Artificial stars

Single white pixels on a black screen. The camera sees them as stars, so the YAN field inspector can check sensor tilt, centring and corner sharpness without going out under the sky.

How to use it

• An OLED screen works best (true black). With an LCD, turn off the room lights.
• When the screen fills the frame, one screen pixel covers roughly as many camera pixels as the ratio of their resolutions (e.g. a camera 6000 pixels wide on a Full HD screen: about 3 pixels). That is similar to stars on the sky. A 4K screen gives smaller dots.
• Point the camera straight at the screen at the distance from the calculator and focus on the dots.
• Choose brightness and exposure so the dots are not saturated. A few tens of percent and a short exposure are usually enough.
• Take a frame and open the field inspector in YAN. It needs dots in each of the 9 fields of the frame, so the screen should fill the frame.

What it does not replace

The screen is a few metres from the lens, not at infinity, so focus sits a few millimetres further out than on stars (the calculator shows how much). Field curvature and coma look different up close than on the sky. It is enough to check sensor tilt, centring and to compare lenses. The final test still happens on stars.