansion....this is the case in 3 of my long telephoto lenses.
     
  6. Dieter, I just set it to infinity "blindly". Was just testing out ... At least I shot my fllm with a 18-35mm which isn't that bad... Some times it may be too dark for AF to work .. or let you know .....
     
  7. The AF-S 18~70mm, like many zoom lenses, is vari-focal. It is not a true zoom, i.e. it doesn't hold focus as you change focal length from 18mm through to 70mm. This is very obvious in the close up focusing range. Furthermore, as Dieter notes, lenses like the 18~70mm don't have hard stops aligned to the center of the infinity index mark, and even if they did they would be quite useless as it would only be accurate for one extreme of the zoom range.
    You can't scale focus these zoom lenses, because the infinity marking at opposite ends of the zoom range is not going to be the same.
    The AF 50mm f/1.8D on the other hand does have a hard stop for infinity, and presuming it is correctly calibrated, being a single focal length lens you can rely on the stop to set infinity focus.
    Your problem is with technique, not your lens. :)
     
  8. Would manual primes have hard stops? Including a series e lens? I still shoot film also ....

    Yeah but at times it may be too dark that AF confirmation may not work .....
     
  9. Yes, all of the Nikon Series E primes have hard stops at infinity. All of these lenses can be "calibrated" so that the focusing ring hits the stop at proper infinity focus.
    Most Nikkor Ai/AiS primes have hard stops at infinity (the long telephoto ED primes often focus "beyond" infinity to accommodate thermal expansion). Again, the infinity stop is adjustable and can be accurately calibrated to stop at proper infinity focus.
     
  10. What would happen to the dof markings on the lens if someone wanted to employ hyperfocal focussing? Just curious.
    Will play with it more, maybe the 50mm I tested using the AF feature. Because today I tested both the 50 and 85 primes, if I turn it right to the end until it stops the pictures are quite fussy. But AF works fine.
     
  11. Cheers - tested again, the above sign post was maybe not far enough away to be infinity. I AF the camera/lens at the distant hills street lights. The only lens that works properly and stops hard for me is the 50/1.8D and the 85/1.8D.
    The wide angle zooms are ok at the longest FL setting but not the widest FL. The others like the 18-70 or 18-200 was like some millimeters off it. The Sigma 10-20 at the 10mm said infinity (AF) was 1m on the lens barrel - ditto the 18-200mm at the 18mm - LOL. The Nikon 35mm 1.8 DX doesn't have any markings anyway but if you kept turning the ring .. it's not focused either.
    Hopefully didn't mess too much slides up.
     
  12. Because of the way ED glass changes with temperature (as I've been told), a lens with ED glass needs to focus past infinity, because infinity focus "changes" with these lenses. That's the reason.
     
  13. I've never had an MF Ai, series E or Ai-S Nikkor that needed "calibrating" to infinity. They're usually all bang-on correct unless they've been dropped or otherwise abused. Some of the longer Ai-S Nikkors, especially IF types, focus past infinity, but even those lenses are correct when the cursor is set to the infinity mark. It's only the new plastic fantastic zooms and AF lenses that are off.
    So the thermal coefficient of ED glass is greater than the expansion or contraction of the barrel it's mounted in? I think someone's been feeding you a load of porkies Peter.
     
  14. I've never had an MF Ai, series E or Ai-S Nikkor that needed "calibrating" to infinity.
    I've not come across one either. But the focus stop is usually adjustable, so the potential for even a "mis-calibrated" factory setting does exist, particularly since on many prime lenses the mechanism is not very "sophisticated" and all it would take would be a simple almost imperceptible slip of the ring before the final screw is tightened (or gasp, adhesive tape is applied). Even Nikon workers from the bygone era when quality mattered above all else were not 100% perfect. :)
     
  15. I found this thread because of a problem with one of mine -- an early '80s 200-600 f/9.5 zoom. I have tried to shoot the moon with it a number of times and always had severe sharpness issues which I believe are due to a lack of collimation at infinity rather than planetary motion. I had initially thought the lens was just poorly paired with my D200, but I used it for some sports work last week and it was awesome for me.
    Presumably this adjustment is something the lens needs to go back to Nikon for?

    The lens has always been in the family and so I know it has never been so much as knocked.
     

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