Difficulties achieving back focus with STF-8300M and Canon adapter

Discussion in 'STF Series' started by Scott Sloka, Sep 19, 2019.

  1. Scott Sloka

    Scott Sloka Standard User

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    I purchased my STF-8300M in 2015 and have been able to take excellent images with my Canon 200mm fixed lens, with no problems achieving back focus. I have not really used this setup for about 3 years or so, but recently have tested my setup with my nifty 50, 50mm lens, and I can achieve good focus with that. I have a Rokinon 14mm lens with a Nikon to Canon adapter that achieves good focus on a Nikon body w/o the adapter and a Canon body with the adapter. I tried to get this system to achieve focus with my STF-8300M/CLA-83-CN system and could not. I started getting disks (not yet stars) as I turned the focus ring away from the infinity focus line to the opposite end of focus, and still got out of focus disks and not stars, although it was getting better. Foolishly, I purchased the same lens but with a Canon mount off Kijiji and it had the exact same problem. I am not certain if the back focus with my 200mm lens places the focus within the infinity focus range on the focus ring, but with my 50mm it does not.

    Here is an example that it works with my 200mm lens:

    https://astrob.in/203224/0/

    I know I am able to get relatively good focus using my Canon 5DmkII body and BackyardEOS, so I am fairly certain both lenses are "working" and that focus is possible, but I am perplexed why I cannot achieve backfocus.

    I had a few questions:

    1. From what I have described, is the lens too far or too close to the sensor? If it is too close, I can test a different flange focal distance by taking the CLA-83-CN apart and placing spacer washers between the rings to move the lens further away. If the lens is too far away, it would mean a ring with a smaller width to push the lens closer.

    2. Has anybody been able to achieve good focus with a similar setup?

    Thanks for any help you can give.

    Scott
     
  2. Doug

    Doug Staff Member

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    Check first that the lens isn’t going slightly past infinity. This is quite possible. Start with it dialed to infinity and then come back slightly and see if it improves. If so then tweak to best focus.

    When a lens is dialed out to infinity, the focus plane is closest to the back of the lens. So if you haven’t gone past infinity as suggested above, it will be because the lens is too far from the camera sensor.
     
  3. Scott Sloka

    Scott Sloka Standard User

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    Thank you very much for your very quick reply. Last night I started at infinity focus and slowly backed the focus ring down taking 10 second captures with small focus dial increments and it was quite blurry when it was dialed to infinity and only when it was close to the other end of the focus range could I begin to see the hints of disks of stars other than the brightest ones. I saw no hint of sudden improvement near infinity focus.

    So I guess the lens is too far from the sensor. Do you have any suggestions of how to arrange the system or will it have to involve a narrower spacer ring on the CLA-83-CN? The current spacer ring between the silver Canon connector and the mount to the filter wheel is 6.53mm if that helps.

    Scott
     
  4. Doug

    Doug Staff Member

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    The lens adapters are designed to be used with a filter wheel installed. If you do not have a filter wheel installed then you need to use the "Filter Wheel Spacer" (slightly odd name - it replaces the thickness of the filter wheel). I suspect you have one of these adapters and forgot to install it.
     
  5. Scott Sloka

    Scott Sloka Standard User

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    Thank you again for your help.

    I forgot to mention above that I have the FW5-8300 filter wheel. The CLA-83-CN is attached to the filter wheel. Do I still need to use the filter wheel spacer if I have the adapter attached to the filter wheel? That would also mean that my lens is too close and not too far away, which would be okay if the spacer fixed the problem.

    Scott
     
  6. Doug

    Doug Staff Member

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    You use either the filter wheel OR the filter wheel spacer. Not both. Not neither.

    If you do use one or the other it should focus correctly.

    If you still can't get it to work, please post a picture of your camera with adapter attached.
     
  7. Scott Sloka

    Scott Sloka Standard User

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    STF8300 1.jpg STF8300 2.jpg STF8300 3.jpg Thank you very much. You have been very patient, and I appreciate it.

    Here are some photos of my setup. I am still uncertain why these two lenses do not work.

    Scott
     
  8. Doug

    Doug Staff Member

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    I don't see anything obviously wrong there. Does any other lens work? Like the last lens you used?
     
