Image Circle Article

Image circles and why they matter.

By Paul Slotboom  Summer 2010.

An image circle is simply the area that a given lens projects, measured in millimeters, while focused at infinity.   It's important that a lens projects a large enough image area to adequately cover the film or sensor size that is being used.  It's of even greater importance to pay particular attention to the image circle of a given lens, if you are using a camera such as an Alpa SWA, Max or XY.  These cameras allow the lens plane or the film/CCD plane to move up and down and left and right, depending on the particular model.  With these cameras, it's particularly critical to know the capabilitiy of the camera, compared to the limitation of the lens due to its image circle.  It's common for cameras to offer more movement than a lens will allow.

For example, if you're working with a Phase One P25 digital back, you will  require a lens with a minimum image circle of 61mm.  If you want to use the Alpa Schneider 24mm XL lens, which projects a circle of 60mm, you will barely be able to cover the CCD sensor with that lens (it is actually a bit too small for this sensor and it does cause some vignetting).  This will be a workable set-up on a camera such as a TC, which offers no movement.  But it would not be adequate for a SWA or Max, except in a zero non displaced mode.  If on the other hand, you want to work with an Alpa Schneider Apo Digitar 120mm lens with an image circle of 110mm, now you have the ability to use The SWA or Max with considerable movement employed.

Below is a chart of popular digital backs and the minimum image circles they require.  It's worth noting that many lenses produce a larger image circle at smaller apertures, such as f 11, than they do at wide open. 

*This chart is designed to be a practical and simple guide.  It is not intended to be 100% precise.  

Please Email me for more information or for further clarification.