what is ISO?
ISO is your camera's sensitivity to light. The lower the ISO number, the less sensitive it is. Essentially, photos with lower ISO settings will turn out smooth & creamy, for lack of better of words! If you're confused, don't stop reading!
Back in the days of film, a roll of film only came in one speed (like 100 or 400, for example.) When you put a roll of film in your camera, you were committed to one film speed for the entire roll. Low numbers, such as film speed 100, indicated that the photos would have a much better quality & were suited for brightly lit environments. Rolls of film with speed 400 would turn out a little bit grainy, but could be used indoors. When digital cameras came out, you still had that option of film speed in the form of ISO, & the same principles applied. The lower the ISO setting, the better quality your photo would turn out.
the cost of ISO
As I mentioned in this article, cameras need a whole lot more light than you may initially think. By increasing your camera's sensitivity (increasing your ISO), you are better able to photograph in low-light situations. Meaning, your camera will better pick up on light sources, however small they may be. However, that does come with a cost!
Have you ever seen a photo & it just looked really gritty? Grainy photos or photos with a lot of noise are results of high ISO settings. When you increase your ISO, the quality of your photo will go down. To get more light, you sacrifice quality.
Another cost of shooting with high ISO is that it becomes more difficult to salvage photos during the editing process. In situations where you may not have had time to properly adjust your settings, especially if your ISO was too high, the grain will start to become more & more pronounced as you edit the photo to look like you originally intended.
But, not all grain is bad! Sometimes a little bit of grain adds to the aesthetic of the photo.
Your camera has a set range of ISO settings, which is something like 100-6400. My rule of thumb is to always always keep my ISO at the lowest setting possible, then adjust aperture & shutter speed. I only change the ISO when I have to. If you're shooting outside on a bright, sunny day, keep the ISO as low as it can go. When you step indoors, bump the ISO up to about 400 & stay close to the windows. If you find yourself in the middle of a super dark dance floor with only flashing lights as your source of light, the highest ISO possible is your best bet.
Every now & then, I'll find myself at an outdoor reception around sunset. To let as much light in as possible, I keep my aperture wide open & choose a shutter speed that freezes the action. As the sun sets, I'll notice that my exposure is slowly becoming underexposed. It is then that I bump up my ISO one stop (while leaving my f-stop & shutter speed the same) to keep my exposure the same, even if that means the photos are a little bit grainier than before.
I really hope this helps you step away from auto! Contact me if you have any questions & I'll do my best to answer them.
The main thing to remember is to figure out which of the three take priority, then make adjustments to the other two. Every photographer is different with his or her style, & there is no right or wrong way to tackle a photo project. Good luck!