Page 1 of 3 123 LastLast
Results 1 to 10 of 28

Thread: Aperture placement

  1. #1
    45-57-617
    Join Date
    Oct 2006
    Location
    Toowoomba, Queensland
    Posts
    440

    Aperture placement

    Hi,

    Is it important where an aperture iris sits along the barrel of a lens? I presume it should be placed at a 'nodal' point - which I assume is located precisely between the two innermost elements ...

    Is it as easy as that ? Or is this an ideal - in which case there is some compromise if it is not centrally mounted ...

    Information on any compromise and / or placement is much appreciated.

    Thanks,

  2. #2
    Big Negs Rock!
    Join Date
    Mar 2000
    Location
    Pasadena
    Posts
    1,082

    Re: Aperture placement

    The nodal point of the lens may or may not be the center of the center of the lens design. Nodal point relates to the relationship of the taking lens to the world where the pan/tilt don't shift the image in relationship the foreground to the background. The center of the lens relates to the design of the lens. Regarding about where the aperture is placed is a decision of the lens designer. I know at the turn of the century, those iris fade outs done on screen were with an iris placed in front of the lens. A little funky, but it worked.
    Mark Woods

    Large Format B&W
    Cinematography Mentor at the American Film Institute
    Past President of the Pasadena Society of Artists
    Director of Photography
    Pasadena, CA
    www.markwoods.com

  3. #3
    come to the dark s(l)ide..... Carsten Wolff's Avatar
    Join Date
    Jul 2004
    Location
    UK at the moment
    Posts
    399

    Re: Aperture placement

    Mark has already pointed out that aperture placement depends on the lens design and does not have to sit in the nodal point plane. Placement is usu. more critical with closely spaced cells and wide-angles as to where the aperture sits.
    Many lenses can be mounted in different shutters, e.g. either a Copal, OR Copal-Press without modification or shims; looking at these two shutters for example, you'll know that their aperture diaphragms are in different positions: Front to iris 10.2mm vs. 11.7mm, respectively.

    I'm sure you're thinking about a particular lens though; feel free to enlighten us...

    Carsten
    http://www.jeffbridges.com/perception.html "Whether you think you can, or think you can't, you are right."

  4. #4

    Join Date
    Nov 2007
    Location
    Nuremberg Germany
    Posts
    1,051

    Re: Aperture placement

    At first we have to ask why an (iris-)aperture at all? First to regulate the brightness in the image plane, second to cut outer rays that decrease the perfomance of the lens and third to decrease the "numerical aperture" and so to increase depth of field. All this without decrease the eveness of the image brightness. Or with certain lenses like WA-lenses to increase eveness of the image brightness.

    If the aperture plane is shiftet from the "right" plane, the brightness of the image will not decrease evenly but the outer areas will show a shadow of the aperture, a vignetting.

    Every lens, also a simple biconvex lens has a front- and a rear-nodal plane. This are not the same planes used as no-parallax-planes the lens (and also the camera) has to be mounted above the pan/tilt axis of the tripod head, so during moving the camera no image point moves relative to one other. This is the plane of either the front- or rear-entrance-pupill.

    Peter
    I'm not weird. I'm limited edition.

  5. #5

    Join Date
    Jul 2004
    Location
    Tucson, Arizona
    Posts
    4,257

    Re: Aperture placement

    Quote Originally Posted by Mark Woods View Post
    The nodal point of the lens may or may not be the center of the center of the lens design. Nodal point relates to the relationship of the taking lens to the world where the pan/tilt don't shift the image in relationship the foreground to the background. The center of the lens relates to the design of the lens. Regarding about where the aperture is placed is a decision of the lens designer. I know at the turn of the century, those iris fade outs done on screen were with an iris placed in front of the lens. A little funky, but it worked.
    Are you sure about this? I believe the center of the entrance pupil is the point where the background and foreground stayed aligned during rotation. Lenses have two nodal points, a front nodal point and a rear nodal point, (where a ray enters and where it apperars to exit.)

    But yup, the correct aperture placement moves around sightly on different lens designs, (a "degree of freedom" for the designer), and its placement affects, and is used to correct, field curvature and coma in that particular design.

    When an aperture is used to vignette an image, then it's a field stop rather than an f/stop, and conventional f/stops are in play as well. F/stops can be placed close in front of a lens without causing vignetting, as with the old landscape lenses or the Kodak Portrait Lens or Imagon.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  6. #6
    Big Negs Rock!
    Join Date
    Mar 2000
    Location
    Pasadena
    Posts
    1,082

    Re: Aperture placement

    I think the discussion of front and rear nodal points refer to the center of the respective elements. The nodal point of the lens has to do with perspective and movement. We encounter this issue all the time in visual effects when shooting miniatures. The aperture can be placed in any number of positions determined by the lens designer. The vignetting caused by an aperture I would think relates to the image circle, and if the lens vignettes the image circle is too small for the format. I hope this helps. :-)
    Mark Woods

    Large Format B&W
    Cinematography Mentor at the American Film Institute
    Past President of the Pasadena Society of Artists
    Director of Photography
    Pasadena, CA
    www.markwoods.com

  7. #7

    Join Date
    Sep 2008
    Location
    Greater Toronto Area - Mississauga
    Posts
    558

    Re: Aperture placement

    Steve,

    For some lens designs (telephoto, retro-focus wide angles), the nodal point may be outside the body of the lens all together (e.g. in front of telephoto lens, behind at retro-focus lens).

    Therefore it may be impossible to locate the iris at that point without some sort of extension that defeats the rationale of the lens design e.g. retro-focus wide angle lens for a SLR so it does not interfer with the mirror.

    Hope that provides some examples that it is not necessarily where we think it should be,

    Len

  8. #8
    Big Negs Rock!
    Join Date
    Mar 2000
    Location
    Pasadena
    Posts
    1,082

    Re: Aperture placement

    With the use of computer design, there are somethings in lens design that don't seem rational to what we experience on a day to day basis, but the lenses work and do the job. For example, I had an 8mm lens on a 35mm motion picture camera last night. The lens was about 24" long. Worked great on the miniature we shot.
    Mark Woods

    Large Format B&W
    Cinematography Mentor at the American Film Institute
    Past President of the Pasadena Society of Artists
    Director of Photography
    Pasadena, CA
    www.markwoods.com

  9. #9

    Join Date
    Nov 2007
    Location
    Nuremberg Germany
    Posts
    1,051

    Re: Aperture placement

    Quote Originally Posted by Mark Sawyer View Post
    Are you sure about this? I believe the center of the entrance pupil is the point where the background and foreground stayed aligned during rotation. Lenses have two nodal points, a front nodal point and a rear nodal point, (where a ray enters and where it apperars to exit.)
    The cardinal or nodal points of a lens are definite as the point a ray which enters the lens at a small angle strikes as well the subject sided nodal point as the image sided nodal point.

    To determinate the nodals points one has to focus the lens at a distant subject. Now the lens - not the camera - will be panned, moved perpendicular to the optical axis of the lens, and moved alonge the optical axis up to the point the image on the ground-glass doesn't moves during panning. (The position of the ground-glass has to be realigned to compensate the moving of the lens along the optical axis.) Now the plane perpendicular to the panning axis is the front nodal point.

    For determinating the rear nodal point the same procedure has to repeated with the rear cell of the lens.

    To mount camera and lens nodaly, the panning axis has to cut the plane of the entrance pupil.
    Quote Originally Posted by Mark Sawyer View Post
    When an aperture is used to vignette an image, then it's a field stop rather than an f/stop, and conventional f/stops are in play as well. F/stops can be placed close in front of a lens without causing vignetting, as with the old landscape lenses or the Kodak Portrait Lens or Imagon.
    The Wollaston landscape lens was the first photographic lens with a front diaphragm, later used in many box-cameras. This has the advantage over a rear diaphragm of a better spherical correction. This is also the reason the single cell of a convertible lens like the Symmar has to be mounted with the diaphragm in front of the cell.

    Of course an Imagon mounted with the sieve behind the shutter isn't so easy to use too.

    Peter
    Last edited by Peter K; 12-Mar-2010 at 15:38. Reason: link added
    I'm not weird. I'm limited edition.

  10. #10

    Join Date
    Jul 2004
    Location
    Tucson, Arizona
    Posts
    4,257

    Re: Aperture placement

    Quote Originally Posted by Peter K View Post
    The cardinal or nodal points of a lens...
    Well, the nodal points are among the cardinal points, but there are othe cardinal points, so the terms aren't quite interchangeable.

    I'm not sure I followed your description on how to find the nodal points by moving the lens elements around, but trial and error are for approximates at best. The diagram of the simple thick lens at the wikipedia page shows where the nodal points are. The formula for finding them in a single element lens is 1/n-th the way from the surface to the center, where n is the index of refraction. I'm sure the formula for finding them in a complex lens would be way over my head! (My trig isn't what it used to be...)

    But the Wiki page did confirm about where to rotate a lens:

    "The nodal points are widely misunderstood in photography, where it is commonly asserted that the light rays "intersect" at "the nodal point", that the iris diaphragm of the lens is located there, and that this is the correct pivot point for panoramic photography, so as to avoid parallax error. These claims are all false, and generally arise from confusion about the optics of camera lenses, as well as confusion between the nodal points and the other cardinal points of the system. The correct pivot point for panoramic photography can be shown to be the centre of the system's entrance pupil."
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

Similar Threads

  1. Replies: 13
    Last Post: 19-Mar-2009, 23:39
  2. Apple Aperture for LF?
    By Rory_5244 in forum Digital Processing
    Replies: 4
    Last Post: 20-Jun-2007, 17:06
  3. Giant Lens Aperture Problem
    By Andrew van Bommel in forum Lenses & Lens Accessories
    Replies: 3
    Last Post: 24-Sep-2004, 20:01
  4. Two different aperture readings from Copal #0 shutter--which one is correct?
    By Rafil Kroll-Zaidi in forum Lenses & Lens Accessories
    Replies: 4
    Last Post: 3-Sep-2004, 08:27
  5. Scals of Zeiss Protar aperture
    By Cheng Lu in forum Lenses & Lens Accessories
    Replies: 4
    Last Post: 18-Jul-1999, 04:24

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
  •