ISO setting in regards to film is a rating that expresses the film’s sensitivity to light. Generally speaking a lower ISO such as 100 will give sharper images, more vivid colors, etc. In film days, the roll was rated at a certain speed, which could not be changed between shots.
With a digital camera ISO is a rating of the camera sensor’s sensitivity to light. The beauty of this is that the setting can be changed between shots to meet the photographer’s needs. In many cases the camera will select the speed automatically depending on the lighting conditions. In the earlier models of DSLRS (Digital Single Lens Reflex Cameras) this was not the case. The Canon 10D did not have this feature, but the 40D does. (I still prefer to make the decision myself). If the picture quality were better at the lower settings, why would one choose a higher one that results in diminished quality?
Things are fine as long as lighting conditions are excellent, but when one is photographing action scenes they must have sufficient shutter speeds to deal with the motion. In wildlife photography one often has to contend with both poor lighting conditions and subject movement. This involves compromises such as raising the ISO setting so that a satisfactory shutter speed may be obtained to stop the action.
Using a higher ISO may result in a softer looking picture with digital “noise” being visible. This usually appears as red and blue or green dots in the image with it being worse in shadow areas. DSLRS have made large strides in dealing with the noise through improved image processing in recent years so that it is possible to shoot at higher ISO settings and get more acceptable results than was previously possible. In fact some have said it is feasible to use many DSLRS at ISO 400 or 800 in all instances with little noticeable hit in quality.
One may also use post-processing tools, such as the noise reduction utilities built into Adobe Photoshop, or pug-ins such as Noise Ninja to remove noise, but we’ll try to keep this simple as possible and not go into that subject at this time.
We can address this in practical terms by talking about the photograph posted below.
It was very early this morning when these whitetail bucks came near me and started sparring. I decided to try IS0 500, which gave me a shutter speed of 1/180 at f2.8 with a Canon 70-200mm f2.8 L lens. Basically doubling the ISO rating will enable one to double the shutter speed, and cutting the ISO by one half will decrease the shutter speed by ½. Had I used IS0 200 I would have been at 1/90 or less and at 100 I would have been about 1/45. Even 1/180 is marginal for stopping this type of motion and 1/45 would certainly have resulted in blur unless I caught the animals at a moment of perfect stillness.
The good news is that if one shoots in decent light and situations that do not cause focus problems, that they do not need to concern themselves with many of these issues. As Wom Tigley pointed out in his response to my previous post he shoots a Canon S1, and uses a few basic settings and takes excellent photographs. By looking at his pictures I could not tell that he was using a point and shoot. “Salty” of Country Captures is another case in point and my Daughter “ASH”. Both have used the Canon S2 extensively with excellent results. Salty had a picture published in The Pennsylvania Game Commission Calendar and it fits right in with the others. No one would guess it was taken with a point and shoot camera. The main downside to these cameras is that ISO 400 is very noisy in poor light because of their small sensor size (the smaller the sensor all things being equal the more noise there will be) and their manual focus is difficult to use. This makes them less than ideal for wildlife photography, but they are an excellent choice for many tasks.
So much for the rant on ISO. I hope that clears things up for Wom, and others interested in the technical matters. If I am wrong or confusing on some of the above points, perhaps some of the knowledgeable DSLR photographers such as Abraham Lincoln, Salty, Chad Oneil, etc. can chime in an clarify things.