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Question regarding focal length and adapters

Discussion in 'Adapted Lenses' started by Believer, Jan 11, 2013.

  1. Believer

    Believer TalkEmount Regular

    80
    Nov 28, 2012
    New Jersey, US
    Ryan
    Hello all,

    I am wondering - Does the length of the adapter effect the focal length of the lens because it moves the glass further away from the sensor?

    Thanks!
     
  2. Bimjo

    Bimjo Super Moderator

    Oct 28, 2011
    Washington State
    Jim
    No.

    Each camera system has a set distance that the lens sits from the media (film/sensor). This is referred to as the registration distance. As long as a lens can maintain its registration distance it will function on any camera like it did on the camera it was designed for.

    If you increase the registration distance you allow the lens to focus more closely than normal, up to and including macro range. This also eliminates the ability to focus to infinity.

    If you decrease the registration distance you may have some very limited range that might be in focus, but it is unlikely.

    To increase the apparent focal length of a given lens usually requires that some adapter that includes optics (one or more lens elements) be used (teleconverter).

    hth :)
     
  3. Believer

    Believer TalkEmount Regular

    80
    Nov 28, 2012
    New Jersey, US
    Ryan
    Interesting information Bimjo, thank you!

    So to make sure I understand correctly - when I slap my 135mm legacy lens on my Nex with the adapter, I have inhibited it's ability to focus to infinity? That makes sense because I experienced this but chalked it up to a sub-par lens.
     
  4. Deadbear77

    Deadbear77 TalkEmount Hall of Famer

    Sep 14, 2012
    Northeast Ohio
    Kevin
    I don't believe so, some of my md adapters are not the right length. Makes lenses not work correctly. I actually thought some of my lines were crap, it was just a crap adapter.
     
  5. Bimjo

    Bimjo Super Moderator

    Oct 28, 2011
    Washington State
    Jim
    No, what the adapter does is maintain the correct registration distance so the lens will focus correctly on the NEX body. The registration distance for the average dslr is somewhere around 40-45mm (give or take a bit) and the registration distance for the NEX is 18mm. Any lens with a registration distance longer than 18mm can be adapted to the NEX and focus correctly. Trying to mount a NEX lens on a dslr body wouldn't work because the lens would not focus.

    Here's a list of camera/lens registration distances. Using a long registration lens on a short registration camera is fairly easy (basically you're just adding a tube between the camera and lens). Using a short registration lens on a long registration body is difficult if not impossible (here you may need to actually place the lens inside the camera, obviously not possible in most cameras with a mirror).

    Most adapters allow the lens to focus slightly past infinity. They do this to allow for manufacturing tolerances between lenses/cameras/adapters. Better to focus slightly past infinity vs. not reaching infinity seems to be the thought. If a firm stop at infinity is absolutely required then the lens/adapter can be shimmed to allow for that. Usually not worth the hassle.
     
  6. Deadbear77

    Deadbear77 TalkEmount Hall of Famer

    Sep 14, 2012
    Northeast Ohio
    Kevin
    Yes the one adapter I have will not focus to infinity at all.
     
  7. Bimjo

    Bimjo Super Moderator

    Oct 28, 2011
    Washington State
    Jim
    Unusual, mostly it's the other way around. You could always mill/sand of a bit at a time to get it to work, but unless you really like to tinker it isn't cost effective considering you can pick up an adapter for less than $20 almost anywhere. ;)
     
  8. Bimjo

    Bimjo Super Moderator

    Oct 28, 2011
    Washington State
    Jim
    Ok, here's an interesting site that will allow you to visualise the difference between various camera registration distances. Kinda fun to play with, you control the view with the options in the box in the upper left of the screen.

    If you put the two cameras (in this case a Pentax K5 and a NEX 5N) on the top view you can really see how much deeper the K5 body is than the 5N. The lens adapter makes up the difference between the two, allowing a Pentax lens to focus properly on a NEX body.
     
  9. Orange

    Orange TalkEmount Veteran

    203
    Jan 4, 2013
    England
    It's not Orange!
    Great stuff.

    When shopping for an adapter, FD in my case, how can you tell a good one from a bad one?
     
  10. Deadbear77

    Deadbear77 TalkEmount Hall of Famer

    Sep 14, 2012
    Northeast Ohio
    Kevin
    I have not had a problem with any rainbow image adapters
     
  11. Bimjo

    Bimjo Super Moderator

    Oct 28, 2011
    Washington State
    Jim
    Flip a coin? :p

    I really have no idea, I've never gotten a bad one. Haven't bought a FD one yet though. Adapters are either really expensive or rather cheap. You can spend $150+ and be assured that it will work correctly or you can spend $20 and get one that probably will work fine.

    What's your budget?
     
  12. Orange

    Orange TalkEmount Veteran

    203
    Jan 4, 2013
    England
    It's not Orange!
    Haha thought as much!

    Budget is just a cheapo one to start, so probably around the £20 mark.
     
  13. Believer

    Believer TalkEmount Regular

    80
    Nov 28, 2012
    New Jersey, US
    Ryan
    I purchased my M42 mount on eBay for < $7.00, and it seems to do the job just fine.

    Thanks for the input from everyone, much helpful in understanding what the adapters do besides just adapt the mount.
     
  14. jcdoss

    jcdoss TalkEmount Regular

    118
    Jul 6, 2013
    How is minimum focus affected? A little longer than spec?
     
  15. Bimjo

    Bimjo Super Moderator

    Oct 28, 2011
    Washington State
    Jim
    I have no idea, but I have a hard time believing that it would even be as much as an inch. It may not even be measurable outside of a lab environment. Hardly significant enough to worry about for the average shooter in any case I don't think. :)