When I scanned photos from the Fuji Superia 400 film I was very unhappy with grainy shadows. The grain seems to be really worse there than in brighter areas even when considering the effect of higher noise visibility in dark areas. I couldn’t get rid of it even when I tried to significantly overexpose. By chance I found explanation of the problem in an old issue of the Czech PHOTO life magazine.
Actual sensitivity of negative amateur Fuji films (and this is likely to apply to other vendors’ films as well) is very different from what one could assume looking at the declared ISO number. First, it is actually significantly higher than declared, protecting you from underexposition. Second, maximum sensitivity (i.e. the ability to capture minimum light without underexposition) is about the same regardless of the declared sensitivity, the difference between Fuji Superia 100 and 400 is reported to be only about 0,5 EV. When you expose Fuji Superia 100 at ISO 50, you can go up to 4 EV from the middle towards darkness before the material gets underexposed, while when you expose Fuji Superia 400 as ISO 200, you’ve got only about 2,5 EV.
This explains what I observe – even when overexposing the ISO 400 film twice, there is a big risk of underexposed shadows, resulting in overgrown grain (note that according to Fujifilm specifications standard grain size should be about the same in properly exposed Superia 100 and 400). Indeed, scanning my latest Fuji Superia 100 film seems to confirm the facts. When I compare results from an ISO 400 film exposed as ISO 200 and from an ISO 100 film exposed as ISO 70, there is not much difference in the look of the grain in the highlights. But the grain is much worse in shadows of the ISO 400 film, while in the ISO 100 film the grain is about the same as in highlights there. So there is no point for me to use an ISO 400 film which is more expensive and provides lower dynamic range, while it offers only slight advantage in maximum sensitivity.
Sensitivity is one of the areas where digital easily beats film. Where I’m limited to ISO 50-100 with film, good digital cameras can go safely up to ISO 400-800. Additionally, the higher depth of field allows to use one step wider aperture. And finally, image stabilizers become common, which adds further 2-3 steps. Summed up, DSLRs can provide at least 6 EV advantage over film in nature photography.