Lens Image Circle Explained
Even though we work with images on a rectangular canvas, lenses project their images in a circular shape. All of the lenses designed for a particular format or imager size will roughly project the same size circle, or at least a minimum circle to cover the rectangular imager within the diameter of that circle. This is called the image circle, or covering power. In order for a lens to work with a given format, the diameter of the projected image circle must be equal to, or greater than, the hypotenuse (diagonal) of the rectangular imager target.
If the image circle isn’t sufficient to cover the fun imager, there will be darkening, known as vignetting (or shading), or even complete blackness, in the corners of the image where the circle doesn’t cover. If the darkening is substantial enough, it can be seen as a portholing effect (named as such because it looks as if you’re viewing the world through a ship’s porthole).
The image circle of a lens is dictated by the optical design of that lens, the size of its rear elements, and the distance the rear elements are from the imager, called the optical back focal distance. Every lens designed for a specific camera format, regardless of the focal length, must project a minimum image circle that covers that format’s imager completely.
As with all aspects of a lens, its performance will be better at the center of the lens than off to the sides. This decline in performance occurs gradually at first and then degrades progressively out toward the edges of its image circle. Attributes such as light transmission, aberration corrections, resolution and contrast will all be better at the center of the lens than at the edges. Lens designers often take this factor into account by creating a lens with a conservative circle of good definition (CoGD) that covers the imager size for the designated format. This is the area of the image circle with the best performance and least aberrations.
It is possible, and even fairly common, that there is additional image information projected beyond this circle of good definition, exceeding the dimensions of the intended imager, that the optical designer in most cases does not consider in the overall performance evaluation and optimization of the lens.
Within this additional area of the image circle – called extra covering power – the optical performance of the lens can degrade rather quickly. Until fairly recently, this additional image area was completely ignored by optical designers and filmmakers alike. In the 2010s, however, as larger digital cinematography imagers were being introduced, more and more the extra covering power of an existing lens became a major benefit for allowing that lens to be used on a larger format than it was originally intended.
Lenses with larger overall image circles can be used on larger imager formats than they were intended for, but at the risk of seeing a potential degradation to the performance and increased aberrations of the lens by utilizing the weaker portion of the extra covering power. These different diameters are also sometimes referred to as the image circle (aka circle of good definition) and illumination circle (the total of CoGD and extra covering power).
It should be pointed out that actual physical limitations within a lens barrel (its metal housing) can also affect the transmitted image circle. The barrel itself can obstruct the outer reaches of a lens° angle of view capabilities, especially the front of wide angle lenses and the back of longer focal length lenses. This physical confinement of an image circle, from within a lens, is not necessarily the same as other forms of vignetting occurring outside the lens, such as mechanical vignettes, which will be discussed later.
Additionally, it is possible that a lens designer deliberately implements physical obstructions within the lens to limit the light rays passing through it. These internal mechanical vignetting elements improve the performance of the lens by restricting the light path at the expense of the size of the image circle.
Because we deal in rectangular (rarely square) imager sizes, by simple geometry, the diagonal of the imager will always be larger than the horizontal or vertical dimension.
The image circle’s diameter, therefore, must be large enough to fully cover this diagonal, which is why we often talk about imager sizes in terms of diagonals when we’re talking about lenses.
The Subjectivity of Image Circle and Illumination Circle
Many manufacturers of cinema lenses advertise the specific format that the lens is intended to be used with: 16mm, Super 16, Academy 35, Super 35, full frame, etc. Some will go so far as to specify a specific diameter to the lens’ image circle: 27mm, 33mm, 43mm, etc.
As noted above, many lenses (most perhaps) have image circles beyond their circles of good definition and therefore can “cover” a larger imager than they were designed for (with a potential loss in overall performance in this “bonus” illumination circle/extra covering power area. As camera imagers grow in size and the extreme variety of sizes continues to permeate the marketplace, cinematographers are constantly asking “will this lens cover my particular sensor?”
The answer to that question is complex, subjective and completely up to the individual aesthetics of the cinematographer and the particular project they’re shooting. Performance of any lens is probable to drop off toward the edges. If you’re using a lens on a larger imager than it was designed for, you’re almost certainly going to see a drop in performance (contrast, resolution, sharpness, flare control and increased aberrations) at the edges of the image. Whether or not this drop in performance is acceptable is wholly up to the individual cinematographer.
“Coverage” may just mean that the image circle is large enough to have illumination at the corners of the image with no portholing. Within extra covering power there can be a drop-off in resolution and contrast, an increase in aberrations and a falloff in overall intensity of illumination/exposure. Yet, even that measurement is up to individual aesthetics. One cinematographer may feel that 50% of the illumination at the corners of the image, compared to the center, is too dark, while another may feel that 20% is perfect as they prefer the vignetting at the edges of the frame.
Likewise, some cinematographers may demand greater edge-to-edge performance than others. This discrepancy in personal creative aesthetics makes scientific measurement of these specifications difficult to impossible. Also, the aesthetics for one cinematographer may change drastically between projects. A commercial shot on a white cyc background may be a situation where nearly any falloff in the corners is unacceptable, whereas the same cinematographer shooting a period drama may openly embrace significant vignetting as part of the look.
Additionally, each manufacturer has their own specifications as to what defines their circle of good definition. Some manufacturers may allow a more significant drop in performance – measured by its modulation transfer function response – than another manufacturer in order to increase the advertised coverage of their lenses.
Others will be extremely conservative and only report the image circle for the best performance of the lens in terms of resolution, contrast, aberration control and lack of falloff, even though their extra covering power may be sufficiently large to cover a much larger imager.
There is no standard, nor any specific metric, by which to measure what the definition of “coverage” is. This is partially due to individual manufacturer practices, but it is also due to the fact that this is a relatively new conundrum.
For several decades prior to the digital revolution, a lens covered Super 35 or it didn’t. That was about all that You had to be concerned with. Lenses for 65mm cameras were in their own category and lenses for 16mm cameras likewise stood alone.
— — — —
This has been an excerpt from the excellent book “The Cine Lens Manual” by Jay Holben and Christopher Probst, ASC. Visit to https://www.cinelensmanual.com to see more details and learn more.
— — — —
You can visit the CineD Lens Coverage Tool to see lens coverage of a large database of modern and vintage lenses, combined with most modern professional and semi-professional camera sensors.