Everything About “ISO” in Photography

The ISO function on your camera controls the sensitivity of the sensor to light. It comes in handy in a number of situations, especially when shooting in low light and when shooting action photos. But ISO has far greater applicability. In this video, Tony Northrup explains both the basics and the advanced theories behind ISO:

If you find the concepts in this tutorial difficult to digest, don’t sweat it. Just switch to auto ISO and fire away happily.

Many photographers leave their cameras on auto ISO, because it’s easy to adjust exposure in post-processing, even if the camera goofs up—provided, you shot in RAW. Usually, the camera does a good job of figuring out the applicable ISO in a given situation.

There’s one situation when using auto ISO may not be a good idea, and that’s night photography. Additionally, Northrup also avoids using auto ISO when shooting in his studio where the light don’t communicate well with the auto exposure settings on his camera.

What is ISO?

Let’s say that your camera is set to a certain aperture and shutter speed. You manually set the ISO to 400, which gives you a good enough exposure. Dropping the ISO two stops to 100 will produce an image that is darker. Alternately, if you increase the ISO by two stops to 1600 the resulting image will be extremely bright.

comparing images shot at different ISO

Same exposure value but different ISO

ISO and Noise

A downside of higher ISOs is the associated digital noise. The higher you crank the ISO number the more noise you’re going to see in the image. But even at lower ISO numbers, your images are bound to have some amount of noise.

Dynamic Range

Higher ISO impacts dynamic range in an adverse way. At higher ISOs, the dynamic range on your photos will be be a lot less than when shooting at a lower ISO.

Extended ISO

All cameras have a native ISO range, and many cameras have what is called as the extended ISO range. For example, he lowest ISO that your camera can shoot at is ISO 100, but there may be an extended ISO of 50. Usually, shooting at ISO 100 and ISO 50 doesn’t produce anything different. The camera just overexposes the image by one stop and adjusts the brightness. If you’re shooting in RAW, all you can do is just make the image a stop darker and you have ISO 50. For JPEG shooting it does make a difference though.

ISO Invariance

More often than not, the difference between shooting at ISO 100 and ISO 6400 is just a digital amplification of the amount of light that is captured. Northrup shows how an image shot at ISO 100 and another at ISO 6400 were identical at first look. Yes, that’s right.

ISO Invariance

ISO Invariance

The trick was that he cranked the ISO 100 exposure in post by six stops! The two resulting images look identical. This goes to show that in many cases it doesn’t matter when it comes to shooting at low ISO. As long as the information is there, it can be retrieved during post-processing. Just a word of caution though: Nikon (at least the D810) and Sony cameras do tend to perform better than Canon and other brands.

Noise Performance APS-C vs Full-Frame Cameras

At a given ISO an APS-C camera will have twice as much noise as a full-frame camera. You can counter this by using a lens that uses a faster aperture. In other words, if you switch from an f/5.6 lens to an f/4 lens you will get twice the amount of light.

Did the video answer all your questions about ISO? What else are you wondering?

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