This technique is more challenging to master than direct phoneoscopy however when mastered it can give excellent results as illustrtrated above with Houman's beautful images taken with the iPhoneX and a Volk panretinal 2.2 lens.
Suitable for a much wider range of phones as the distance between the light source and the lens is much less critical than in the direct technique.
It is also less dependent on pupil diameter however, as with the traditional indirect ophthalmoscopy technique, it is easier to get good images with a dilated pupil.
1) Install a camera app which allows the light to be on continuously as for the direct technique. (e.g. Camera + for the iPhone or DSLR app for the Android phone) OR use the native video app and select the illuminating light to be on.
2) work out the minimum focal distance for your phone camera.
3) Open the camera or video app
4) turn the camera light on.
5) looking at the phone screen use the phone camera to obtain a tapetal reflection
6) Hold the diagnostic lens between thumb and forefinger with the little finger resting on the patients temple and the lens held away from the eye.
6) Position the diagnostic lens between the phone and the eye as you would for indirect ophthalmoscopy ensuring:
a) the lens is held perpendicular to the light path between the phone camera and the pupil.
b) the lens is the optimum distance from the pupil so that the retinal view fills the entire lens.