Good advice, but since you say you enjoy learning how technology works, it might be worth pointing out that the difficulty in detecting motion, moving towards the camera, has nothing to do with the lens, or the software. Indeed the lens isn’t even active during this process, nor the Blink image analysis software, because the cameras aren’t even turned on.
Instead they rely entirely on the PIR sensor, which is a very low power method of detecting motion, and is pretty much the only part of the camera which is not in deep sleep mode at this point. This front on difficulty is an inherent limitation of most all PIR sensors and is a result of how they detect motion.
They are sensitive to infrared, and have two slits through which the IR passes. When the IR passes through a slit, it falls on a sensor and that creates a small voltage. It then compares that voltage to that from the sensor behind the second slit, and if the difference is above a certain adjustable threshold, the sensor triggers. This is what you adjust in the sensitivity setting in the app. When something approaches from the front, it’s common for both slits to be exposed to exactly the same amount of IR, and thus the change in voltage from the sensor is the same, so you do not pass the trigger threshold, unless it is set on very high sensitivity. When something moves across the sensor, invariably, each slit will be exposed to a different IR signal causing a voltage difference between the two sensors. If this difference is above the adjustable trigger threshold, the PIR will trip.
How this works in practice is that the output from the two sensors becomes the input to a differential amplifier, which when both inputs are the same it has a net output of 0. This allows the sensor to ignore the general background IR signal, which is present all the time. It also allows it to ignore the change you would get, if for example, a light was turned on, illuminating the entire scene, where the entire background IR would suddenly increase, but the increase would be the same for both slits, so the output from the differential amplifier would remain 0.
In the case of Blink, the PIR sensor triggering will wake up the camera, and then images can be taken, which will result in a recording, or, if you have activity zones set up, it will allow the camera software to perform image analysis, in line with how you have your zones set. If it now passes this further test, the recording will continue, and will be uploaded to the cloud.
More sophisticated PIRs have two pairs of slits, or even more, helping them to be sensitive in more directions, but for whatever reason, it would appear, based on observed behaviour, that Blink have gone with by far the most common type. You might even say the ‘bog standard’ version with a single pair of slits. Could be for any one of a number of reasons, but I suspect the most likely is cost. They are so common, in huge numbers, such as Blink would require, they would be incredibly cheap. With no trouble at all you can find them for under £1 each, delivered, on ebay, and that will hardly compare to what Blink/Amazon could achieve.
EDIT: Good picture that sums it up below: