How can I know the actual brightness of xxx lumens (or vice versa)? All I know is that all the flashlights I've bought have much less lumens than claimed (an "1800 lumens" flashlight is dim enough for anyone not to believe its claim). It is hard to tell how bright a light is from videos.
MTBR's light shootout now has an integrating sphere to measure the total light output. Of course, that doesn't say how wide the beam is, or how evenly it lights up the road.
Their review navigation between pages is confusing. Try googling "MTBR shootout + your light" to find it's page.
For example:
Magicshine 808 Claimed 1000, actual 648 lumens
Lezyne Mega Drive: clained 1000, actual 1140 lumens.
There's more lights on the
2012 shootout pages
The Backyard Beam shots are good for comparing the light's coverage.
I don't know of any tests for the under $50 lights. Their actual lumens might be 1/2 or 1/3 of their claim. But that's still a lot. Their beam patterns are probably fairly uneven, with a bright hot spot in the center. Not bad for the price, but I'd bring a spare light in case the primary light fails on a ride.