Auto ISO

Gone are the film days and gone with that is the fixed light sensitivity of films. Now the ISO can be changed with just a turn of the dial or through a simple setting in the menu. How does this help?


(Mountains when a dark patch of clouds cleared up – I don’t even remember if I was using Auto-ISO at that time or not. When the clouds clear up!)


What is Auto ISO?

ISO in present day cameras roughly indicates the sensitivity of the sensor (What is ISO? Read here – Basics of Exposure). However being electronic in nature this sensitivity can be boosted or lowered, thereby allowing the ISO to be changed. Auto-ISO is a very simple setting that lets the camera decide the ideal ISO for the scene in front. There are a three main factors that the camera considers – the camera settings, lens being used and available light.

Camera settings like aperture and shutter speed decide the amount of light that is finally going to hit the sensor. Auto-ISO settings takes the cue from built-in light meter and decides if the light entering the camera with the settings is sufficient for a good exposure. If the ambient light is low and camera settings will lead to an under-exposed or dark image, Auto-ISO just increases the ISO so as to compensate for the low light. Like I said earlier, Auto-ISO also has a third component – the lens being used. I’ll tell you about it a little later.



(Cart-wheel – f/5.6 and shutter speed of 1/200 sec with 50mm lens, on aperture priority with center-weighted metering. Auto-ISO kept the setting at 100 ISO. Read on to understand more)


Camera Modes and Auto-ISO

With different camera modes (Camera modes (PSAM and more)), the time when Auto-ISO kicks in is very different.

Shutter Priority : When the light is low, the camera first keeps on opening up the aperture. Once the limit to how much the aperture can be opened (which depends on how fast or slow the lens is) is reached, Auto-ISO starts raising the ISO depending on the available light.

Aperture Priority : This is my favorite when it comes to Auto-ISO. What camera does is to calculate the exposure and if the shutter-speed appears to be falling below the 1/focal length rule, it automatically enhances the sensitivity of the sensor (increases the ISO). For example- with a 50 mm lens, when the shutter priority mode is being used with Auto-ISO, the ISO will start increasing once a lowest shutter speed of 1/50 sec is reached. The time when Auto-ISO kicks in is also configurable. Those with steady hands or using any of the stabilizing aids can opt for a lower than 1/focal length limit. I use Auto-ISO at -1, which effectively means that camera pushes the ISO in such a manner that the shutter-speed is never below 1 stop from the 1/focal length or about ‘1/ half of focal length’ being used. Thankfully, my hands are still quite stable.

Manual Mode : Both aperture and shutter-speed are set by user and the camera tends to boost up the ISO when the camera’s light meters sees an underexposure with the used settings.  I do not recommend using Auto-ISO with manual mode in most cases. (Obsession with Manual Mode). The only exceptions being the wildlife and bird photography.

Automated modes – Program Mode, Auto and various other automated modes just try to balance out everything on their own. If you use any of these modes and read my articles too, it’s time to shift to the above three camera modes.


(Burnt-down forest – captured in aperture priority mode with Auto-ISO. I had set the aperture to f/8. Camera set the ISO to 720 so as to maintain a minimum shutter speed of 1/50 with Nikkor 50mm lens. My hands were not that steady on that day due to the emotional impact the burnt forest had on me. I therefore set the minimum shutter speed to 1/focal length in Auto-ISO setting. Do read this – Forest Fire)



Too high an ISO causes noise. So, be on the look-out. Don’t shoot at ISOs which cause so much of noise that the final image gets spoilt. If possible, set an upper limit too in Auto-ISO.

Auto-ISO does not lower the ISO beyond the lowest setting on your camera. If you plan to shoot on a bright day with 1/60 sec and f/2.8 exposure, the ambient light maybe too bright for your camera at an ISO of 100. Auto-ISO will not move the ISO down to 25 or lower to compensate. It will be limited by the ISO set by you and by the camera itself.

Switch off Auto-ISO if you are planning on making HDRs. It can interfere with your exposure bracketing. (HDR Photography)


Chain and Ring

(Chain and Ring – f/8 at 1/50 sec. Auto ISO set the ISO to 560 when I had dialed in -1EV exposure compensation with center-weighted metering)


Now, armed with this powerful yet simple setting – go ahead and capture the compositions that you visualize without bothering too much about low available light. Just don’t be completely dependent on Auto-ISO!


Did you know that most mobile phone cameras use Auto-ISO as a default setting? In most phone cameras this setting is hidden and can’t even be disabled!


Some food for thought for the advanced photographers who happen to find this article – High ISO causes noise, ETTR or exposing to the right reduces noise. Astro-photographers use high ISOs to reduce noise (shot noise) So, which of these scenarios are better – Photographing a bird at 1/500 sec, f/8 with Auto-ISO setting the ISO to 1600 and capturing as raw, or, dialing in +1 EV compensation and letting the ISO to be set to 3200 with everything else remaining the same?


Further Reading:
Basics of Exposure
Camera Shake – How to avoid it?

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