Image Circle Test

[Ari]

I had a look on Google, but with LFPF down, all the relevant articles are unavailable.
I have a Graphic Kowa 210 here, I'd like to know what image circle it provides, useful or otherwise, since there are reputedly several versions of this lens, all with varying ICs.

How do I find this out? I have mounted it on my 8x10 with the back removed, lens at infinity with light coming through the lens into a darkened room. I had a white cardboard behind the camera, but the projected image didn't look like much.
I know I'm missing something here, what is it?

Thanks in advance.

 

[Dan Fromm]

Put the back on, focus on a distant scene using the ground glass and use a loupe to examine the image in the corners. If there is good image in the corners, put in as much shift and rise and possible and repeat.

 

[Ari]

Thank you, Mr Fromm.

 

[Ian C]

Photography—Its Materials and Practices
C. B. Neblette 1945
D. Van Nostrand Company: Princeton, New Jersey, Toronto, London, New York

Page 128

“The field of view of a lens is circular in character and this may be seen if a lens of relatively sort focal length forms an image on a large ground glass. The larger circle, as seen in Fig. 7.13, is often called the circle of illumination. It is limited in size because, since corrected lenses are thick, light beyond a certain angle is cut off, or vignetted, by the lens mount. The circle of illumination is almost always larger in diameter than the circle of good definition, sometimes considerably larger. The size of the circle of good definition depends upon the design of the lens.”

See also Circle of Good Definition, Camera Craft 1918, p240

https://books.google.com/books?id=E...v=onepage&q=circle of good definition&f=false


The lens maker’s stated image circle is generally given at the aperture for which the circle of good definition based on some standard of resolution reaches its maximum size. For many large-format lenses this is f/22. Some 4” x 5” format lenses reach this condition at f/16.


For example, Nikon customarily gave the usable image circle wide open and a larger circle at f/22 (or f/16 for some shorter 4” x 5” format lenses). What constitutes the circle of good definition is determined by the lens maker.


Lens users sometimes claim larger coverage than that given by the lens maker’s data. The difference is: the lens maker establishes the required resolution and states the maximum image circle diameter that complies based on careful testing.


Users have no way to accurately measure the resolution at all apertures and at all parts of the image. Consequently, they often claim the diameter of the observed circle projected onto a surface or focusing screen at the plane of focus as the image circle diameter.

Trust the lens maker’s data. It was determined by a careful process using special equipment.

 

[Ari]

Thank you, Ian.

 

[LJH]

Trust the lens maker’s data. It was determined by a careful process using special equipment.

Unless you're using some process lenses such as G Clarons and APO Nikkors. They have much bigger ICs than stated based on their initial intended use.

 

[ic-racer]

You are probably interested in the image circle with the lens stopped down. Ground glass view of the corners (assuming you even have corners) with a loupe, when stopped down, can be difficult. So you will likely need to use film. However, the edges of the circle will likely fall outside of the 8x10 film with the lens centered. To avoid having to construct a 16x20 back to test the lens you can center the lens on the film and then raise it all the way up. Measure the displacement. When you process the film add the displacement to the center of the film and that is your circle center. This allows the measurement of a Radius. The image circle will be twice the radius.

 

[Ari]

Simple and elegant; thank you, IC.