What Are Native and Base ISO?

If you are looking to purchase a new camera, you will probably see the terms "native ISO" and "base ISO" thrown around. ISO, being one of the three fundamental exposure parameters, is a term that all photographers should understand. This great video tutorial will explain the differences between native and base ISO and the consequences each have on your photography.

Coming to you from David Bergman with Adorama TV, this helpful video tutorial will show you the differences between base and native ISO. In the days of film, ISO simply referred directly to the sensitivity of the film you were using and was easy to understand, but in the digital era, it gets a bit more nuanced. Most digital sensors actually only have one sensitivity level. When you change the ISO, what you are roughly doing is changing the amount of a post-sensor gain that is applied to the signal, unlike film, where if you changed ISO by swapping out the cartridge, you were actually directly changing the sensitivity. Practically, this distinction doesn't really affect your day-to-day work as long as you understand the consequences of raising the ISO on image quality, but it can still be quite helpful to learn more about how ISO works. Check out the video above for the full rundown from Bergman.

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3 Comments
Naruto Uzumaki's picture

While my experience is more in the RF space, there are many similarities to image sensors when it comes to the back end processing and managing of the signal path, since in both cases, you are essentially converting whatever is being senses into a tiny current at a very low voltage in both the case of RF and and image sensors.

Basically when since the sensor is dealing with extremely weak signals (very tiny amounts of current generated when light hits the sensor), the goal is to begin amplifying that signal as soon as possible. This is often done with fixed gain low noise amplifiers that are largely there to counter the signal losses of the analog signal path before you get to your more robust variable gain low noise amplifier.
Variable gain amplifiers will be more noisy than fixed gain ones, thus the harder you drive that VGA, the more noise it will introduce (this applied to extremely expensive lab equipment as well).

Usually if more sophisticated analog processing hardware cannot be located directly behind a sensor element, then you may have simpler hardware for picking multiple signal paths for different fixed gains to then feed into your VGA.

The areas where companies developing those sensors show off their engineering prowess, is developing more efficient photosites, lower loss signal paths, and better amplifiers given the limited space, which has to be really difficult, especially considering that it is a huge challenge even in equipment that doesn't have those same space constraints for the first amplification stage.

Brian Howard's picture

Nice video! Minus the light triangle, I haven't seen much with regards to how to use ISO and the rough "yea, it gets noisy".

I have a lot of photos where I needed this information to help make them a little better. A little noise is fine, but some of my photos have too much noise - as I was not understanding how much ISO becoming an issue. The one thing I find missing from a lot of learning material I have are rules of thumb on ISO. We have them for f stops (f4-f11 for most work, etc.) and speed (e.g. 2x focal length). But ISO is just lower is better.

Now to do some more experimentation with my Nikon .... thanks!

Naruto Uzumaki's picture

Given how modern cameras work, it is better to think of the exposure triangle as more of a mental framework for easy compensation for adjustments made.
Don't think of it as a way to get the proper exposure, and instead think of it as a framework for adjusting settings while maintaining the lightness that you want.

For example, if your image is properly exposed but the shutter speed is too low for your subject, then you may want to increase ths shutter speed. Suppose you double your shutter speed, then you lose a stop of light and thus need to compensate for that by the other 2 parts of the exposure triangle. For example, you may increase the aperture by one stop, or you may increase the ISO by one stop, or do a combination of the 2 and do a little extra aperture and a little extra ISO to add up to one stop.

Same applies if your image is too noisy. If the environment and subject allows for it, then you may lower the ISO by one or 2 stops, and then see if you can get the equivalent of the 1 to 2 stops from another part of the triangle. For example, dropping the shutter speed to gather 1 to 2 stops more light, or opening up the aperture if possible, or a combination of the 2.

Everything is done in context of the environment and subject e.g., if the high ISO noise is too much and you can compensate for lowering it by opening the aperture, it may not be right for the subject. For example, a group photo shot at f/1.8 may not look right.