  9. Scott Sloka

    Scott Sloka Standard User

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    Thank you again for your help.

    I went over everything again last night to be sure I wasn't doing something obviously stupid (although I still might be). Here is what I know again from tonight.

    My 200mm and 50mm lenses achieve excellent focus with both my Canon 5DMkII and STF-8300M. The focus with the 200mm lens is moderately out of the infinity range for the STF.
    Both of my 14mm lenses (my Canon and my Nikon with Canon adapter) achieve excellent focus with my Canon 5DmkII. The focus ring location for both of these is much less than the infinity range.
    Neither of my 14mm lenses focus with the STF. I slowly incremented the focus from the opposite range of infinity and the focus steadily worsened from the start with no improvement (bummer).

    I suspect that the CLA-83-CN is off in tolerance for some reason (even though I have taken it apart twice to see if there is anything jammed in between the ring connections) that is enough to throw off the 14mm lenses completely out of range and the 200mm lens somewhat but not enough that the focus cannot compensate for it in a lower setting.

    I am grateful to rely on your experience, which is gold, and ask that if the lens is too far away from the sensor (flange focal distance is too great), then would reducing (?machining down) the spacer ring attached to the silver Canon connector be the easiest way to solve the issue? Another approach could be to test whether a Nikon adapter would be more accurate for the setup that I have as I do have a 14mm lens with Nikon attachment.

    I am otherwise still very happy with the setup and the quality of the components as you can see from the images that I am able to get from it.

    Please, what is your advice? Am I missing something?

    Scott
     
  10. Doug

    Doug Staff Member

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    There is a certain tolerance on the position of the CCD sensor itself inside the camera, due to stack-up tolerances on the cooler stack. However I've not heard of this being far enough off to cause a problem.

    You could try machining down the spacer ring, if that is something that is easy for you to do. Obviously you should take off only a small amount.
     
  11. Scott Sloka

    Scott Sloka Standard User

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    Machining down the spacer ring would not be something that is easy for me to do and I do not know what type of metal it is. I would have to figure out how to retap the threads as well. I also might not be able to guess how much to take off. Randomly throwing it on a belt sander will likely make it uneven. If I do try something (foolish) and mess it up, is it possible to obtain a replacement ring?

    Do you have any further suggestions?
     
  12. Doug

    Doug Staff Member

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    The material is aluminum. If you don't have access to machine tools then I do not recommend modifying it.

    Do you have a caliper that you could use to measure the distance from the front of the ring to the front surface of the filter wheel?
     
  13. Scott Sloka

    Scott Sloka Standard User

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    Strangely, I built a homemade CNC machine from a Raspberry Pi in my basement, but it is made out of wood and I only use it for wood so I do not trust it to be calibrated accurately (although I am tempted). It sits on a wooden spoilboard that is not flat, which it would need to be. I have no access to machine tools otherwise.

    The distance from the silver front ring of the Canon adapter to the filter wheel is 8.86mm. The attached image should show where I measured it from.

    Thank you very much. I know this can work, and I really appreciate your help.

    Scott STF8300 4.jpg
     
  14. Doug

    Doug Staff Member

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    We measured a ring assembly that we have here, and the height was 8.61 mm. It's only a difference of 0.25 mm or 10 thousandths of an inch. That's within tolerances. I'm not sure why it's not working with some of your lenses.

    If I was going to machine that piece slightly, I would do it on the bottom side - the surface that sits on the filter wheel. But that's not a easy job without the proper tools.
     
  15. Scott Sloka

    Scott Sloka Standard User

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    I found a similar difficulty from another user on your forums from a few years ago. Not sure how or if this gentleman's problems were resolved.

    https://forum.diffractionlimited.com/threads/canon-lens-not-coming-to-focus.1295/

    His focal distances are suspiciously similar to mine (10-22mm and 8mm vs (mine) 14mm), which could suggest a similar problem with backfocus on wider angle lenses.

    Although I know that the flange to sensor distance has to be as exact as possible, I am not sure what that tolerance should be. Do you think that a difference of 0.25mm is enough to make a difference, or how much of a difference do you think would necessary to get to focus? This also goes along with the question whether or not it is possible to obtain a spacer ring separately.

