Thread: "Back Focus" Infinity or a Specific Ft Mark?

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  1. #1 "Back Focus" Infinity or a Specific Ft Mark? 
    Senior Member Dustin Hoye's Avatar
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    *** EDIT: Okay, lots of views, no responses, so I am going to try and rephrase the questions:

    Some people suggest a wider angle lens at 3' for setting back focus others a mid-range lens at 7'-10'. Is there a better practice here? Seems like the same thing, just different ways of getting there?

    Some say pick a distance as marked on the lens for setting the back focus, then measure the distance from the sensor to the Siemens Star chart to match- but my question is how does this distance relate to the Infinity marking on the lens? If 7' is the last marking, is that distance okay for setting, so that Infinity will still function okay? Or is this diffferent for each lens? Thanks in advance..

    Original Post:

    (Ok. I know there are countless threads covering this in some specific fashion or another. I have looked, but still have my question. I originally meant this for Jacek from Optitek as a PM, but the quota limit has left me with just trying my luck in public. Here goes. Please, be gentle.

    I recently purchased an Optitek Nikon Pro Mount- I just wanted to say it has been really great. I am setting the back focus (I will refer to it as such, in accordance to the Red Manual/PDF) with a set of Tokina/Nikon still lenses. I realize this isn't the best situation, but it is what I have to work with for the moment.

    I have the Red One back focused. Sharp focus on zooms, and on primes at 7' and 3' marks, matching on several lenses. But here is my dilemma- I set the back focus on the Red One at 7'. Yes it matches between lenses at 7' and 3', but this has the 7' mark now acting like the Infinity mark.. is this an issue? I would have set them at infinity, but I was worried (because I don't know) that these lenses are probably different distances at their infinity setting, so I didn't know how far to set/measure the sensor from the chart.. whereas, a 7' foot mark proved more exact, at least to me at the time. :)

    Is there a better way to do this, but actually take advantage of the infinity mark? Am I losing anything by setting it at 7' mark and not the infinity mark, other than not being able to blindly roll the focus ring, to it's stopping point, say, if I am shouldered without a follow focus?

    I really appreciate your time and thoughts.

    Very Best,
    Dustin )
    Last edited by Dustin Hoye; 03-03-2013 at 03:09 PM. Reason: More Info
    Dustin Hoye
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  2. #2  
    Senior Member Jacek Zakowicz's Avatar
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    No it does not mean that your 7' (or any other mark for that matter) becomes infinity mark. Only the actual infinity mark is it.
    If it matches between lenses there is a good chance that it's correct.
    Easy to verify- just point the camera at infinity target at distance and see if the image is sharp.....
    Jacek Zakowicz, Optitek-dot-org, jacek2@optitek.org
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  3. #3  
    Senior Member Dustin Hoye's Avatar
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    Jacek, thanks for your response. I really believe I was just over thinking this.. I used the following YouTube vid (Very good explanation for when making do) from Doug Underdahl (found in a post you were involved with also) to help me work through this.

    http://www.youtube.com/watch?v=pRicc1HU44c


    The confusion for me started with the "Infinity" mark versus the actual distance you can roll focus to, physically, on any given still lens (on some lenses, past the actual Infinity mark) and that is what was giving me grief. After watching Doug's tutorial and testing myself, I have determined that I am matching well between lenses and sharp close to the infinity mark for greater distances, but not sharp for the full physical distance that each lens can turn past it's given infinity mark.

    I chalk this up, possibly, to the room that is left at infinity for temperature changes that I have read about (again assuming) and/or the simple fact that these are cheaper (not cheap, but cheaper) still lenses so the 'markings' can be a bit wavering. (I used a Tokina DX II 11-16mm, a Rokinon Cinelens 35 prime, Nikon Macro 60mm, and a Nikon 80-200 2.8 zoom to test between.)

    I just completed a commercial shoot and everything was tack sharp between lenses, so I guess if it's not broke, don't fix it. ;)

    Thanks.
    D
    Dustin Hoye
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  4. #4  
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    You've answered your own question correctly!
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  5. #5  
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    Hey Dustin, How are you liking that Rokinon 35 lens? I have the 14mm Cine and have had good results so far. I was curious about the 35.
    Thanks,
    Chris
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  6. #6  
    Senior Member Dustin Hoye's Avatar
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    Quote Originally Posted by Chris Pyle View Post
    Hey Dustin, How are you liking that Rokinon 35 lens? I have the 14mm Cine and have had good results so far. I was curious about the 35.
    Thanks,
    Chris
    Hi Chris,

    I've just had the one shoot with it, but so far I really like the lens. Sharp, pretty smooth and well built, IMHO. Produced a clean image all around on the R1MX for me. I was outside in 20 weather on frozen lake, so it was very bright- shot around F8-F11 mostly. (Lens cap can be finicky, NBD, just saying.) While I don't have a good deal of varied shooting experiences with it yet, (taking it with me to Sweden here in a week for a doc shoot) there was at least one thing worth mentioning, unique to my setup here, that I had to adapt to. I use the Nikon Pro Lock mount with a Cinevate follow focus, so even though the Rokinon 35 has follow focus grips on the lens' focus ring itself, I still had to use one of my Cinevate focus rings over it, because of the Pro Lock Mount's shape prevents a lens shaped like the 35 from having the follow focus reach it. Not really a big deal, just something to be aware of, if applicable. Other than that, so good so far!
    Dustin Hoye
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  7. #7  
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    Thanks for the detailed response Dustin, I appreciate you taking the time. I am definitely looking at that lens and the 85mm. What part of Sweden are you shooting in? I was born in Stockholm and try to get back there when I can.... Love it.
    Cheers,
    Chris
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  8. #8  
    Senior Member Dustin Hoye's Avatar
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    Quote Originally Posted by Chris Pyle View Post
    Thanks for the detailed response Dustin, I appreciate you taking the time. I am definitely looking at that lens and the 85mm. What part of Sweden are you shooting in? I was born in Stockholm and try to get back there when I can.... Love it.
    Cheers,
    Chris
    Hey Chris,

    Np. We are filming in Svangsta and on the Morrum River. :)

    Cheers,
    D
    Dustin Hoye
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