Exercise - Higher and lower sensitivity

 

Exercise - Higher and lower sensitivity

Surrey Street Market (1)
f/13  1/40  ISO-100

Surrey Street Market (2)
f/18  1/60  ISO-400

Surrey Street Market (3)
f/11  1/40  ISO-100

Surrey Street Market (4)
f/18  1/60  ISO-800

Surrey Street Market (5)
f/13  1/40  ISO-100

Surrey Street Market (6)
f/22  1/60  ISO-800

Surrey Street Market (7)
f/18  1/25  ISO-100

Surrey Street Market (8)
f/22  1/50  ISO-400

Surrey Street Market (9)
f/14  1/30  ISO-100

Surrey Street Market (10)
f/20  1/60  ISO-800 

Surrey Street Market (11)
f/13  1/40  ISO-100

Surrey Street Market (12)
f/18  1/80  ISO-800

Surrey Street Market (13)
f/14  1/40  ISO-100

Surrey Street Market (14)
f/18  1/80  ISO-400

I took these photos with the intention of achieving a similar exposure at an ISO of 100 and then at 400 / 800.  Reviewing the photos now, the second shots at ISO-400/800 are noticably lighter than the similar shots taken at ISO-100, particularly in the shadow areas.
In hindsight, I got a bit carried away with the situation, enjoying shooting the scenes below my high viewpoint.  It would have been better for this exercise to take more marginal / extreme shots, at the highest shutter speed or aperture possible.  Increasing all images on screen to 100% there was little noise on any of the photographs.  Obviously, with more marginal shots a clear difference would be apparent between the low and higher ISO images.  Increasing the image sizes further, the higher ISO photos retained their quality to a large size, but this was marginal due to minimal noise on all images. 
What was very clear after taking a photo at ISO-400 or 800, and then changing the ISO to 100, was how the screen or viewfinder was thrown into darkness.  Vice versa, increasing the ISO meant that the shutter speed and aperture could be increased for the next photo, with greater light available.
Changing the ISO from 100 to 400 increases the cameras sensitivity to light by four times.  In this series of photos, the aperture has been around mid-range.  Looking at the shutter speeds though, you can see that they are relatively low through the shots at ISO-100.  There would not have been much scope to increase the shutter speed without then compromising on the depth of field, by selecting a low aperture.  At ISO 400, it is possible to increase the shutter speed by four times, or reduce the aperture by four times.
Up until doing this exercise, I have got into the habit of setting the ISO to auto.  Now that I have a greater understanding of its function, and the extra control it allows, I will now set the ISO manually.