    Thanks again for hanging in there with me.

    Scott
     
  16. Doug

    Doug Staff Member

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    I'm not an expert on DSLR optics. It probably makes sense that the shift in the focal plane position versus focus setting is greater on long focal length lenses than short ones. In other words, short lenses might be "fussier" about the spacing.

    The thickness of the filters is also a variable. When you put a filter behind the lens, it effectively reduces the back focus distance by 1/3 of its thickness. So if you put a 3 mm thick filter behind the lens it reduces the effective (optical) spacing by 1 mm. If you select an open slot (no glass) the back focus distance will appear larger.

    This gives me an idea. If you could mount a 3 mm thick clear or UV/IR filter at the back of the adapter, it would effectively shim you by 1 mm.
     
  17. Scott Sloka

    Scott Sloka Standard User

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    Thanks to your guidance, I have been doing some reading about wide angle lenses. It appears that they may be fussier about finding the optimal flange focal distance than regular lenses. This (long) article suggests that:

    https://www.dpreview.com/forums/thread/4141652

    and wikipedia suggests that the tolerance is important and could be as little as 0.01mm. As per wikipedia:

    "Flange focal distance is one of the most important variables in a system camera, as lens seating errors of as little as 0.01 mm will manifest themselves critically on the imaging plane and focus will not match the lens marks."

    https://en.wikipedia.org/wiki/Flange_focal_distance

    What a buncha craziness.

    If you have a system with a difference of 0.25mm, would you be willing (please) to lend that to me for a short while so that I can use it to confirm how fussy I need to be to make the system work? I might be able to extrapolate an estimate of how much to take off if the 0.25mm less does not get me there.

    Scott
     
  18. Doug

    Doug Staff Member

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    Okay that confirms my suspicions about fast lenses, though not matching the lens marks is one thing. Not focusing at all is entirely another. We can't hold those sorts of tolerances - simply using a different thickness of optical filter in the wheel will have an effect of as much as 1 mm.

    I don't think 0.25 mm is going to get you there.

    Can you try, as an experiment, putting an extra piece of glass behind the lens? Maybe you can find a spare camera lens filter or something. Whether or not that gets you to focus will help us understand how much adjustment is needed. You may be able to just rest a camera lens filter underneath the lens, as long as you point the camera straight up.
     
  19. Scott Sloka

    Scott Sloka Standard User

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    Great, creative idea. However, something tickled me in the back of my head so I did some research into this and found that inserting a glass plate pushes the plane of focus away from the lens, which is the opposite of what I need. I remember going through this when trying to get my other imaging setup to achieve focus and having to account for the filters, which was a painful process with great rewards.

    http://dougkerr.net/Pumpkin/articles/Glass_Plate.pdf
    and page 8 on this one has a cool rule of thumb:
    https://pdfs.semanticscholar.org/8cd9/dc01f5a2c3bca3cc07bdeb0f4ac665280c2f.pdf

    I trust your experience, and continue to be grateful for your help. If you say the above is wrong, I will still hunt around and look for a piece of glass.

    Here is a long post that eventually makes it less confusing for me:

    https://www.cloudynights.com/topic/475406-considering-filter-thickness-when-calculating-backfocus/

    I shudder to ask if there is an easy way to adjust the location of the CCD within the camera. I am guessing not.

    Without knowing more of the physics and engineering of the lenses, it is very difficult to estimate the amount that I need to move the lens to the sensor. The opposite problem would be easier as I could devise a way to shim the ring up.

    Do you have any other suggestions to help get this adapter to work with the 14mm lenses?
     
  20. Scott Sloka

    Scott Sloka Standard User

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    In further reading the cloudy nights article above, over to page 2:

    https://www.cloudynights.com/topic/...r-thickness-when-calculating-backfocus/page-2

    the last post suggests that "1/10th of a millimeter can make a huge difference." The gentleman, WesC, is describing this in reference to fast scopes, which is similar to the faster lenses I am using. Seems he had a bit of a hair pulling experience with it. Also, the gentleman above, Akwilliams, seems to feel that sub millimeter tolerance is important.

    The 0.25mm difference might get me there, or closer enough to estimate how close I need to be.

    Scott
     

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