Image circle

Image circle of a lens (calculated for sensors in APS-C format) that becomes visible when the lens is attached to a camera with a sensor in small image format .

In photography , the image circle describes the area that a lens can illuminate on the image side. In order for the film or the sensor to be fully illuminated, the diameter of the image circle must be at least as large as the diagonal of the film or sensor format.

It is possible to use lenses with a large image circle (e.g. for 35mm format ) on a camera with the smaller sensor (e.g. APS-C ). If, on the other hand, a lens with a small image circle is used together with a camera with a large sensor, the image circle becomes visible in the image (see illustration). The transition zone at the edge of the image circle from the illuminated to the unlit area is very small and its effect should not be confused with vignetting or the edge light drop .

View cameras in particular require lenses with a particularly large image circle in order to permit extensive adjustments. Tilt and shift lenses , which allow adjustment options similar to those of the view cameras, have a built-in limitation that only allows adjustment within the image circle.

calculation

The minimum required image circle diameter of a lens is calculated as the diagonal of the negative or rectangular sensor according to the Pythagorean theorem with the formula

${\displaystyle \varnothing _{\text{Bildkreis}}=D_{\text{min}}={\sqrt {B^{2}+H^{2}}}}$

format	Width (B) [mm]	Height (H) [mm]	Diagonal (ø) [mm]
35mm	36.0	24.0	43.27
Analogous to APS-C	25.1	16.7	30.1
Analogous to APS-H	30.2	16.7	34.5
Analogous to APS-P	30.2	9.5	31.7
Four thirds	17.3	13	21.6
Nikon DX format	23.6 ... 24.0	15.8 ... 16.0	28.4 ... 28.8
Canon xx0D, x0D, 7D	22.2 ... 22.5	14.8 ... 15.0	26.7 ... 27.0
Canon 1D	28.8	19.2	34.6
Canon 1Ds, 5D, Nikon D700, D3	35.8 ... 36.0	23.9	43.2

Land use and sensor format

Sensors and films are usually rectangular, but the lenses provide a circular image. If you take a film or sensor whose diagonal D corresponds exactly to the image circle, you can see that only part of the image provided by the lens is actually recorded by the sensor or film. This ratio can be calculated using the length of the sides of the rectangular sensor (B, H) and related to the area of ​​the image circle that the lens must at least provide in order not to vignette :

${\displaystyle {\frac {A_{\text{Sensor}}}{A_{\text{Bildkreis}}}}={\frac {B\cdot H}{\frac {\pi \,D_{\text{min}}^{2}}{4}}}={\frac {4}{\pi \cdot ({\frac {B}{H}}+{\frac {H}{B}})}}}$

From this it can be deduced that a sensor in 3: 2 format (35mm film, full format, APS formats) can in the best case utilize around 59% of the area depicted by the lens in the image circle; a sensor in 4: 3 format (uses of most compact digital cameras) to about 61% (16: 9: only about 54%) and a square sensor surface (e.g. 6 × 6 film) would be geometrically ideal with a use of about 64%.

literature

• Peter Bauernschmid (Ed.): Image Circle. A textbook and picture book for creative specialist photography . Linhof, Munich 2002