ISO is the one camera setting you can change without it affecting how your image looks too much, at least for lower values. At higher values, visible digital noise can become an issue. So, let’s look at how to choose the right value for different situations.
The Default: Your Camera’s Base ISO
Every camera has a base ISO. This is the baseline sensitivity of the sensor, and it’s the value at which it operates best with the highest dynamic range. At every other value, the camera amplifies the signal generated by light hitting the sensor which in turn amplifies the amount of digital noise in the image.
For the vast majority of DSLRs and mirrorless cameras, the base ISO is 100, although a few high-end Nikon cameras have a base ISO of 64.
The base ISO isn’t necessarily the lowest ISO setting. For example, my Canon 5D III has an ISO 50 setting, but this is achieved by reducing the gain on the sensor.
Since you get the highest quality images at the base ISO, it should be your default for any situation you can use it. If you can get the shutter speed you want and the aperture you want with ISO 100 (or ISO 64, check your camera’s manual to be sure), then that’s what you should use.
Note: The image above was shot on a Canon 650D at ISO 100. The sample images for each ISO value below are cropped versions of the same image shot at the stated ISO value.
Digital cameras are incredible. They’ve come along in leaps and bounds over the years, and the reality is, any modern camera can take incredible images between ISO 200 and ISO 800 with almost no discernible drop in image quality—or at least, not without you looking for one.
If you need to use a faster shutter speed or narrower aperture than your base ISO will allow, you can confidently increase the ISO to around 800 without it having too much of an effect on the image. I regularly shoot portraits at ISO 400 so that I can guarantee my shutter speed won’t drop too low.
I’m kind of arbitrarily calling ISO 800 the top of this range because it’s as high as most entry level crop sensor cameras can go without seeing some reduction in image quality, but on some newer and full-frame cameras, you’ll be able to push it higher. The best thing to do is play around with your camera and see how it operates at different values.
Somewhere between ISO 800 and ISO 3200 range, you will start to see visible digital noise in your image even if you aren’t looking too closely for it. Again, it’s kind of camera-specific; with lower end or older cameras, you’ll see it at lower ISOs than with higher end or newer cameras.
This range is kind of the highest you can push your camera in most situations without making a definite sacrifice in image quality. It’s not the highest you can push it, but it’s the highest you can go and reliably get good images.
Increasing ISO to this point is a tradeoff. You’re almost certainly shooting at night or working somewhere dark and, if you can’t reduce your shutter speed or widen your aperture any more, then upping the ISO is your only option. In this range, you’re still going to get usable images, but they just won’t be the highest quality. Still, a good photo is better than no photo.
ISO 6400 and Beyond
Once you start to push past ISO 3200, you will see a dramatic increase in noise. As always, the exact value depends on your camera but, at some point, the images will become unusable, at least for professional contexts.
Where also depends on what you’re shooting. I did a series of night portraits at high ISO values and, because I embraced the noisy look, I was able to shoot them at ISO 6400 without worrying too much.
On the other hand, if you’re going for a super clean look, then you’re probably out of luck.
The other option is to look at other ways of reducing noise. Astrophotographers regularly shoot multiple photos at ISO 6400 and then combine use them in post-production to offset the noise from the other images. Since noise is random, it’s unlikely that the same spots will show noise in every image.
ISO is often the first setting to get changed when you need to increase an exposure, and that’s fine—up to a point. Once you see a visible decrease in image quality, you need to start thinking more carefully.