TTL vs Flip-up
What is the difference between TTL and flip-up Loupes?
TTL = "through the lens". This means the Loupes are made with the telescopes (oculars/magnifiers) literally inside the lens. They cannot be removed or modified - they are truly custom made for each face.
Flip-up Loupes are different in that the telescopes are not attached to the lens. On flip-up Loupes, the telescopes are generally attached by an "arm" mechanism that attaches to your frame. This allows for the telescopes to be adjusted at the wearers discretion.
Although there are a few other factors, the key difference in functionality comes down to the ability to adjust the Loupes.
TTL Loupes feature limited adjustments. While you can adjust the fit (nose pads and temple arms), you cannot adjust the position of the telescopes. Flip-up Loupes on the other hand can be adjusted just about any way you'd like. You can adjust the declination angle, the IPD, and the height of the telescopes - they're so adjustable that you can even share them with other people.
You can begin working the moment you put on your TTL Loupes but if others try and use your TTL Loupes, they will likely find it very uncomfortable and ineffective. This is because TTL Loupes are custom built for each individual face. The IPD (inter-pupillary distance) and declination angle varies from person to person depending on how tall they are, how far apart their eyes are and their working ergonomics. Flip-up Loupes can easily overcome these obstacles for each individual as they can be adjusted to meet these varying requirements.
The main trade-off for these adjustments is weight. Flip-up Loupes weigh significantly more than TTL Loupes because of the "arm" mechanism used to attach the telescopes to the frame.
TTL Loupes - Truly custom made for each face, lightweight and features only "fit" adjustments.
Flip-up Loupes - Heavier, but features more adjustments and shareable.