Having received the 12x telelens for a smartphone, I have got a first impression and taken a few test photos.
Take into account that this is only a small test using the most common settings with the telelens attached.
I settled for a budget priced lens kit with a simple clamp to hold the lens.
Looking at the kit, it seems intended for (I)phones 6-10mm thick.
Two clamps are included, a slim one covering a single lens and a second one intended for dual lenses phones.
The fp4 is not slim enough for the two lenses clamp, but fortunately it is possible to make the single lens clamp fit with a small fix. And it does not cover the light sensor.
It is light, the lens and clamp weight less then 60 grams.
A small fix.
To avoid the lens to rotate or being displaced while adjusting the focus ring, I printed a mount to hold the clamp in place.
The FairPhone 4 case and dummy by nainno - Thingiverse was used as base, and modified to hold the clamp firmly in place.
Even using the manual focusing assisted by the fp4 auto focus it is very difficult to obtain a sharp picture.
Mounting the fp4 on a small tripod only stabilise the images, but the optical problems persist.
Only a small area appears focused while edges are blurred.
First a 12 MP picture.
Then 12x zoom.
I happened to get an area of a 48MP picture exposed well enough to compare a part of the picture with the 12MP and 12X Zoom of the same scenario.
Note that all pictures has been saved using 75 pct. compression to save a bit of space.
No other corrections than showing the area of interest are made.
12MP.
12x zoom.
48MP.
From these test shots, it seems a bad idea to use an attached lens unless you find one which matches the optics of the phone/camera.
However, the lens can be used as a very small 12x monocular.
You might have more success with other brands or less zoom lenses.