Instoscope

the extinction meter image by Mario Groleau (Image rights)

Drem Instoscope image by Anssi Puisto (Image rights)

The Drem Instoscope is an extinction meter. When viewing through the ocular one could see a number of letters. The least visible letter is explained on the table which is wrapped around the instrument. The table shows the possible shutter-speed/aperture combinations for the least visible letter.