Image circle

Simulator

What the drawing shows

• The image circle

  Every lens projects a round image onto the sensor plane. Its diameter is the image circle. Outside it there is no picture at all; just inside it the picture is dim and soft. The figure a maker quotes is the diameter they will stand behind for sharpness and brightness, so many lenses show a usable picture a little beyond it.

• Coverage

  A lens covers a sensor when the sensor's diagonal is shorter than the image circle. The corners are the first thing to go: if the diagonal is longer than the circle, the corners of the frame are dark (vignetting), and the drawing shows them in grey outside the circle.

• The margin

  Half the difference between the circle and the diagonal is how far each corner sits inside the edge of the circle. Two or three millimetres is comfortable. Under a millimetre, the corners sit where illumination is already falling off, so expect darker and softer corners wide open, and a change when you focus close, because the image circle grows a little as a lens focuses nearer.

• One circle per series

  A lens series is designed for one format, so every focal length in it is built to the same circle, give or take a millimetre. That is why there is one chip per series. A Super 35 series covers about 31 mm; a full frame series 43 to 47 mm, which also takes in ALEXA LF and RED VV; medium format and 65 mm lenses cover 55 mm and more.

• Anamorphic

  An anamorphic lens squeezes the picture sideways before it reaches the sensor, by 1.33, 1.5, 1.8 or 2 times. The circle its maker quotes is still a plain diameter on the sensor, so coverage is judged the same way. The squeeze is shown with the series name; it changes the final frame's width, not the circle.

• Photo lenses

  Makers of photo lenses do not publish image circles. Their series are listed with the circle of the format they are sold for: 43.3 mm for full frame, about 28 mm for APS-C, about 21.6 mm for Micro Four Thirds, 55 mm for the 44 by 33 mm medium format backs. Most cover a little more than that, some a lot more, so a full frame photo lens on an ALEXA LF or a RED VV is worth a test rather than a refusal.

• The same lens, a bigger sensor

  Moving a lens to a bigger sensor does not change the lens; it changes how much of its circle you keep. A full frame 50 mm on a Super 35 sensor uses the centre of its circle and frames like a 75 mm would on full frame. The crop factor is the ratio of the two diagonals: the full frame diagonal of 43.3 mm divided by the sensor's.

• Where the figures come from

  Sensor sizes are the makers' active areas in the modes named under each button. Image circles are the makers' published figures where there is one; the rest are taken from the format, and the chip says "about". Measured circles vary by a millimetre or two between copies and between focal lengths in a series.