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