Your camera's ISO setting
Cameras work with light. However, light comes in different strengths, influenced by weather, location, time of year and time of day. This may sound obvious, and it is.
In the old days of using film, each film would come with a rating of how light sensitive it is. This is expressed as “ISO” and a number, e.g. ISO 200. The “ISO” part means “International Standards Organisation” (derived from “International Organisation for Standardisation”) and is just an agreed upon standard.
The number part refers to just how sensitive to light a film is. The higher the figure, the more sensitive to light a film is.
A film or setting that is more sensitive to light allows higher shutter speeds which, in turn, can prevent shaky images.
When I first started out in photography, back in the early 80's (1980's, not 1880's, thank you very much), film came in 25, 50, 64, 100 and 400 ISO.
Now, ask yourself: why should anyone want to use a 25 ISO film when he or she could use a 400 ISO film and avoid carrying a tripod?
The short answer is because of “grain” (and with digital, noise). All photos are made up of small dots (for film) or small blocks (pixels) for digital.
In the case of film, the higher the ISO, the larger these dots are. Eventually, this starts to show. Sharpness inevitably suffers and grain can look ugly, depending upon the subject. For instance, a grainy sky in a black and white landscape can look great, but if you were shooting a portrait, then grain usually looks terrible.
For digital, instead of grain, we have “digital noise”. Whilst film grain can have a certain charm, noise is just plain ugly. Noise is simply a mass of multicoloured dots or specks, usually in darker areas and often also in highlights. Colours often suffer at high ISO settings and inevitably, so does sharpness. Put simply, noise is best avoided.
Which is why I used to use a film called Kodachrome ISO 25........and a tripod in all but the brightest weather.
The main differences between film and modern digital is that for digital, ISO is not “fixed”. You can almost always adjust this. However, most cameras arrive from the factory with their ISO set to “AUTO”. This is where the camera's meter automatically works out when to push the ISO (light sensitivity) up. Obviously, some do this better than others.
But plainly, cameras are not infallible. They are not to know that you're using a tripod or support, for instance. Most cameras will assume that you're holding the camera in your hands and hence will set a high ISO to stop camera shake by setting a higher shutter speed.
As I've already mentioned, though, digital noise is rarely attractive. So, you're left to choose whether to leave the camera on AUTO ISO or control it yourself, maybe resorting to a support of some kind. A monopod, tripod or similar.
Some more expensive cameras do have noise control features built in. However, use these carefully, as noise removal almost always causes a loss of detail. Some cameras are better at this than others, and full frame DSLRs tend to have the edge.
In photography, it's rare to get something for nothing. You can keep the ISO high and risk noise, or you can keep the ISO low and endure carrying a tripod. Sometimes, you may be forced into raising the ISO because you really need that high shutter speed. To catch a fact moving object, for instance. Or you may need the slowest possible shutter speed to allow those “blurred water” shots beloved of landscape photographers.
The choice is yours, but my advice is to work out your own ways of working with ISO- but don't just leave it to the camera. Adjust to suit your aims and working conditions.
As a rule, I keep my camera's ISO usually at ISO 400, dropping to ISO 200 on very bright days or conditions. Only rarely will I go above this.
See what works for you. Study your photographs at least at 1:1 (full size) and see how the noise level is.
Most areas of photography are a compromise. Find your own way of working, but you'll find this best by experimenting and finding out exactly what these settings do. Come back here often, and I'll hopefully help with this.