Every photographer would love to shoot at the lowest ISO their camera allows all the time to maintain the highest possible levels of detail and dynamic range. But sometimes, that just isn't possible; in fact, sometimes, you have to pump up the ISO quite high to get the shot. Do not be afraid to do that.
We all wish we could shoot every photo at ISO 100 to maximize the quality of our images, and while certain genres have that luxury, others sometimes have to pump that setting well above the base level. Because of that, it can be very tempting to sacrifice the other two exposure parameters, particularly shutter speed. When I was new to photography, I certainly tended to try to keep my shutter speed as slow as possible, almost to the point of obsession. But it was the wrong way to think about about exposure, and it caused me to miss images both for technical reasons and because my mind was preoccupied with the wrong thing.
The majority of nonprofessionals (and a reasonable proportion of professionals) shoot with crop sensor cameras. Crop sensor cameras tend to have about a stop worse noise performance than a comparable full frame camera, and as such, they tend to start showing noise sooner.
Similarly, the older the camera, generally, the worse the noise performance. The earliest digital cameras (even the first full frame models), amazing as they were at the time, were quite horrible by today's standards. For example, a few years ago, I purchased a Canon 1D Mark II N just for the fun of playing with an older camera. The 1D Mark II N came out in 2004 and features an APS-H sensor (1.3x crop factor), smaller than full frame, but larger than today's APS-C sensors. Nonetheless, it tops out at ISO 1,600, and above about ISO 400, the results are noticeably bad. I even purchased a Canon D30, Canon's first homegrown DSLR, which came out way back in 2000 with a 3.1-megapixel APS-C sensor. By today's standards, those specs are laughable, but back in 2000, it was an incredible camera.
And yet, even with those extremely limited feature sets, photographers have been creating great images with them and similar cameras for the last two decades.
Why You Should Not Be Afraid to Bump That ISO
So, now that we have established that even older cameras with relatively horrible ISO capabilities can still take decent images, let's talk about some of the reasons why you should not be afraid to bump up the ISO on the likely more modern camera you are using.
Camera Shake and Motion
When you are shooting moving subjects, it is crucial to have a fast enough shutter speed to freeze them. Similarly, for a given focal length, you can only slow the shutter down so much before your hands start to blur the image due to camera shake. Many photographers err too far on the side of a slower shutter speed in an effort to keep the ISO down and end up introducing one or both of these two issues. Remember, you can always apply noise reduction, but there is very little you can do to fix a photo that is blurry due to camera shake or motion blur. On the other hand, if your subject is still, but camera shake is an issue, grab a tripod.
Wide Open Isn't Always Best
Sharp and Grainy Is Always Better Than Blurry
The Proper Exposure in Camera Is Almost Always Better Than Fixing in Post
Try taking your camera out and shooting one photo at ISO 3,200, then the same photo with the same shutter speed and aperture settings at ISO 100. Then, bring both photos into Lightroom and raise the ISO 100 photo five stops to match the ISO 3,200 photo. Which image is noisier?
This is because when you increase exposure in post, you are increasing both read noise and shot noise. On the other hand, increasing ISO in camera only increases shot noise. Of course, you may want to protect highlights by underexposing a bit, but when it comes to noise, it is generally best to get your exposure as close as possible in camera.
Have a Top ISO in Mind and Set Your Camera Accordingly
Many photographers choose a top ISO they're comfortable with given their camera equipment and what they're shooting. Many cameras have built-in functionality to handle this, such as an auto ISO mode that allows you to set the maximum allowable ISO or the ability to set the maximum ISO in partially automated modes such as aperture priority. Digging into these options on your camera can be very useful for a lot of situations.
Learn Artificial Lighting
All of the discussion up to this point has hinged on the assumption that it was not possible to add more light to the situations that were being discussed. After all, I don't think I can plop a flash on a baseball field during a game, and some wedding clients specifically request no flash. But in situations where adding light is possible, if it's a lack of understanding holding you back, it's time to start learning. You don't need ultra-expensive equipment; just a cheap speedlight and softbox are great to learn the fundamentals of how light behaves and how you can shape it.
Don't be afraid of bumping up your ISO when you need to. You can work with a sharp image that has noise in post, but an image that's blurry due to subject motion or camera shake or that doesn't have enough depth of field can't be brought back. Sharp and noisy is always better than blurry.
Lead image was taken in a room lit only by candlelight and a few small LEDs: 1/80 s, 200mm, f/2.8, ISO 6,400.