Scope focus question

Discussion in 'Hunter (HFT) & Field Target (FT)' started by Sparky, Jul 15, 2009.

  1. Sparky

    Sparky Ich dien Staff Member

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    As some of you know decided to give the Bushnell 6500 a go this year for HFT, I always have the focus wound all the way in, but after M.A.D I wound it out a bit to get a sharper cross hair image, now since then at the club I seem to have been going further left than others at 45 yards (all other ranges spot on) so me and “Sulky Del Boy” Stone went up the range yesterday and again although there was a right to left wind I was definitely going more left than I should have been, and too be honest wasn’t far off putting the EB back on, but remembering something Dave Hatfield told me decided to wind the focus(eye piece) all the way back in and to my surprise the pellets were going where they should have been at 45 yards???? Now I’ve never experienced by altering the focus causing parallax error before but it definitely is the case this time.

    Anyone else ever come across this before?

    Cheers
    Pete
     
  2. Scooby

    Scooby Pete Dutton

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    I still think it's likely that your fast focus was altering your zero which might not have been as apparent at 35y or less in the wind but may have been visible at 45y....
     
  3. Levidog

    Levidog Member

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    I'm being thick, but whats a fast focus
     
  4. RobF

    RobF Administrator Staff Member

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    Hmm, all the eye focus does is focus the eye on the crosshair... the parallax is the thing that moves the focal plane of the focused image on the same plane as the crosshair, so the two cannot slide apart when you move your head when the scope is paralaxed to the correct range... so really, altering the ret's focus should not have any effect on parallax, because the difference in focal planes between the ret and the image is still the same, and a function of the lenses in front, not behind.

    However, i have noticed that changing the fast focus on my custom shops has a magnifying effect on the picture... in extremes there is slight, but definate change in the size of the picture/ret... it's small, but noticable... i think it's too small to be noticable on POI shift in wind... but it's there... dunno, have a look at that. I haven't looked to see if it affects the amount of relative parallax error, because for FT the scope is always pa'd to the range... but it might be interesting to see if that can exaggerate that.

    But i find it odd that you've not got the hairs focused to their optimum anyway... is there some secret reason for this? It's just that the eye will try and dive to focus on the best image, so if you have a weakly focused hair, and weakly focused image, it can jump to the image, and make the hair go out of focus... if the hair is focused better it prevents this to a greater extent.
     
  5. alan-aitch

    alan-aitch I'll practice tomorrow...............

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    The new RS is meant to be quite nippy :D:D
     
  6. alan-aitch

    alan-aitch I'll practice tomorrow...............

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    I think I would go with Mr Dutton on this, simply because of an experience I had with a Bushnell Trophy a couple of years back.

    Stuck it on a gun, zeroed it up and all was fine until I decided I wanted to move the parallax a little. As this was a non PA scope so I had to loosen the front and rotate the lens, after I had done this, the POI had shifted dramatically, inches not just a little bit. Now I know we are talking about lenses at opposite ends of the scope but I can easily imagine that once you take one lens of a group out of alignment (with the turrets) rotating another lens may well change things if the lenses are not 100% accurately ground.

    I am prepared to be told I'm talking rubbish though :D

    Alan.
     
  7. Dave Ramshead

    Dave Ramshead England HFT Team squatter

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    excellent:D
     
  8. Levidog

    Levidog Member

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    Cheers Alan :p
     
  9. RobF

    RobF Administrator Staff Member

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  10. Sam Vimes

    Sam Vimes New Member

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    I haven't got a clue.;)
     
  11. Sparky

    Sparky Ich dien Staff Member

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    Did you get many bunnies after:D

    Pete
     
  12. Sam Vimes

    Sam Vimes New Member

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    Three little buggers came out to play 25 yards and down wind of us seconds after we hung up. So much for being quiet and watching the wind direction!;):D
     
  13. Levidog

    Levidog Member

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  14. nurek

    nurek Member

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    Pete,

    I had a similar experience with an old Hertel and Reuss 6x46 scope that I got from Barry McCraw. The fast focus eyepiece changed the POI, but on that particular scope you could actually see the fast focus move up and down and from side to side, now before anyone says anything I got that scope free from Barry with a gun I had bought from him.

    Fast focus affects the magnification/parallax and both parameters affect the POI on some scopes so it has to be about lens alignment?

    I like the fast focus feature (as opposed to the screw in/locking screw type) and was quite happy to see it on the new Bushnells, but maybe it isn t such a good idea, but than again it seems to work ok on other Bushnells like the Elite 3200 10x40 and there is a lot of them about on the HFT circuit?
     
  15. RobF

    RobF Administrator Staff Member

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    I know lee had problems at byley...he was fiddling with his scope enchancer around the course and it pushed his fast focus around... and his ranges went out as a consequence.
     

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