Re: Image diameter ....?

Art Reitsch wrote:

> I can't find a reference on this, so am I thinking straight?  A 4x5 
> film, taking the longer dimension, is 5inches = 127mm.  So if  a lens 
> has an image diameter of 127mm or more it will cover 4x5, right?  


Wrong.  The diagonal, not the length of any side is the important 
dimension.  So, using SQRT(4^2 + 5^2) = SQRT 41 = 6.403 inches = 162.6 
mm, rounding to 163 mm.

The term usually used is 'image circle' or 'circle of good definition' 
(which is smaller than the 'circle of illumination').  But remember that 
image circle is at its maximum at the largest f-stop number.  It is also 
at its minimum when focused on infinity.  As the rear standard is pulled 
back to focus on closer objects the image circle increases. 

> And the larger it is, the more movements permitted?  

Yes

> So on, say, a 75mm lens what would be a reasonable minimum image 
> diameter for some modest movements for landscapes?
> Thanks.
> Art

Lens design plays a role in determining the angle of coverage of a 
lens.  Modern lenses tend to have larger angles of coverage than do 
older lenses.  But to indicate the point, Calumet has two 75 mm lenses 
known as Caltar II Wide Angles.  One has an angle of coverage of 102° 
(and an image circle of 187 mm @ f/22) while the other has an angle of 
coverage of 105° (and an image circle of 195 mm @ f/22).   So at f/22 
these lenses would accommodate a very small amount of movement.   On the 
other hand, a 75 mm NIkkor-SW has an angle of coverage of 106° and an 
image circle of 200 mm, allowing for more movement than the previous two 
lenses.  Vignetting will occur beyond a certain range of movement. 

Tables for currently sold lenses can be found at the back of Stroebel, 
View Camera Technique. 


Francis A. Miniter