There are two factors working here. The first, as
Pat mentioned, is that monitors are limited as to what they can show. The DPI is set, and generally 73 for the most
part, but that only limits the finest detail that can be shown, and if you compare the pictures, tha same detail is there. The other
part is the light amplification. Going from "ISO" 50 to 400 is a 8 x step in amplification, however amplifiers can't be selective as to what they are amplifing, and there is always background noise, this is increased also, and becomeds more noticeable with the higher ISO settings. It is the electronic equivalent of "grain" in traditional photography.
If you want to improve the overall
quality of the picture, us
good strong lighting, the weake the light sources the less information is transmitted to the camera sensors and and detail is lost. In engineering terms, it is: signal
quality = log [ signal + noise] /noise, when the
quality number is 10 or highter, it is all signal, and the noise does not produce a significant factor, but below that, noise is an important factor, and color, saturation, and detail all suffer from
poor light. Changing the ISO can have some effect, but changing the lights or increasing the brightness has a bigger impact. There are trade of issues, too much light or too harsh, will burn out details, too little light, the details are not registered, as it is all shadow. Take some time and read Ansel Adams book on the Zone System, it is harder to
work with in the digital form than in traditional photography.
The intensity of the light is an a inverse square to the distance from the object, halving the distance, doubles the intensity and is equal to 1 "F" stop or one shutter speed change, but it can make a difference. Also being able to place secondary reflectors to change or modify shadows can
help.
Bruce