Each different film stock has a specific ISO/ASA(American Standards Association) rating, that lets photographers know how sensitive to light the film is. Films with lower ISO such as ISO 50, ISO 100 or ISO 200 are considered “slower” film, these films are less sensitive to light and will require more light to properly expose the image. They are known for having finer grain, more accurate color reproduction and greater dynamic range which means that they can distinguish between shades and tones of a particular color better than a faster film. They are also ideal for landscape and portraiture.
On the other hand, films with higher ISO such as ISO 800, ISO 1600 or ISO 3200 are considered “faster” film. These films are more sensitive to light, so they require less light to properly expose an image. They are more suitable for action and low-light photography and are characterized by having bigger grains and color shifts.
Grain can be reduced by increasing the resolution of the photograph, but it cannot be completely eliminated. This is why older photographs, which were taken on film, often appear grainy compared to photographs taken with digital cameras. With the advancements in computer vision and neural network technology, tools like digmypics' FaceEnhance are able to improve the quality of old photographs, reducing the visibility of grain and increasing the overall resolution of the image, making the memories captured decades ago more vivid and true to life.