A faintblood aasimar results when all the descendents of a half-celestial mate with humans (to create aasimars) to the point where, several generations later, the human features begin to emerge again and the aasimar traits begin to fade. Most faintblood aasimars don't know their own heritage and consider themselves human.
Faintblood aasimars lose all the benefits of an aasimar's spell-like abilities, skill bonuses, and certain aasimar energy resistances. They have darkvision like aasimars, but at a reduced range due to having a lower density of darkvision retinal receptors.
Depending on the gender of the human parent, a faintblood aasimar gains one human racial benefit (either faster skill rank growth, or 1 bonus feat). A male human parent causes the human feat to dominate, and a female human parent causes the human skill bonuses to dominate.
A male aasimar parent leaves dominant only one type of energy resistance whereas the female aasimar parent causes all three to manifest in a milder way.
As with aasimars, Common and Celestial are automatic languages for faintbloods. As with humans, all other languages are available as bonus languages.
A faintblood aasimar cannot multiclass with impunity like humans can, but neither do they have a favored class like aasimars; rather, a faintblood aasimar's first class is his favored class.
What sort of vision lies in between an aasimar's darkvision and a human's standard vision? Intuitively, you'd simply average together human standard vision with aasimar darkvision, resulting in low-light vision, right? Wrong. Vision doesn't work that way.
Standard vision, low-light vision, and darkvision are all real-world phenomena.
Standard vision and low-light vision result from retinal nerve receptors that fire in response to intercepting photons of light. There's a continuum of visions possible between "standard" and "low-light." Real-world creatures have a mixture of both in varying degrees. Darkvision is a completely different thing — the ability to see in the absence of light. It's a completely different biological function of vision. They don't average. If even a small amount of darkvision retinal receptors remain, darkvision will still work.
Real-world human eyes have both standard and low-light vision, but our low-light vision is peripheral, not direct, therefore not useful for direct-viewing something in low-light situations. You can demonstrate this by closing yourself in a dark bathroom with no other light source besides a phosphorescent dial on a watch that glows too dimly to see at first. After your eyes adjust to the dark, you notice that you can see the watch dial only when you look away from it, not when you look directly at it. It's almost like a black circle obscures your view, blinding you in whatever direction you look.
We have two kinds of optical receptors. The "rods" respond to brightness (flux density of photons) and fire at a rate that depends on how many photons they intercept. The "cones" fire in response to intercepting photons of specific wavebands, but only when the flux density exceeds a certain threshold. When we look directly at something, the image focuses on the area of our retina where we have a high density of cones (color), and few rods (brightness). Cones don't respond in very dark environments, leaving us blind in whatever direction we look. This is also why we don't easily discern colors while looking at nebulae and planets through a telescope, although photographs bring the colors out. Rods always fire in response to the photons still in the environment, so they allow us to see in nearly-dark situations, but only peripherally because they concentrate in the peripheral areas of our retinas.
A creature having mostly rod receptors would have direct low-light vision. They would see light and shadow in daylight and in low-light, but have difficulty seeing colors. Dogs and octopi are examples. A creature with mostly cones would see colors vividly, and see light and shadow as long as the ambient brightness exceeds the threshold for the cones to respond — but they cannot see in darkened environments. Most birds are examples of creatures who are blind in low-light situations. When I worked at a zoo, and a bird got out of its cage (indoors), you could dim the lights, approach the now-blind bird, and grab it. Humans are in between, having only low-light peripheral vision.
Now what's darkvision? It exists in the real world too, artificially. In the total absence of visible light, objects above absolute zero temperature (-273°C) always radiate invisible electromagnetic energy at infrared and longer wavelengths. Objects in a room will radiate and bathe each other in their illuminations, creating shadows and areas of higher illumination, making the objects discernible to infrared sensors. The dominant wavebands for such radiation are near-infrared (NIR), midwave-infrared (MWIR), and longwave infrared (LWIR). Night-vision goggles typically work at NIR. Thermal imaging cameras and military sensors work at MWIR and LWIR. The amount of visible light in the environment is irrelevant — to these sensors, a scene will look about the same in the day or night, although brighter in the day due to solar warming.
A creature with darkvision would have retinal receptors sensitive to infrared, likely NIR because NIR wavelengths won't require too-large nerve cells and too-large eyes. Such a creature could see just as well in the dark as in daylight. NIR wavelengths are about 2X longer than visible wavelengths, so the receptors would have to be twice the size of visible-light receptors. NIR receptors would take up more space in the retina, and they will necessarily crowd out the normal cones and rods in order to realize vision of sufficient quality for clear sight at long distances.
So, back to a human/aasimar mixture. Suppose all receptor types (rods, cones, and infrared) were mixed together in equal amounts. He'd still have darkvision because of the infrared receptors. He'd have normal and low-light too, but darkvision would dominate in low-light situations.
Such mixed vision would have lower resolution because of the reduced density of each kind of receptor in the retina — so the creature may have trouble resolving objects at a distance. He'd still have darkvision unless the density of infrared receptors got so low that he couldn't make out objects even at close range. However, if the darkvision receptors had even 1/100 the density of the rods and cones, it would still be functional and useful at typical dungeoneering ranges (60 feet or less), albeit at lesser resolution. "Low resolution" in this case would be like a human viewing a daylight scene on a television rather than directly with our eyes. Human eyes have many times higher resolution than a television.
That's why faintblood aasimars have darkvision, albeit at lesser resolution, and therefore less range, than normal aasimars and celestials. This ability would persist generation after generation, getting lower in resolution, until it's too diluted to be called "vision" anymore, leaving the descendents with standard human vision. By that time, the other faintblood aasimar qualities will have disappeared also.
Alex Matulich