The PhotoPlug Shutter Speed Tester
With this little plug, you can measure the shutter speeds of analog cameras - just with your smartphone! It contains a small light-sensor that detects the actual amount of light passing through your camera. Just connect it to the headphone jack of your smartphone and turn it into a portable shutter-speed-tester. More information: www.filmomat.eu/photoplug
Your smartphone has no headphone jack? Just use an adapter cable - the PhotoPlug will work perfectly fine.
Please note: In order to use this item, you need to download the "Shutter-Speed" app first. The app is available for free on the iOS and android AppStore.