Today’s Question: What would be the best way to determine at what point the higher ISO settings start to degrade image quality? I am guessing take the same exact shot at the lowest ISO setting and then work your way up to a high ISO setting. Then compare at 200% or 300% magnification. What exactly should I be looking for in the magnified image?
Tim’s Quick Answer: Yes, I recommend capturing test photos at different ISO settings, and then evaluating the results. It is a subjective decision, but you’re both learning how your camera behaves in terms of noise at different ISO settings, and also getting a sense of at what setting the noise tends to be beyond your comfort level.
More Detail: With most cameras you’ll see increased levels of noise as you increase to higher ISO settings, and with many cameras the noise can really start to be a problem as you get up above around 1600 ISO or so.
There is also variation in terms of the light levels in the scene, as well as other factors such as increased noise with long exposures. I therefore recommend capturing test photos of a scene that includes a relatively wide range of tonal values, from dark shadows to relatively bright highlights. I also recommend avoiding long exposures for the baseline testing, such as by using a relatively wide-open lens aperture so that even at a low ISO setting the shutter speed will be reasonably fast. You can separate the exposures by one stop, which means doubling (or halving) the ISO setting for each photo. For example, you could set the ISO to 100, then 200, then 400, and so on.
I then recommend zooming in to evaluate the images at about 100% as well as at about 400%. It can be helpful to cycle through the images multiple times to get a better sense of the noise behavior. I also suggest making sure you have captured a sample photo at the highest ISO setting your camera offers. There’s a good chance that when you look at that image up close, you’ll immediately conclude there is too much noise. This can help provide some context when evaluating the other photos.
There’s a very good chance (depending on your camera) that you’ll start to see that the noise is more obvious especially as you get up to the image at around 800 or 1600 ISO. Some cameras will have relatively problematic noise at lower ISO settings, and some provide relatively low noise at high settings.
Noise can be exhibited as both significant tonal variations and color variations at the pixel level. Noise will also be more prevalent in the shadow areas of the image rather than highlight areas. It can be helpful to brighten the images you’re evaluating so you can better see the noise that may be present in the shadow areas.
Again, this is a somewhat subjective evaluation, but the aim is to get a sense of what ISO setting start to be problematic in terms of noise from your perspective. Based on that, you can do your best to try to stay below the threshold you’ve decided on, to the extent that the circumstances will allow you to keep the ISO value below that level.
