Hi Yogy. When I say that the camera will 'see' more, this is a generalisation, as the senisitivity of the camera, depending on lens and conditions (see below), and especially with film, as opposed to electronic imaging, will, IN GENERAL, register more than the human eye will see. This is not to say that the human eye does not actually see an object, more that the brain registers what it thinks it sees, or wants to see, whereas, in photography, what is actually there is recorded in its true state, if it is possible to record it in the first place. An example of this would be, for instance, in photographing model aircraft. What we think is a perfect, pristine model, may exhibit flaws in paintwork, dust etc., when seen in a good, clear, close -up photograph. These imperfections will probably have been missed by the viewer.
Yes, you are right; the viewing conditions, night or day, will have a very large bearing on the ability to see tracer at a distance. Viewing from the rear, in other words, from the direction of the shot(s), will always appear brighter, as the phosphor compound is in the tail of the projectile and, of course, the whole purpose of tracer is to aid the gunner in seeing the track and fall of shot. Most tracer will appear a lot brighter in dull, or neutral, lighting conditions, and more so in clear, but not overly bright, night conditions.
In conclusion, because of the viewing conditions, it MIGHT be possible to see tracer, at night in particular, and at altitude certainly, at greater distances than I have already mentioned, but, of course, the calibre of the ammunition would have a bearing on this; the larger the calibre, the more chance of seeing it a greater distances.
If you have never seen tracer for real, regardless of calibre, a reasonable comparison would be a firework (the airborne type) that simulates tracer fire, as used in some movies, or at firework displays. If you see a 'star burst' type firework at great distance, this is not disimilar to a 'flak ' burst at similar distance and, roughly, the light emission from an 'average' tracer round below, say 40mm, is similar in intensity to some fireworks. There are many variables in trying to provide a full answer to your question. These include type of tracer compound, burn rate, ignition range, day or night tracer, trajectory, colour, weight ratio to balistic requirements, and, of course, the actual calibre of the round, to name a few.
Hopefully though, this will have provided some information that I hope has proved useful and helpful.
Terry.