Before making my initial post I wrote to "pj", who I always considered to be the expert on lighting and several other aspects of gear on BPL. Here is his reply, and I hope you find it as informative as I have.
"25 lumen, 40 lumen, 'nn' lumen - what's the right amount of light?
no fixed answer.
based upon several criteria:
1) your own personal physiological and psychological abilities, meaning age, diet, natural unaffected ability to see in low light, prescription meds, & habits (e.g. smoking), health - all of which can affect low light vision; plus how much (or little) light do i feel comfortable using w/o getting the feeling that i might miss a "blaze" or turn in a trail, or feel that the darkness is closing in on me ("lions and tigers and bears, oh my!").
2) am i moving somewhat quickly (let's say, 4-5 mph) and i want to feel confident that i can pick out low contrast, faded "blazes" on tree trunks and rocks at least 30 feet away in a sufficient amount of time to make any adjustments in your direction of travel (at 4mph you're moving ~6 feet per second, so you only have 5 seconds to pick out the "blaze" if you have a somewhat arbitrary 30' viewing distance).
3) how much burntime do i want. do i want to change batteries in the middle of a night hike? if so, make sure that you have a Photon Freedom Microlight for use to facilitate the changing of the battery. or if you're not "solo" hiking, then your mate's light will suffice for this purpose. i like at least 8h of burntime on one "refueling", so to speak. i could live with 3.5-4.0 hours on a single (or pair/three/four - if the light takes more than one cell/battery). AA/AAA/CR123A, however, which would necessitate one battery change during 8h of continuous night hiking.
4) what is the beam pattern of the headlamp or flashlight like? is it very floody (e.g. an array 5mm LEDs, like the PrincetonTec Corona headlamp & the two new models/generations PT Eos HL )? is it a tight spot (e.g. the original PT Eos HL)? is it a mix w/a decent spillbeam in addition to a fairly bright spot (Fenix flashlights, and most other flashlights for instance). more floody lights generall require a higher lumen output since the brightness of the light is is spread over a larger area. a light w/too tight a spot and little relatively bright spillbeam means that you might miss something or that you will need to "chicken walk" w/your head bobbing about placing the tight, bright spot on individual trees and rocks to make sure that you don't miss a "blaze", for instance.
my little Fenix P1D-CE and P1D-Q5 lights are the bare minimum i can get away with on their minimum setting (12 & 16 lumen respectively - though my claimed 12 lumen P1D-CE is a "hot" performer and brighter than some other "Q5" versions that i've owned, and have since given away, with the exception of my current Q5 which is very bright).
a Fenix L0D-Q4 still available (checkout 4sevens.com, BatteryJunction.com, BrightGuy.com, or Lighthound.com - i've made many purchase from each of these over a period of 2-5 years; no problems with any of them; all top notch and honorable - i've spoken via telcon w/all of the owners of these businesses - nice fellas, all), or the current model LD01 will work fine on its medium setting, but your burntime will be at most 3.5 hours.
additionally, while we're on the subject, i no longer use coin cell powered microlights (even e+LITE or Scout). they ALL after just 15min of continued use on HIGH drop to just 25% of their starting light output (see BPL's Rick Drehrer's tests and runtime plots). they'd be fine for in camp task lighting or reading, however, but not for nighttime hiking, IMO. here are two MUCH BETTER choices for a low power, long burning light:
a) the older Fenix E0 or its current replacement, the Fenix E01. expect >10 hours of burntime and closer to 16-20 hours on a single AAA cell. however, neither will probably acceptable for night hiking. dead flat regulated o.p. (output) for most of that time, then "moon mode" low light o.p. for a few hours which is at least as bright as a coin cell powered light after its been on for 2-4 hours, depending upon the particular coin cell powered light.
b) the 9V "transistor" battery powered Pak-Lite (available fr/Lighthound.com or www.9voltlight.com) has both high & low modes (except for the "basic" model), and comes in various models (i own 'bout 9 or 10 of them - all different models except for two of their brightest 2 WHITE LED models) and burn so long that i haven't timed them yet, easily 20+ hours on HIGH is my qualitative reckoning (they claim ~60hours). however, not sure that you could hike on HIGH with this one either - i haven't tried.
both the E01 and the 9V Pak-Lite can EASILY be used, however, for walking/strolling down a simple foot path in pitch black. even my old, age degraded low light vision has no problem. if you're a young'un you'll have no trouble, but since they both use 5mm LEDs, you won't see 30' with them. 15' yes, but NOT for the entire burntime as the battery runs down.
lastly, please keep in mind that most flashlights rate their burntime down to 50% of the initial starting brightness. however, the three major HL Mfrs (PT, BD, & Petzl) all now use the same spec & measure both distance/throw and burntime down to an unrealistic 0.25 lux. this value is fine for a small task light, but is worthless for hiking a trail at night. you can only very dimly see your feet with 0.25 lux (~6 feet dimly). they claim that this is the amount of light produced by a full moon on a clear, cloudLESS night. hence, the very long burntimes claimed by some HL Mfr's are only good if you are going to use the light for in camp task lighting or reading in your shelter. you'll get nowhere near their claimed time in UNREGULATED lights, but regulated light ought to, and generally will, perform closer to the Mfr's claims.
the best thing to do is to borrow someone's and try it out to see if it fits your needs. hopefully, the info i gave you here will prove somewhat helpful in making a good decision.
please feel free to share this info on BPL, but please remove my email address before posting this 'PM'. however, leave this request in so that no one gives you any "flak" for sharing "private"/PM info publicly."