Obscured apertures when wide open?

Discussion in 'Large Format Cameras and Accessories' started by MattCarey, Oct 25, 2005.

  1. MattCarey

    MattCarey Member

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    I have two lenses mounted on boards for my 8x10--an old 27cm Dagor and a 159mm Wollensack, both in barrel. I find that when I check through the clipped corners of the ground glass that the aperture is partially obstructed for both when they are wide open. I.e. I have to stop down a few clicks to see a round aperture. Is this normal?

    Thanks,

    Matt
     
  2. BradS

    BradS Subscriber

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    From my little bit of experience with this, yes. It seems normal especially when focussed at infinity (i.e. short bellows). Even the 150mm Sironar-N exhibits this same behavior - albeit, on 4x5. Thing is, I have shot the thing wide open and no vignetting was noticable.
     
  3. MattCarey

    MattCarey Member

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    But, did the sharpness drop off towards the corners?

    Matt
     
  4. sanking

    sanking Restricted Access

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    This is how many of us check for vignetting. If you look thorugh the corner of the ground glass and see an elipse you do not have full coverage and there will be some vignetting at the corners. You should, as you have done, stop down until you see a round aperture. It is especially important that you do this when using wide angle lenses, when using lots of movements, and when using filters and lens hoods.

    The fact that you have a round aperture only means that there will be no vignetting on the corners. It does not indicate the quality of the image on the edges, which you can only know from experience.

    Sandy
     
  5. BradS

    BradS Subscriber

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    Hard to say. Not much depth of field when wide open. This photo for example, was shot with a Symmar-S wide open.
     
  6. Lee L

    Lee L Member

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    Think about it this way. You'd have to have the apparent aperture blocked down to 50% of it's on-axis area to lose one stop from the center to the corner of the negative. When you stop down so that you can see the whole aperture, you're not vignetting, just getting the natural geometric fall-off, which is greater with wide angle lenses.

    Lee
     
  7. MattCarey

    MattCarey Member

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    Wouldn't this mean that I am working with a non-round aperture towards the edges? Shouldn't that cause some effect on the image quality?

    Matt
     
  8. Ole

    Ole Moderator Staff Member

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    Matt, it does. That's the cause of the "swirly" corners of many famous fast Leica lenses!
     
  9. Struan Gray

    Struan Gray Member

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    With a relatively wide angle (like 270 mm or 159 mm on 8x10) you should always see an ellipse in the corners, simply because you are looking at the aperture at an angle. The important thing is to make sure that the ellipse is formed by the aperture blades and not, for example, by the rim of the lens barrel or any attached filters.

    The apparant narrowing of the aperture at wide angles from the optic axis is one of the causes of 'normal' light falloff. It is also responsible for one type of 'swirly' bokeh, and for the way the saggital and tangential MTF curves seperate at wide angles.

    In some lenses vignetting by the barrel appears to be deliberate. I have been playing with a 14" Verito John Stafford lent me, and the barrel vignettes all the way down to the point where the deliberate diffusion is lost.