Page 1 of 2 12 LastLast
Results 1 to 10 of 20

Thread: Focusing issues with certain lens

  1. #1

    Join Date
    Nov 2009
    Location
    Germany
    Posts
    23

    Focusing issues with certain lens

    Hello,

    first, I want to apologize if I'm posting in the wrong sub forum. I don't really know which one is the right for my kind of "problem". And here it comes:

    I have a nice 7x11 Chamonix which I use with a few different lenses. I focus with a loupe on the ground glass, and usually, the part I focus on the glass is the part that is sharp on the plate. As it should be.

    Now I have this nice old petzval portrait lens, and whenever I focus correctly on something on the glass, it shows a front focus of about 3 centimetres on the plate/negative.

    But only with THIS lens. Where is the problem?! I don't understand?!

    If the focal plane would be wrong with my back or the holders, it would be the case with any used lens, not only this certain one?

    Can anybody help me? It freaks me out. I don't know how this could happen. How can it be sharp on the ground glass but then be greatly misfocused on the final plate or negative? And it also happens with still motives that can not move, so... it is really weird.

    What do you think? Your help is greatly appreciated.

    Thank you!

    Best regards!

  2. #2

    Join Date
    Sep 2003
    Location
    Massachusetts USA
    Posts
    8,236

    Re: Focusing issues with certain lens

    If you stop down the lens after focusing - and then the image goes out of focus - that's called focus shift. With some lenses you need to focus at the taking aperture. Perhaps that old lens exhibits focus shift.

  3. #3

    Join Date
    Nov 2009
    Location
    Germany
    Posts
    23

    Re: Focusing issues with certain lens

    Thanks for the reply Ken,

    I forgot to mention that the lens does not have an aperture. I use it wide open, no Waterhouse Stops. I think the aperture is about f/4.

  4. #4

    Re: Focusing issues with certain lens

    "Now I have this nice old petzval portrait lens, ... it shows a front focus of about 3 centimetres on the plate/negative."

    Petzval lenses have a curved field of focus both inside and outside the camera, can be a problem.
    This could also be a focus shift from NOT being color corrected, which would changed off-set by focus distance.
    Easiest way to check is to use panchro then non-panchro film on the same set-up.

  5. #5

    Join Date
    Dec 2006
    Location
    Hamilton, Canada
    Posts
    1,569

    Re: Focusing issues with certain lens

    I think what Paul is referring to is chromatic aberration, which in Panchromatic film would lead to an overall defocussing of the image rather than a focus shift (as the different wavelengths of light would be focussed in a grouping around the point of intended focus.
    Ortho film would show a focus shift as the focus point might be on a red yellow basis but the film would register on a green blue focus point.
    At least that's my understanding, real practice may well be different.
    Bill
    "There are a great many things I am in doubt about at the moment, and I should consider myself favoured if you would kindly enlighten me. Signed, Doubtful, off to Canada." (BJP 1914).

  6. #6

    Join Date
    Mar 1999
    Posts
    769

    Re: Focusing issues with certain lens

    I remember reading about differences between chemical focus and visual focus - something about actinic light being refracted more than the visual part of the spectrum in simple lenses. In other words, we tend to focus typically based on yellow green. However, unless the lens is corrected for lateral color, the blue and UV will focus in a slightly different region. This can be reduced by stopping down or by performing experiments and correcting (or more accurately adjusting) for them. Cheers, DJ

  7. #7

    Join Date
    Nov 2009
    Location
    Germany
    Posts
    23

    Re: Focusing issues with certain lens

    Hello all,

    this sounds really interesting, esp. the part with the blue and actinic light. I was using a few black light bulbs together with my normal lights.

    This is the image I'm talking about:

    Click image for larger version. 

Name:	thorsten_WWPD.jpg 
Views:	49 
Size:	36.3 KB 
ID:	94623

    I did it twice because I thought I misfocused or the subject moved (he didn't!). Two times perfect focus on the eyes on the ground glass and the resulting plate has a sharp nose
    I noticed this with that certain lens before but then I wasn't sure I focused right. This time I am.

  8. #8
    Alex Timmermans
    Join Date
    May 2008
    Location
    Holland
    Posts
    578

    Re: Focusing issues with certain lens

    Ines, you just can't test this on a living model. Try a still ife and see if you have the same problem.
    As I mentioned before. Some old petzvals have this problem. I have one myself.
    "You dont take a picture, it's given to you"

    www.alextimmermans.com
    www.collodion-art.blogspot.com
    email : collodion-art dot onsmail dot nl

  9. #9

    Join Date
    Nov 2009
    Location
    Germany
    Posts
    23

    Re: Focusing issues with certain lens

    Yes, I DID test it on a still life and the problem IS there. I just had this image to show what I mean, I did not scan the still life plates because I thought I'd misfocused 4 times in a row

  10. #10
    All metric sizes to 24x30 Ole Tjugen's Avatar
    Join Date
    Mar 2002
    Location
    Norway
    Posts
    3,377

    Re: Focusing issues with certain lens

    Another possibility is that your ground glass could be very slightly out of position. That would only be visible on the final print/negative with a lens used at a large aperture, like a f:3.7 Petzval. If your pther lenses are used at f:8 or so, that small a focus error won't be visible.

Similar Threads

  1. Focusing issues with Polaroid Back?
    By krisliu77 in forum Cameras & Camera Accessories
    Replies: 2
    Last Post: 8-Apr-2011, 17:20
  2. Fair price for lens? coverage issues?
    By Jon Paul in forum Lenses & Lens Accessories
    Replies: 6
    Last Post: 16-Jun-2010, 14:19
  3. Polaroid 900 4x5 - Composing and Focusing issues..
    By lambis in forum Cameras & Camera Accessories
    Replies: 9
    Last Post: 8-Oct-2009, 20:22
  4. Ground glass and fresnel lens issues...
    By G.R.Reddy in forum Cameras & Camera Accessories
    Replies: 3
    Last Post: 1-May-1999, 12:13

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •