Farther Vs. Further.

Further along the learning curve ... I was wondering which version to use.

"Throughout history, farther and further have been used interchangeably. Even the Merriam-Webster Dictionary (Must be written by a Yank????) states that the words are interchangeable; they are the inflected form of far.
It is not until recent history that they are becoming distinguished by grammarians."
Farther
Farther shows a relation to physical distance. If you can replace the word farther with "more miles" then you have done it correctly.
I wanted to run farther, but I became too exhausted.
Further
Further relates to metaphorical distance or depth. It is a time, degree, or quantity. It is also another way of saying "additional".

Gaffer was asking me recently about a Starbust(?) filter. I didn't shoot a damthing worthy of note, or humour, today so I thought I'd mess around. What led me to the initial messing was the set of new lights (LED).

First observation (Sans camera) ... Some appeared to be double and some single, for which I blame refraction. On some the light, of whatever colour, is seen through the side of what passes for a bulb, others are seen through the "end": the favoured few are seen at just the correct angle to see both the direct LED and the refracted one.

While messing further (With camera) I reached a setting where the lights get a star, to the detriment of the tree.

Phase ll ... Bring out the fluffy-water filter (Variable N.D.) and the tripod and have another bugrabout.