Page 1 of 2 12 LastLast
Results 1 to 10 of 16

Thread: Curious Minds – Image Circle, Circle of Illumination, ULF, and Contact Prints

  1. #1

    Join Date
    Sep 2008
    Location
    Sarnia, Ontario, Canada
    Posts
    756

    Curious Minds – Image Circle, Circle of Illumination, ULF, and Contact Prints

    I did have a good look at the Large Format Home Page and a look at some of the other information available, so looking for some additional insight from the collective knowledge here.

    Actually in my mind all the above items are related. I have an 8x20 camera (nominal diagonal of 21.5” / 550mm, and when looking at suitable lenses continue to be confronted by data sheet specifications that say you cannot use that lens for 8x20 e.g. 450 Nikkor M IC= 440mm, 480 Apo-Ronar IC=396mm, 355 G-Claron IC=442mm, yet are commonly used by other ULF photographers.

    There is some confusion in the information available, versus my understanding. There seems to be information out there that implies the image circle and circle of illumination are one and the same e.g. Wikipedia and Ebony web sites.

    • It is my understanding that the image circle at infinity is the diameter of the circle of acceptable image quality as defined by the lens manufacturer e.g. high enough MTF value (Modulus Transfer Function) and is defined at a specific aperture (usually f16 or f22).

    • It seems to me that circle of illumination is a vignetting issue, where one can even use the optical equivalent to a “Coke bottle bottom” could illuminate, yet not provide suitable image quality.

    Given that ULF are contact printed, then the lack of enlargement will reduce the relative image quality requirements, then image circle would seem to be less of an issue and it impact will be in the corners and one can burn those in…

    So the questions are:

    1. Is my understanding of the difference in image circle and circle of illumination correct, or is there something I am missing?

    2. How much difference is there in definition of lens quality e.g. different MTF values, between manufacturers i.e. Nikon versus Rodenstock?

    3. Given the difference in application i.e. reproduction / graphic industry versus general photography, is there difference in the definition of image circle e.g. MTF value for Apo Ronar versus Sironar? Bob throwing that one your way…

    4. How rapid typically is the fall off in MTF at defined image circle and beyond?

    I strongly suspect that I am not the only one lacking knowledge in this area.

    Thank you for your insights,

    Len

  2. #2

    Join Date
    Dec 2001
    Location
    NJ
    Posts
    7,624

    Re: Curious Minds – Image Circle, Circle of Illumination, ULF, and Contact Prints

    4. How rapid typically is the fall off in MTF at defined image circle and beyond?
    Len, this depends on the lens and the manufacturer so there's no really solid answer.

    Example one. Rodenstock 75.4 Apo-Rodagon D. The MTF plots here http://www.rodenstock-photo.com/medi...3-62__8230.pdf (p. 61) are flat to the edge. Coverage has to be limited by mechanical vignetting.

    Example two. Rodenstock 150/9 Apo-Ronar. MTF at 16 lp/mm is near zero at the edge. See https://skydrive.live.com/#cid=8D71B...33C77D1008!182 p.3-6

    Example three. Schneider 150/5.6 Apo-Symmar XL. In the MTF charts here https://www.schneideroptics.com/pdfs...L_56_150_2.pdf MTF is falling fast at the edge of the field. Schneider no longer posts MTF charts for the 210/6.1 Xenar, but as I remember them MTF was near zero at the edge.

  3. #3
    David Lobato David Lobato's Avatar
    Join Date
    Feb 2011
    Location
    Baltimore MD
    Posts
    1,053

    Re: Curious Minds – Image Circle, Circle of Illumination, ULF, and Contact Prints

    Image circle criteria is also dependent on the f-stop, why f22 typically accompanies an image circle spec. Stopping down further will gain you a little more coverage. It's because the edge of the circle goes from a blurred zone to a more sharply defined edge as the lens is stopped down. The useable image circle thus gets larger.

    This partly explains why some photographers get away with using a lens considered too limiting for the larger format. If the lens is focused closer than infinity you also get a bigger image circle, and sometimes just a little bit more is all you need.

  4. #4

    Join Date
    Jul 2007
    Location
    Austin TX
    Posts
    2,051

    Re: Curious Minds – Image Circle, Circle of Illumination, ULF, and Contact Prints

    Len, here's my answer to some of the questions you pose. One needs to generalize a bit though.

    1. Yes I've always taken the circle of illumination to be the inside edge of vignetting at a define aperture whereas the image circle at infinity is that radial distance from lens center where the resolution approaches a minimum acceptable by the manufacturer. Different manufacturers have different standards and AFAIK no one posts those standards.

    2. For the top quality lens designs there is, on average not a large difference in resolution/contrast between the big four, Nikon, Schneider, Fuji and Rodenstock for similar focal length lenses. There seems to be a bigger variation within the same model than between different manufacturers for the same focal lengths. Most all quality normal focal lengths (90 to 210 mm) show resolution/contrast numbers from 40 to 60 lp/mm across the field of view.

