I always wondered why there is regular ISO and then there is low. Most of the time I found that there is no difference and the low iso produced cleaner image. That’s what I thought, then I took series of images to find the difference. ISO low can be used to get clean image as long its there is no need for blown out details in highlight.

 

1. Well lit image

In medium light area

In most of the properly exposed areas.. they looks the same..

In the highlight  area

In Highlighted ares..the whites are more blown out in ISO 64 where as ISO 200 retains the details.

In Shadows

Not much difference between low ISO and ISO 200

2. Image in the dark or low lit

This is 100% view of the building more than 1.5 mile away from the camera, viewed at 100%, 45mm zoom. Though there is no details in the highlight area in this image there is no place where is there is a gradual transition. In this kind of places, it is better to use ISO (low) 64 to get little cleaner image.

Though there is a tiny bit of noise in the sky, a very slight noise reduction will fix that.

Though there slight advantage to ISO 200, this is hardly noticeable.

Conclusion

Use low ISO when there is no gradual detail from highlight to shadow, otherwise stick with ISO 200, which is on the safer side.