I only buy LED's with 15 - 20 candela output and I've found that a 10K current limiting resistor provides enough light from the 20 candela LED's to be useful with normal room lighting. I build devices that are battery operated and low current is a requirement. I just plugged a green LED into a breadboard. Cathode grounded a 10K resistor from +5V (4.1Vdc measured) and measured 2.5V across the LED subtracting that from 4.1V = 1.6V divided by 10000 = 160 uA and provides a visible light that could be more visible and draw half the 160uA too if it was pulsed off and on at a 50% duty cycle. There is a small increase in price for the 5 mm high output LED's.. I buy the 5 color assortments and surprisingly the light output from batch to batch has been strangely uniform... An odditie when buying loose parts from China.. IMO
Bob