What's going on with my 85mm?

Discussion in 'Photo Assignments & Technical Challenges' started by CThomas817, Jul 22, 2019.

  1. CThomas817

    CThomas817 TPF Noob!

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    I have a Nikon 85 1.8G.

    I have encountered this issue a few times now. When I am at a considerable distance away from the subject (ie. 20 feet), the lens deeply back focuses. I am not missing focus, I am steady and dead set on the eye. When I move closer, say 5 feet, lens is tack sharp and focus falls just where I intend.

    Yesterday's example. I did a maternity shoot in a flower field. The flowers came up about thigh high. Focus was placed on the mother's face, several feet higher than the flowers. The background space behind her head was over 100 feet away. Shutter was at 320, light was excellent (sun at golden hour to the side falling beautifully on the face), plenty of contrast on the face. I took several full body shots at 2.8, zoomed in, and realized she was completely OOF. Focus fell on the flowers about a foot or two behind her. I recomposed, refocused, tried again and the same thing happened. There is no way I moved and caught the background behind her head accidentally because that was far behind the flowers and not in focus either.

    This same thing has happened several times before in different locations.

    So I moved closer to do half body shots and my focus fell perfectly on the eyes. This lens is already manually fine tuned/calibrated to my camera.

    Am I missing something here?


     
  2. tirediron

    tirediron Watch the Birdy! Staff Member Supporting Member

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    What AF mode and what camera body?

    The first thing you need to do is verify that it is the gear and not you. Mount the camera on a tripod, and then make a nice, large, high-contrast focusing target. Say 2'x2' and mount that on a second tripod. Set your lens to f4 and AF and then start with the target 5' away, and make an exposure, and repeat until the target is 25' away. If these are all sharp, chances are, it's you. If one or more isn't sharp, then repeat using manual focus. If all of those are good, something's out of whack, likely in the AF system. If the manual focus is off as well, then it's almost certainly the lens.
     
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  3. CThomas817

    CThomas817 TPF Noob!

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    Nikon D800, AF-S.

    I am definitely not ruling out that's it's me. It just seems unlikely that it was due to my movement because what caught focus was several feet below my focal point. If I had accidentally focused anywhere else other than her face in the top half of the frame, I would have expected the background to be in focus instead, but it wasn't. I will experiment with the tripod and see what I find. I ended up switching to my Tamron 24-70 for the rest of the shots as I have never had an issue with it.

    Thanks!
     
  4. Original katomi

    Original katomi No longer a newbie, moving up!

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    I only speak canon but can I ask what sort of focus points were you using. I use only the center point having turned the others off or use the manual focus because I had problems just like yours.
     
  5. CThomas817

    CThomas817 TPF Noob!

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    Single point AF. I did not try to focus manually. I'm going to do a test with the autofocus vs manual focus.
    Thanks
     
  6. Derrel

    Derrel Mr. Rain Cloud

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    Sometimes the auto focus point covers an area that is wider than a human is. At times the autofocus bracket is only a rough approximation of the actual area that the focus sensor actually "sees".

    The type of problem that you describe is fairly common with short focal length lenses such as 24 to 35 millimeters.

    Believe me, you are not the first person to have noticed this issue.One way to determine exactly where the actual focus sensor is located is to focus upon very narrow objects such as poles, laundry Drying line polechain link fence poles etc.
     
  7. CThomas817

    CThomas817 TPF Noob!

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    Thanks for the info. This is my 85 prime so I'm not sure if it's still common. I do understand that the focal sensor may expand beyond the focal square in the viewfinder, however, in the entire top half of the frame, the only thing in the foreground were her head and shoulders, and the only thing in the background were the trees and some farm houses over 100 feet away. Lens focused about 2 feet behind her, yet there was nothing in the focal plane two feet behind her head close to the focal point, if that makes sense. So even if I had moved or the sensor was off a little, I would have basically have to have moved about 4 feet lower to the ground to actually focus where the focus fell. I'll try to post the image if I get a chance.
     
  8. JBPhotog

    JBPhotog No longer a newbie, moving up!

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    I am unfamiliar with the D800 but I have some questions for you.

    1. You mention Single Point AF but how many AF points have you implemented? AF11 or AF51?
    2. Although Single Point AF might be selected on the body via the button near the lens mount, there is also Auto where the camera chooses the Single Point to focus on, have you checked that setting through the Sub-Command dial?
    3. Are you using the Centre AF point and recomposing? Generally the centre point is the most accurate.
    4. Have you done any sort of focus calibration on your lenses to check for back focusing?
    5. Which focus method do you use, Back Button or Shutter Button?

    By defining these parameters you may be able to track down why the focus is off. Other than a defective lens, the AF system resides in the viewfinder and should response similarly between lenses.
     
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  9. CThomas817

    CThomas817 TPF Noob!

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    Hi -

    1. 51 points always
    2. Focal point is not in auto in either setting
    3. I do not always use the center point then recompose. It depends on the image composition, how much I would need to move to recompose, and how wide open I am shooting..
    4. As mentioned in my original post, yes this lens is already calibrated and fined tuned to this body. I am at -12 for my 85.
    5. Focus is on shutter, I have considered switching to back button

    Thanks for the tips! I will consider each point
     
    Last edited: Jul 22, 2019
  10. Derrel

    Derrel Mr. Rain Cloud

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    I thought you said earlier that you were on single point AF, but you were on 51 point AF, so that means a pretty wide spread of focusing points. for most people work I find 11-point AF works better for me than does 51-point focus
     
  11. weepete

    weepete TPF Supporters Supporting Member

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    Are you recomposing after you've focused?

    also I'm sure I've heard that you need to check the calibration of your lens at different distances, as if you calibrate for a short distance it may throw focus out at the long end etc
     
  12. JBPhotog

    JBPhotog No longer a newbie, moving up!

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    I am wondering if the shutter button AF is the issue as it may have refocused after you recomposed?

    Anyway, best to do some testing with a stand in human to see if there is an error with that particular focus point trying both shutter and back button focus methods. Keep us posted. :)
     
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