What Is a Siemens Star?
A Siemens star is a circle of black and white wedges — spokes — that converge at the centre. Near the edge the spokes are wide; towards the middle they become infinitely fine. Photographed straight-on, the centre turns into a gray blur circle where the camera can no longer separate the spokes. Because the spokes run in every direction, the star tests sharpness at all angles at once.
Using the Star to Check Focus
- Go fullscreen and place the camera square to the screen, with the star filling most of the frame height.
- Focus on the centre, using manual focus and magnification if your camera has it.
- Adjust focus until the blur circle is as small as possible — much easier to judge than a page of text.
Important — keep the camera far enough back. The chart can only be as fine as your screen’s pixels. If the screen covers more camera pixels than it has screen pixels, you are measuring the monitor, not the camera. Move back (or zoom out) until the pattern spans fewer pixels in the photo than it does on screen.
Measuring Resolution from the Blur Circle
Open the photo at 100 % and measure the diameter of the gray centre in photo pixels. At that diameter one black-and-white cycle spans π × diameter ÷ spoke pairs pixels, so:
- Cycles per pixel = spoke pairs ÷ (π × blur diameter).
- % of Nyquist = cycles per pixel ÷ 0.5.
- LW/PH = 2 × cycles per pixel × photo height.
Example: 72 spoke pairs and a 60-pixel blur circle → 72 ÷ (π × 60) = 0.38 cycles/pixel, or 76 % of Nyquist.
Reading the Reference Rings
The red rings mark the radius at which one black-and-white cycle spans 2, 4, 8 and 16 screen pixels. Inside the 2 px ring the star is finer than your monitor can draw, so any detail there is not meaningful. If the camera’s blur circle is smaller than that ring in the photo, move the camera back. The rings are off by default so the photographed star stays a clean target; switch on Reference rings in the panel when you want them.
Oval Blur: Astigmatism, Stabilization and Scaling
A perfectly round blur circle means equal sharpness in every direction. An oval one points to lens astigmatism, camera shake or image stabilization moving during the exposure, or non-uniform scaling in the camera’s processing. Colored fringes near the blur are moiré from the sensor’s color filter.
Siemens Star FAQ
How many spokes should I use?
Use 72 spoke pairs for most cameras. Choose 144 for very high-resolution cameras photographed from further away, and 36 for low-resolution webcams so the blur circle is not too large to measure.
Why is the blur circle oval?
Sharpness differs by direction. Common causes are lens astigmatism, motion or image stabilization during the exposure, and processing that scales the image differently horizontally and vertically.
Can I use the star for autofocus fine-tuning?
It is an excellent visual target for checking whether focus is accurate, but autofocus micro-adjustment is best done with a dedicated angled focus chart that shows front- or back-focus directly.
Why does the centre of the star look gray on screen too?
Near the centre the spokes become thinner than a screen pixel, so the monitor itself blends them into gray. That region is inside the 2 px reference ring and should be ignored.