    3.As far as I know the definition for image circle between a general purpose LF lens and a reproduction lens is about the same. But I would defer to Bob for his comments. The MTF values are determined in a standard way so there should be little difference between MTF values as long as they are determined at the optimum magnification for that particular lens be it 1:1 or ∞.

    4. With lenses I've looked at the edge of the image circle already has a drastic MTF falloff so as a percentage of the lens image circle radius I tend to utilize about 90% of the radius. I can tell you that near the point of vignetting some lenses show as low as 5 LP/mm for MTF50 criteria where at the listed image circle one could see only 15 to 20 LP/mm at MTF50. But the variation is huge at the peripheral region. My sense for this is based entirely with testing a number of lenses quite a few years ago for industrial photography. Those were aerial image tests on an optical bench using a scanning densitometer. For precise macro work on IC chips I tried to stay within the center 4/5 of the image circle.

    Nate Potter, Austin TX.

  5. #5
    Jac@stafford.net's Avatar
    Join Date
    Dec 2012
    Location
    Winona, Minnesota
    Posts
    5,410

    Re: Curious Minds – Image Circle, Circle of Illumination, ULF, and Contact Prints

    Image circle criteria is also dependent on the f-stop, why f22 typically accompanies an image circle spec.
    Is that true for later Biogons?

  6. #6
    Moderator
    Join Date
    Jan 2001
    Posts
    7,339

    Re: Curious Minds – Image Circle, Circle of Illumination, ULF, and Contact Prints

    Quote Originally Posted by David Lobato View Post
    Image circle criteria is also dependent on the f-stop, why f22 typically accompanies an image circle spec. Stopping down further will gain you a little more coverage. It's because the edge of the circle goes from a blurred zone to a more sharply defined edge as the lens is stopped down. The useable image circle thus gets larger.
    This is the nub of the matter for ULF applications. You can't tell the whole story from the specification for f/22, because some lenses gain much more usable coverage than others as you stop down. Thus the apparent anomaly of lenses like the G-Clarons or the 450 Nikkor M, which have unremarkable specifications for coverage at f/22 but in practice are mainstays for ULF photographers.

  7. #7

    Join Date
    Mar 2005
    Location
    Aurora, Colorado
    Posts
    498

    Re: Curious Minds – Image Circle, Circle of Illumination, ULF, and Contact Prints

    Quote Originally Posted by Oren Grad View Post
    You can't tell the whole story from the specification for f/22, because some lenses gain much more usable coverage than others as you stop down.
    Is there a way to calculate the image circle as one stops down to smaller apertures? Just curious.
    Never is always wrong; always is never right.

    www.LostManPhoto.com
    www.MarkStahlkePhotography.com

  8. #8
    Moderator
    Join Date
    Jan 2001
    Posts
    7,339

    Re: Curious Minds – Image Circle, Circle of Illumination, ULF, and Contact Prints

    Quote Originally Posted by Mark Stahlke View Post
    Is there a way to calculate the image circle as one stops down to smaller apertures? Just curious.
    No. It depends on the lens and on one's criteria for acceptable quality.

  9. #9
    8x20 8x10 John Jarosz's Avatar
    Join Date
    May 2006
    Location
    Fairfax Iowa
    Posts
    653

    Re: Curious Minds – Image Circle, Circle of Illumination, ULF, and Contact Prints

    Some process lens' coverage is quoted for a specific resolution (L/mm). They do that so a purchaser who will use it for process work will know exactly what to expect, and the numbers they quote are guaranteed. I use the 420mm Nikkor on 8x20 and the coverage allows movements. You'd never know that by looking at the specs. Contact printing 'art' photography has no relation to process camera work. Try the lens out, but you're right that it's hard to make a purchase without knowing if it will cover.

    John

  10. #10
    Jac@stafford.net's Avatar
    Join Date
    Dec 2012
    Location
    Winona, Minnesota
    Posts
    5,410

    Re: Curious Minds – Image Circle, Circle of Illumination, ULF, and Contact Prints

    Quote Originally Posted by Jac@stafford.net View Post
    Is that true for later Biogons?
    Bumb

Similar Threads

  1. Image circle?
    By Emil Schildt in forum Lenses & Lens Accessories
    Replies: 10
    Last Post: 8-Mar-2011, 03:32
  2. Image circle?
    By Anden in forum Lenses & Lens Accessories
    Replies: 2
    Last Post: 22-Sep-2008, 05:14
  3. image circle
    By Daniele Minetto in forum Lenses & Lens Accessories
    Replies: 9
    Last Post: 28-Apr-2006, 15:48
  4. Image circle
    By jeff schraeder in forum Cameras & Camera Accessories
    Replies: 7
    Last Post: 6-Apr-2002, 17:38
  5. Image Circle
    By A. Mouton in forum Lenses & Lens Accessories
    Replies: 4
    Last Post: 27-Sep-2001, 07:49

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
  •