SLR, 35mm fixed-focus head shot 1.8m How far should the model stand?

The cmos size of a general truncated camera is 24.9×16.6 mm.

Take the horizontal shot as an example.

1.8 meters high is1800 mm.

1800/16.6 =108.43 times

You are using a 35mm lens now.

35 *108.43 = 3795.18mm, 3.795m away from the lens.

Vertical line:

1800/24.9=72.29 times

35 * 72.29 = 2530mm

It takes 2.5 meters

Just divide the camera distance by 1.6.

It should be very comfortable at your present distance. 24 fixed focus