The requirements of this exercise are a little unclear as the opening line refers to the first part but there appears to be no second part. The idea to to consider the effect of increasing sensitivity. I took a series of photographs of the clematis bloom at 100, 200, 400, 800, 1600, and 3200 ISO. I then increased the magnification to 100% and reviewed the results.
Fig. 02 is a screen shot of the ISO 100 image compared with the ISO 1,600 image. The noise level is distinctly noticeable and there is a significant loss of detail. The colours are more saturated but there is a loss of highlights and thereby a loss of contrast.
Fig. 03 compares the 100 ISO photo with the 400 ISO. There is discernible noise at 100% magnification and the colour rendition and contrast has altered but in many circumstances this would be an acceptable starting point for a photograph.
Summary and Comment
Noise is a particular feature of digital photography and whilst it originates in similar circumstances it does not have the same effect on a photograph as film grain. Grain arises as a result of the size of the light sensitive grains in the film. Faster film has larger grains. Fast, i.e. more sensitive, film captures the subject accurately but with less definition. Noise is an artefact introduced into the image when the sensor lacks enough data to process accurately and this lack of accuracy translate into a fuzzy representation especially in the shadows.
Better sensors in DSLRs have reduced the problem of noise and, as can be seen in the example above, ISO 400 will typically cause few issues unless the required print size is very large. In assignment 3 I was often working in very marginal light and, because using long exposures or artificial light was not an option, at times I had to push the camera using very high ISOs. The most exaggerated instance of this approach was in the following image.
Fig. 04 1/125 at f/11, ISO 11,404
If I isolate the tricycle in a A4 sized view (100% magnification) the result is noisy.
There are vertical lines and a loss of definition (although it should be noted that the tricycle is a reflection in a shop window so is already indistinct.
There is no general answer to when there is too much noise. The photograph in fig. 04 works for me because the noise level is acceptable in the context of the subject, the theme and my intent. If it was intended to be printed as a poster it would probably become unacceptable and even at A4 this level of noise would be an issue if quality was a major factor such as a portrait or a landscape photograph for a advertisement or a brochure.
The summary is that the effect of using a very high ISO must be understood and restricted to circumstances where the impact is acceptable. Acceptable to a photo journalist will often be very different than to a landscape photographer so context is usually the most important factor.